General Description
The MAX1836/MAX1837 high-efficiency step-down con-
verters provide a preset 3.3V or 5V output voltage from
supply voltages as high as 24V. Using external feedback
resistors, the output voltage can be adjusted from 1.25V
to VIN. An internal current-limited switching MOSFET
delivers load currents up to 125mA (MAX1836) or 250mA
(MAX1837).
The unique current-limited control scheme, operating
with duty cycles up to 100%, minimizes the dropout volt-
age (120mV at 100mA). Additionally, this control scheme
reduces supply current under light loads to 12μA. High
switching frequencies allow the use of tiny surface-mount
inductors and output capacitors.
The MAX1836/MAX1837 step-down converters with inter-
nal switching MOSFETs are available in 6-pin SOT23
and 3mm x 3mm TDFN packages, making them ideal
for low-cost, low-power, space-sensitive applications.
For increased output drive capability, use the MAX1776
step-down converter that uses an internal 24V switch to
deliver up to 500mA. For even higher currents, use the
MAX1626/MAX1627 step-down controllers that drive an
external P-channel MOSFET to deliver up to 20W.
Applications
●9V Battery Systems
●Notebook Computers
●Distributed Power Systems
●Backup Supplies
●4mA to 20mA Loop Power Supplies
●Industrial Control Supplies
● Handheld Devices
Features
●4.5V to 24V Input Voltage Range
●Preset 3.3V or 5V Output
●Adjustable Output from 1.25V to VIN
●Output Currents Up to 125mA (MAX1836) or
250mA (MAX1837)
●Efficiency Over 90%
● 12μA Quiescent Current
● 3μA Shutdown Current
●100% Maximum Duty Cycle for Low Dropout
●Small 6-Pin SOT23 and TDFN Packages
Selector Guide appears at end of data sheet.
19-1919; Rev 3; 7/06
*EP = Exposed pad.
T = Tape and reel.
PART TEMP RANGE PIN-
PACKAGE
TOP
MARK
MAX1836ETT33-T -40°C to +85°C 6 TDFN-EP* AJG
MAX1836ETT50-T -40°C to +85°C 6 TDFN-EP* AJE
MAX1836EUT33-T -40°C to +85°C 6 SOT23 AANY
MAX1836EUT50-T -40°C to +85°C 6 SOT23 AANW
MAX1837ETT33-T -40°C to +85°C 6 TDFN-EP* AJH
MAX1837ETT50-T -40°C to +85°C 6 TDFN-EP* AJF
MAX1837EUT33-T -40°C to +85°C 6 SOT23 AANZ
MAX1837EUT50-T -40°C to +85°C 6 SOT23 AANX
MAX1836
MAX1837
IN
GND
LX
SHDN
FB
OUT
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
OUTPUT
3.3V OR 5V
INPUT
4.5V TO 24V
GND
LXIN
1 6 OUT
5SHDN
FB
MAX1836
MAX1837
MAX1836
MAX1837
SOT23
TDFN
TOP VIEW
2
3
GND
IN
1FB
2
3
4
LX
6 OUT
5SHDN
4
MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Typical Operating Circuit Pin Congurations
Ordering Information
EVALUATION KIT AVAILABLE
IN, SHDN to GND...................................................-0.3V to +25V
LX to GND.......................................................-2V to (VIN + 0.3V)
OUT, FB to GND.......................................................-0.3V to +6V
Continuous Power Dissipation (TA = +70°C) (Note 1)
6-Pin SOT23 (derate 8.7mW/°C above +70°C)...........696mW
6-Pin TDFN (derate 24.4mW/°C above +70°C).........1951mW
Operating Temperature Range..............................-40°C to +85°C
Junction Temperature.......................................................+150°C
Storage Temperature Range...............................-65°C to +150°C
Lead Temperature (soldering, 10s)...................................+300°C
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = 0°C to +85°C. Typical values are at TA = +25°C,
unless otherwise noted.)
Dual Mode is a trademark of Maxim Integrated Products, Inc.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Input Supply Range VIN 4.5 24 V
Input Undervoltage Lockout
Threshold VUVLO
VIN rising 3.55 4.0 4.4 V
VIN falling 3.45 3.9 4.3
Input Supply Current IIN 12 25 µA
Input Supply Current in Dropout IIN(DROP) VIN = 5V 18 µA
Input Shutdown Current SHDN = GND 3 7 µA
Output Voltage (Preset Mode) VOUT
FB = GND,
ILOAD = 0 to 125mA
(MAX1836) or
250mA (MAX1837)
MAX183_EUT50,
MAX183_ETT50 4.80 5.00 5.20
V
MAX183_EUT33,
MAX183_ETT33 3.168 3.30 3.432
Output Voltage Range
(Adjustable Mode) VOUT (Note 2) 1.25 VIN V
Feedback Set Voltage
(Adjustable Mode) VFB 1.200 1.25 1.300 V
OUT Bias Current VOUT = 5V 2.5 7.4 µA
FB Bias Current IFB VFB = 0 or 1.25V, TA = +25°C -25 +25 nA
FB Dual ModeTM Threshold VFB rising or falling 50 100 150 mV
LX Switch Minimum Off-Time tOFF(MIN) 0.2 0.4 0.6 µs
LX Switch Maximum On-Time tON(MAX) VFB = 1.3V 7 10 13 µs
LX Switch On-Resistance RLX VIN = 6V 1.1 2 Ω
LX Current Limit ILIM
MAX1836 250 312 450 mA
MAX1837 500 625 850
LX Zero-Crossing Threshold -75 +75 mV
MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
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Note 1: Thermal properties are specified with product mounted on PC board with 1in2 of copper area and still air.
Absolute Maximum Ratings
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these
or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect
device reliability.
Electrical Characteristics
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted.) (Note 4)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = 0°C to +85°C. Typical values are at TA = +25°C,
unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Zero-Crossing Timeout LX does not rise above the threshold 30 µs
LX Switch Leakage Current VIN = 18V, LX = GND, TA = +25°C 1 µA
Dropout Voltage VDROPOUT IOUT = 100mA, VIN = 5V 120 mV
Line Regulation VIN = 5V to 24V 0.05 %
Load Regulation IOUT = 0 to 125mA (MAX1836) or 250mA
(MAX1837) 0.3 %
Shutdown Input Threshold VSHDN VIN = 4.5V to 24V (Note 3) 0.8 2.4 V
Shutdown Leakage Current ISHDN VSHDN = 0 or 24V -1 +1 µA
Thermal Shutdown 10°C hysteresis (typ) 160 °C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Input Supply Range VIN 4.5 24 V
Input Undervoltage Lockout
Threshold VUVLO
VIN rising 3.55 4.4 V
VIN falling 3.45 4.3
Input Supply Current IIN 25 µA
Input Shutdown Current SHDN = GND 7 µA
Output Voltage (Preset Mode) VOUT
FB = GND,
ILOAD = 0 to 125mA
(MAX1836) or
250mA (MAX1837)
MAX183_EUT50,
MAX183_ETT50 4.80 5.20
V
MAX183_EUT33,
MAX183_ETT33 3.168 3.432
Output Voltage Range
(Adjustable Mode) VOUT (Note 2) 1.25 VIN V
Feedback Set Voltage
(Adjustable Mode) VFB 1.200 1.300 V
OUT Bias Current VOUT = 5V 7.4 µA
FB Dual Mode Threshold VFB rising or falling 50 150 mV
LX Switch Minimum Off-Time tOFF(MIN) 0.2 0.6 µs
LX Switch Maximum On-Time tON(MAX) VFB = 1.3V 7 13 µs
LX Switch On-Resistance RLX VIN = 6V 2 Ω
LX Current Limit ILIM
MAX1836 250 450 mA
MAX1837 500 900
MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
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Electrical Characteristics
Electrical Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted.) (Note 4)
Note 2: When using the shutdown input, the maximum output voltage allowed with external feedback is 5.5V. If the output voltage is
set above 5.5V, connect shutdown to the input.
Note 3: Shutdown input minimum slew rate (rising or falling) is 10V/ms.
Note 4: Specifications to -40°C are guaranteed by design, not production tested.
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
LX Zero-Crossing Threshold -75 +75 mV
Shutdown Input Threshold VSHDN VIN = 4.5V to 24V (Note 3) 0.8 2.4 V
Shutdown Leakage Current ISHDN VSHDN = 0 or 24V -1 +1 µA
100
95
90
85
80
70
0.1 10 1001 1000
MAX1836EUT33
EFFICIENCY vs. LOAD CURRENT
MAX1836/7 toc02
LOAD CURRENT (mA)
EFFICIENCY (%)
75
VIN = 9V
VIN = 12V
VIN = 5V
FIGURE 1
VOUT = 3.3V
3.27
3.29
3.28
3.31
3.30
3.32
3.33
0 150 20050 100 250 300 350
MAX1837EUT33
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc03
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
VIN = 9V VIN = 5V
VIN = 12V
FIGURE 2
100
95
90
85
80
70
0.1 10 1001 1000
MAX1837EUT33
EFFICIENCY vs. LOAD CURRENT
MAX1836/7 toc04
LOAD CURRENT (mA)
EFFICIENCY (%)
75
VIN = 9V
VIN = 12V
VIN = 5V
FIGURE 2
VOUT = 3.3V
0
40
20
100
80
60
160
140
120
180
0 100 15050 200 250 300 350
MAX1837EUT33
SWITCHING FREQUENCY vs. LOAD CURRENT
MAX1836/7 toc05
LOAD CURRENT (mA)
FREQUENCY (kHz)
VIN = 9V
VIN = 5V
VIN = 12V
FIGURE 2
VOUT = 3.3V
3.27
3.29
3.28
3.31
3.30
3.32
3.33
0 4 8 12 16 20 24
MAX1837EUT33
OUTPUT VOLTAGE vs. INPUT VOLTAGE
MAX1836/7 toc06
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
IOUT = 10mA
IOUT = 200mA
FIGURE 2
VOUT = 3.3V
L1 = 47µH
3.27
3.29
3.28
3.31
3.30
3.32
3.33
0 10050 150 200
MAX1836EUT33
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc01
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
VIN = 5V
VIN = 9V to 12V
FIGURE 1
MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
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Electrical Characteristics (continued)
Typical Operating Characteristics
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.)
70
80
75
90
85
95
100
0 4 8 12 16 20 24
MAX1837EUT33
EFFICIENCY vs. INPUT VOLTAGE
MAX1836/7 toc07
INPUT VOLTAGE (V)
EFFICIENCY (%)
IOUT = 10mA
IOUT = 200mA
FIGURE 2
VOUT = 3.3V
L1 = 47µH
100
1
0 8 12
10
INPUT VOLTAGE (V)
FREQUENCY (kHz)
4
MAX1837EUT33
SWITCHING FREQUENCY vs. INPUT VOLTAGE
16 2420
MAX1836/7 toc08
IOUT = 200mA
IOUT = 10mA
FIGURE 2
VOUT = 3.3V
L1 = 47µH
0
200
600
400
800
1000
0 4 8 12 16 20 24
MAX1837EUT33
PEAK INDUCTOR CURRENT vs. INPUT VOLTAGE
MAX1836/7 toc09
INPUT VOLTAGE (V)
PEAK INDUCTOR CURRENT (mA)
IOUT = 10mA
IOUT = 200mA
FIGURE 2
VOUT = 3.3V
L1 = 47µH
LIMITED BY
tON(MIN)
LIMITED BY
ILIM
4.96
4.98
5.00
5.02
5.04
0 10050 150 200 250 300
MAX1837EUT50
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc10
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
V
IN
= 12V TO 24V
V
IN
= 7V
V
IN
= 9V
FIGURE 6
100
95
90
85
80
70
0.1 10 1001 1000
MAX1837EUT50
EFFICIENCY vs. LOAD CURRENT
MAX1836/7 toc11
LOAD CURRENT (mA)
EFFICIENCY (%)
75
V
IN
= 9V
V
IN
= 24V
V
IN
= 18V
V
IN
= 7V
FIGURE 6
V
OUT
= 5V
V
IN
= 12V
200
250
300
350
400
0 100 200 300
MAX1837EUT50
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1836/7 toc12
LOAD CURRENT (mA)
DROPOUT VOLTAGE (mV)
0
50
100
150
FIGURE 6
V
OUT
= 5V
10
11
12
13
14
15
0 4 8 12 16 20 24
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX1836/7 toc13
INPUT VOLTAGE (V)
SUPPLY CURRENT (µA)
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MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Typical Operating Characteristics (continued)
(Circuits of Figures 1 (MAX1836) and 2 (MAX1837), VIN = 12V, SHDN = IN, TA = +25°C.)
100µs/div
MAX1837EUT50
LOAD TRANSIENT
A: IOUT = 10mA to 250mA, 200mA/div
B: VOUT = 5V, 20mV/div
C: IL, 500mA/div
VIN = 12V, FIGURE 6
400mA
200mA
5.02V
0
B
A
C
MAX1836/7 toc14
5.00V
4.98V
750mA
250mA
0
400µs/div
MAX1837EUT50
LINE TRANSIENT
A: VIN = 9V to 18V, 10V/div
B: VOUT = 5V, ROUT = 100Ω, 100mV/div
C: IL, 500mA/div
FIGURE 6
20V
10V
5.1V
0
B
A
C
MAX1836/7 toc15
5.0V
4.9V
500mA
0
400µs/div
MAX1837EUT50
LINE TRANSIENT NEAR DROPOUT
A: VIN = 5V to 12V, 5V/div
B: VOUT = 5V, ROUT = 100Ω, 100mV/div
C: IL, 500mA/div
FIGURE 6
15V
10V
5.1V
5V
B
A
C
MAX1836/7 toc16
5.0V
4.9V
500mA
0
200µs/div
MAX1837EUT50
STARTUP WAVEFORM
A: VSHDN = 0 to 2V, 2V/div
B: VOUT = 5V, ROUT = 100Ω, 2V/div
C: IL, 500mA/div
VIN = 12V, FIGURE 6
2V
0
2V
4V
B
A
C
MAX1836/7 toc17
0
500mA
0
MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
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Typical Operating Characteristics (continued)
Detailed Description
The MAX1836/MAX1837 step-down converters are
designed primarily for battery-powered devices, notebook
computers, and industrial control applications. A unique
current-limited control scheme provides high efficiency
over a wide load range. Operation up to 100% duty cycle
allows the lowest possible dropout voltage, increasing
the useable supply voltage range. Under no-load, the
MAX1836/MAX1837 draw only 12μA, and in shutdown
mode, they draw only 3μA to further reduce power con-
sumption and extend battery life. Additionally, an internal
24V switching MOSFET, internal current sensing, and a
high switching frequency minimize PC board space and
component cost.
Current-Limited Control Architecture
The MAX1836/MAX1837 use a proprietary current-limited
control scheme that operates with duty cycles up to 100%.
These DC-DC converters pulse as needed to maintain
regulation, resulting in a variable switching frequency that
increases with the load. This eliminates the high supply
currents associated with conventional constant-frequency
pulse-width-modulation (PWM) controllers that switch the
MOSFET unnecessarily.
Figure 1. Typical MAX1836 Application Circuit Figure 2. Typical MAX1837 Application Circuit
PIN NAME FUNCTION
1 FB
Dual-Mode Feedback Input. Connect to GND for the preset 3.3V (MAX183_EUT33) or 5.0V (MAX183_
EUT50) output. Connect to a resistive divider between the output and FB to adjust the output voltage between
1.25V and VIN, and connect the OUT pin to GND. When setting output voltages above 5.5V, permanently
connect SHDN to IN.
2 GND Ground
3 IN Input Voltage. 4.5V to 24V input range. Connected to the internal p-channel power MOSFET’s source.
4 LX Inductor Connection. Connected to the internal p-channel power MOSFET’s drain.
5SHDN
Shutdown Input. A logic-low shuts down the MAX1836/MAX1837 and reduces supply current to 3µA. LX is
high impedance in shutdown. Connect to IN for normal operation. When setting output voltages above 5.5V,
permanently connect SHDN to IN.
6 OUT
Regulated Output Voltage High-Impedance Sense Input. Internally connected to a resistive divider. Connect to
the output when using the preset output voltage. Connect to GND when using an external resistive divider to
adjust the output voltage.
— EP Exposed Metal Pad. Connect to GND. This pad is internally connected to GND through a soft connect. For
proper grounding and good thermal dissipation. Connect the exposed pad to GND.
MAX1836
IN
GND
LX
FB
OUT
OUTPUT
3.3V OR 5V
INPUT
4.5V OR 12V
CIN
10µF
25V
D1
L1
47µH
COUT
100µF
6.3V
CIN = TAIYO YUDEN TMK432BJ106KM
L1 = SUMIDA CDRH5D28-470
COUT = SANYO POSCAP 6TPC100M (SMALLER CAPACITORS CAN BE USED FOR 5V)
D1 = NIHON EP05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
SHDN
MAX1837
IN
GND
LX
SHDN
FB
OUT
OUTPUT
3.3V OR 5V
INPUT
4.5V OR 12V
CIN
10µF
25V
D1
L1
22µH
COUT
150µF
6.3V
CIN = TAIYO YUDEN TMK432BJ106KM
L1 = SUMIDA CDRH5D28-220
COUT = SANYO OS-CON 6SA150M (SMALLER CAPACITORS CAN BE USED FOR 5V)
D1 = NIHON ED05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
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Pin Description
When the output voltage is too low, an error compara-
tor sets a flip-flop, which turns on the internal p-channel
MOSFET and begins a switching cycle (Figure 3). As
shown in Figure 4, the inductor current ramps up linearly,
charging the output capacitor and servicing the load. The
MOSFET turns off when the current limit is reached, or
when the maximum on-time is exceeded while the output
voltage is in regulation. Otherwise, the MOSFET remains
on, allowing a duty cycle up to 100% to ensure the lowest
possible dropout voltage. Once the MOSFET turns off, the
flip-flop resets, diode D1 turns on, and the current through
the inductor ramps back down, transferring the stored
energy to the output capacitor and load. The MOSFET
remains off until the 0.5μs minimum off-time expires and
the inductor current ramps down to zero, and the output
voltage drops back below the set point.
Figure 3. Functional Diagram
Figure 4. Discontinuous-Conduction Operation
INPUT
4.5V OR 24V
CIN
OUTPUT
3.3V OR 5V
D1
L1
COUT
MAX1836
MAX1837
IN
GND
LX
SHDN
FB
OUT
Q
MAXIMUM
ON-TIME
DELAY
TRIG
Q
MAXIMUM
OFF-TIME
DELAY
TRIG
QR
S
VSENSE
VSET
1.25V
100mV
4µs/div
CIRCUIT OF FIGURE 2, VIN = 12V
A. VLX, 5V/div
B. VOUT = 3.3V, 20mV/div, 200mA LOAD
C. INDUCTOR CURRENT, 500mA/div
10V
0
500mA
3.3V
A
B
C
0
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Input-Output (Dropout) Voltage
A step-down converter’s minimum input-to-output volt-
age differential (dropout voltage) determines the lowest
useable input supply voltage. In battery-powered sys-
tems, this limits the useful end-of-life battery voltage. To
maximize battery life, the MAX1836/MAX1837 operate
with duty cycles up to 100%, which minimizes the input-
to-output voltage differential. When the supply voltage
approaches the output voltage, the P-channel MOSFET
remains on continuously to supply the load.
Dropout voltage is defined as the difference between the
input and output voltages when the input is low enough for
the output to drop out of regulation. For a step-down con-
verter with 100% duty cycle, the dropout voltage depends
on the MOSFET drain-to-source on-resistance (RDS(ON))
and inductor series resistance; therefore, it is proportional
to the load current:
( )
DROPOUT OUT DS(ON) INDUCTOR
V IR R=×+
Shutdown (SHDN)
A logic-level low voltage on SHDN shuts down the
MAX1836/MAX1837. When shut down, the supply cur-
rent drops to 3μA to maximize battery life, and the internal
P-channel MOSFET turns off to isolate the output from the
input. The output capacitance and load current determine
the rate at which the output voltage decays. A logic-level
high voltage on SHDN activates the MAX1836/MAX1837.
Do not leave SHDN floating. If unused, connect SHDN to
IN. When setting output voltages above 5.5V, the shut-
down feature cannot be used, so SHDN must be perma-
nently connected to IN. The SHDN input voltage slew rate
must be greater than 10V/ms.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX1836/MAX1837. When the junction tem-
perature exceeds TJ = +160°C, a thermal sensor turns off
the pass transistor, allowing the IC to cool. The thermal
sensor turns the pass transistor on again after the IC’s
junction temperature cools by 10°C, resulting in a pulsed
output during continuous thermal-overload conditions.
Design Information
Output Voltage Selection
The feedback input features dual-mode operation.
Connect the output to OUT and FB to GND for the preset
output voltage. The MAX1836/MAX1837 are supplied
with factory-set output voltages of 3.3V or 5V. The two-
digit part number suffix identifies the output voltage. See
the Selector Guide. For example, the MAX1836EUT33
has a preset 3.3V output voltage.
The MAX1836/MAX1837 output voltage may be adjusted
by connecting a voltage divider from the output to FB
(Figure 5). When externally adjusting the output voltage,
connect OUT to GND. Select R2 in the 10kΩ to 100kΩ
range. Calculate R1 with the following equation:
OUT
FB
V
R1 R2 1
V


= −





where VFB = 1.25V, and VOUT may range from 1.25V to
VIN. When setting output voltages above 5.5V, the shut-
down feature cannot be used, so SHDN must be perma-
nently connected to IN.
Inductor Selection
When selecting the inductor, consider these four param-
eters: inductance value, saturation current rating, series
resistance, and size. The MAX1836/MAX1837 operate
with a wide range of inductance values. For most applica-
tions, values between 10μH and 100μH work best with
the controller’s switching frequency. Calculate the mini-
mum inductance value as follows:
( )
IN(MAX) OUT ON(MIN)
(MIN)
LIM
V Vt
LI
−
=
where tON(MIN) = 1.0μs. Inductor values up to six times
L(MIN) are acceptable. Low-value inductors may be small-
er in physical size and less expensive, but they result in
higher peak-current overshoot due to current-sense com-
parator propagation delay (300ns). Peak-current over-
shoot reduces efficiency and could exceed the current
ratings of the internal switching MOSFET and external
components.
Figure 5. Adjustable Output Voltage
MAX1836
MAX1837
IN
GND
LX
SHDN
FB
OUT
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
OUTPUT
1.25V TO VIN
INPUT
4.5V OR 24V
CIN D1
L1
COUT
R1
R2
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9
The inductor’s saturation current rating must be greater
than the peak switching current, which is determined
by the switch current limit plus the overshoot due to the
300ns current-sense comparator propagation delay:
( )
IN OUT
PEAK LIM
V V 300ns
II
L
−
= +
where the switch current-limit (ILIM) is typically 312mA
(MAX1836) or 625mA (MAX1837). Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support, and the inductance
starts to fall.
Inductor series resistance affects both efficiency and
dropout voltage. See the Input-Output (Dropout) Voltage
section. High series resistance limits the maximum current
available at lower input voltages and increases the drop-
out voltage. For optimum performance, select an inductor
with the lowest possible DC resistance that fits in the
allotted dimensions. Typically, the inductor’s series resis-
tance should be significantly less than that of the internal
P-channel MOSFET’s on-resistance (1.1Ω typ). Inductors
with a ferrite core, or equivalent, are recommended.
The maximum output current of the MAX1836/MAX1837
current-limited converter is limited by the peak inductor
current. For the typical application, the maximum output
current is approximately:
OUT(MAX) PEAK
II
Output Capacitor
Choose the output capacitor to supply the maximum load
current with acceptable voltage ripple. The output ripple
has two components: variations in the charge stored in
the output capacitor with each LX pulse, and the voltage
drop across the capacitor’s equivalent series resistance
(ESR) caused by the current into and out of the capacitor:
RIPPLE RIPPLE(ESR) RIPPLE(C)
VV V≈+
The output voltage ripple as a consequence of the ESR
and output capacitance is:
( )
RIPPLE(ESR) PEAK
2
PEAK OUT IN
RIPPLE(C)
OUT OUT IN OUT
V I ESR
LI I V
V2C V V V
=

−
=

−

where IPEAK is the peak inductor current. See the Inductor
Selection section. These equations are suitable for initial
capacitor selection, but final values should be set by test-
ing a prototype or evaluation circuit. As a general rule, a
smaller amount of charge delivered in each pulse results
in less output ripple. Since the amount of charge deliv-
ered in each oscillator pulse is determined by the inductor
value and input voltage, the voltage ripple increases with
larger inductance but decreases with lower input voltages.
With low-cost aluminum electrolytic capacitors, the ESR-
induced ripple can be larger than that caused by the
current into and out of the capacitor. Consequently, high-
quality low-ESR aluminum-electrolytic, tantalum, polymer,
or ceramic filter capacitors are required to minimize out-
put ripple. Best results at reasonable cost are typically
achieved with an aluminum-electrolytic capacitor in the
100μF range, in parallel with a 0.1μF ceramic capacitor.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching. The
input capacitor must meet the ripple-current requirement
(IRMS) imposed by the switching currents defined by the
following equation:
( )
OUT IN OUT
RMS LOAD
IN
V VV
II V
−
=
For most applications, nontantalum chemistries (ceramic,
aluminum, polymer, or OS-CON) are preferred due to
their robustness with high inrush currents typical of sys-
tems with low-impedance battery inputs. Alternatively,
two (or more) smaller-value low-ESR capacitors can be
connected in parallel for lower cost. Choose an input
capacitor that exhibits < +10°C temperature rise at the
RMS input current for optimal circuit longevity.
Diode Selection
The current in the external diode (D1) changes abruptly
from zero to its peak value each time the LX switch turns
off. To avoid excessive losses, the diode must have a
fast turn-on time and a low forward voltage. Use a diode
with an RMS current rating of 0.5A or greater, and with a
breakdown voltage > VIN. Schottky diodes are preferred.
For high-temperature applications, Schottky diodes may
be inadequate due to their high leakage currents. In
such cases, ultra-high-speed silicon rectifiers are recom-
mended, although a Schottky diode with a higher reverse
voltage rating can often provide acceptable performance.
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MAX1836/MAX1837 Stability
Commonly, instability is caused by excessive noise on the
feedback signal or ground due to poor layout or improper
component selection. When seen, instability typically
manifests itself as “motorboating,” which is characterized
by grouped switching pulses with large gaps and exces-
sive low-frequency output ripple during no-load or light-
load conditions.
PC Board Layout and Grounding
High switching frequencies and large peak currents make
PC board layout an important part of the design. Poor lay-
out may introduce switching noise into the feedback path,
resulting in jitter, instability, or degraded performance.
High-power traces, bolded in the typical application cir-
cuits (Figure 1 and Figure 2), should be as short and wide
as possible. Additionally, the current loops formed by the
power components (CIN, COUT, L1, and D1) should be
as tight as possible to avoid radiated noise. Connect the
ground pins of these power components at a common
node in a star-ground configuration. Separate the noisy
traces, such as the LX node, from the feedback network
with grounded copper. Furthermore, keep the extra cop-
per on the board, and integrate it into a pseudoground
plane. When using external feedback, place the resistors
as close to the feedback pin as possible to minimize noise
coupling. The MAX1837 evaluation kit shows the recom-
mended layout.
Applications Information
High-Voltage Step-Down Converter
The typical application circuits’ (Figure 1 and Figure 2)
components were selected for 9V battery applications.
However, the MAX1836/MAX1837 input voltage range
allows supply voltages up to 24V. Figure 6 shows a modi-
fied application circuit for high-voltage applications. When
using higher input voltages, verify that the input capaci-
tor’s voltage rating exceeds VIN(MAX) and that the induc-
tor value exceeds the minimum inductance recommended
in the Inductor Selection section.
Inverter Conguration
Figure 7 shows the MAX1836/MAX1837 in a floating
ground configuration. By connecting what would nor-
mally be the output to the supply-voltage ground, the IC’s
ground pin is forced to regulate to -5V (MAX183_EUT50)
or -3.3V (MAX183_EUT33). Avoid exceeding the maxi-
mum ratings of 24V between IN and GND, and 5.5V
between OUT and GND. Other negative voltages may be
generated by placing a resistive divider across the output
capacitor and connecting the tap to FB in the same man-
ner as the normal step-down configuration.
Table 1. Component Suppliers
SUPPLIER PHONE FAX WEBSITE
INDUCTORS
Coilcraft 847-639-6400 847-639-1469 www.coilcraft.com
Coiltronics 561-241-7876 561-241-9339 www.coiltronics.com
Sumida USA 847-956-0666 847-956-0702 www.sumida.com
Toko 847-297-0070 847-699-1194 www.tokoam.com
CAPACITORS
AVX 803-946-0690 803-626-3123 www.avxcorp.com
Kemet 408-986-0424 408-986-1442 www.kemet.com
Panasonic 847-468-5624 847-468-5815 www.panasonic.com
Sanyo 619-661-6835 619-661-1055 www.secc.co.jp
Taiyo Yuden 408-573-4150 408-573-4159 www.t-yuden.com
DIODES
Central Semiconductor 516-435-1110 516-435-1824 www.centralsemi.com
International Rectier 310-322-3331 310-322-3332 www.irf.com
Nihon 847-843-7500 847-843-2798 www.niec.co.jp
On Semiconductor 602-303-5454 602-994-6430 www.onsemi.com
Zetex 516-543-7100 516-864-7630 www.zetex.com
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Figure 6. High-Voltage Application
Figure 7. MAX1836/MAX1837 Inverter Configuration
PART PRESET OUTPUT
VOLTAGE (V)
LOAD CURRENT
(mA)
MAX1836ETT33 3.3 125
MAX1836ETT50 5 125
MAX1836EUT33 3.3 125
MAX1836EUT50 5 125
MAX1837ETT33 3.3 250
MAX1837ETT50 5 250
MAX1837EUT33 3.3 250
MAX1837EUT50 5 250
MAX1837
IN
GND
LX
SHDN
FB
OUT
OUTPUT
5V
INPUT
4.5V TO 24V
CIN
10µF
25V
D1
L1
47µH
COUT
68µF
10V
CIN = TAIYO YUDEN TMK432BJ106KM
L1 = SUMIDA CDRH5D28-470
COUT = SANYO POSCAP 10TPC68M
D1 = NIHON EP05Q03L
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
MAX1836
MAX1837
IN
GND
LX
SHDN
FB
OUT
INPUT
3.6V TO 18V
CIN
10µF
D1
L1
47µH
COUT
100µF
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
OUTPUT
-3.3V OR -5V
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Chip Information
TRANSISTOR COUNT: 731
PROCESS: BiCMOS
Selector Guide
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Package Information
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a “+”,
“#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing
pertains to the package regardless of RoHS status.
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Package Information (continued)
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a “+”,
“#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing
pertains to the package regardless of RoHS status.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc. © 2006 Maxim Integrated Products, Inc.
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MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Revision History
Pages changed at Rev 3: 1, 7, 8, 12
Package Information (continued)
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a “+”,
“#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing
pertains to the package regardless of RoHS status.
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.