MP29368
1.2A, 24V, 1.4MHz
Step-Down Converter in a TSOT23-6
MP29368 Rev. 1.2 www.MonolithicPower.com 1
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© 2007 MPS. All Rights Reserved.
The Future of Analog IC Technology
TM
TM
DESCRIPTION
The MP29368 is a monolithic step-down switch
mode converter with a built-in power MOSFET.
It achieves 1.2A peak output current over a
wide input supply range with excellent load and
line regulation. Current mode operation
provides fast transient response and eases loop
stabilization. Fault condition protection includes
cycle-by-cycle current limiting and thermal
shutdown.
The MP29368 requires a minimum number of
readily available standard external components.
The MP29368 is available in TSOT23-6 package.
FEATURES
• 1.2A Peak Output Current
• 0.35 Internal Power MOSFET Switch
• Stable with Low ESR Output Ceramic
Capacitors
• Up to 92% Efficiency
• 0.1A Shutdown Mode
• Fixed 1.4MHz Frequency
• Thermal Shutdown
• Cycle-by-Cycle Over Current Protection
• Wide 4.5V to 24V Operating Input Range
• Output Adjustable from 0.81V to 15V
• Available in TSOT23-6 Package
APPLICATIONS
• Distributed Power Systems
• Battery Charger
• Pre-Regulator for Linear Regulators
• WLED Drivers
“MPS” and “The Future of Analog IC Technology” are Trademarks of Monolithic
Power Systems, Inc.
TYPICAL APPLICATION
MP29368 SW
FB
BST
GND
IN
EN
1
6
3
2
4
5
VOUT
3.3V @ 1.2A
CB
10nF
D1
B230A
VIN
12V
OFF ON
Efficiency vs
Load Currents
V
IN
= 12V
V
IN
= 24V
V
OUT
= 5V
100
90
80
70
60
50
40
30
20
10
0
LOAD CURRENT (A)
EFFICIENCY (%)
0.01 0.1 110
MP29368 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
MP29368 Rev. 1.2 www.MonolithicPower.com 2
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© 2007 MPS. All Rights Reserved.
TM
PACKAGE REFERENCE
BST
GND
FB
1
2
3
6
5
4
SW
IN
EN
TOP VIEW
Part Number* Package Temperature
MP29368DJ TSOT23-6
–40°C to +85°C
* For Tape & Reel, add suffix –Z (eg. MP29368DJ–Z)
For RoHS compliant packaging, add suffix –LF (eg.
MP29368DJ–LF–Z)
ABSOLUTE MAXIMUM RATINGS (1)
Supply Voltage VIN....................................... 26V
VSW....................................... –0.3V to VIN + 0.3V
VBS ....................................................... VSW + 6V
All Other Pins................................. –0.3V to +6V
Junction Temperature...............................150°C
Lead Temperature ....................................260°C
Storage Temperature ..............–65°C to +150°C
Recommended Operating Conditions (2)
Supply Voltage VIN........................... 4.5V to 24V
Output Voltage VOUT ...................... 0.81V to 15V
Ambient Temperature................–40°C to +85°C
Thermal Resistance (3) θJA θJC
TSOT23-6.............................. 220 .... 110.. °C/W
Notes:
1) Exceeding these ratings may damage the device.
2) The device function is not guaranteed outside of the
recommended operating conditions.
3) Measured on approximately 1” square of 1 oz copper.
ELECTRICAL CHARACTERISTICS
VIN = 12V, TA = +25°C, unless otherwise noted.
Parameters Symbol Condition Min Typ Max Units
Feedback Voltage VFB 4.5V ≤ VIN ≤ 24V 0.790 0.810 0.830 V
Feedback Current IFB V
FB = 0.8V 0.1 A
Switch-On Resistance (4) R
DS(ON) 0.35
Switch Leakage VEN = 0V, VSW = 0V 10 A
Current Limit (4) 1.8 A
Oscillator Frequency fSW V
FB = 0.6V 1.2 1.4 1.7 MHz
Fold-back Frequency VFB = 0V 460 KHz
Maximum Duty Cycle VFB = 0.6V 87 %
Minimum On-Time (4) t
ON 100 ns
Under Voltage Lockout Threshold Rising 2.5 2.8 3.1 V
Under Voltage Lockout Threshold Hysteresis 150 mV
EN Input Low Voltage 0.4 V
EN Input High Voltage 1.2 V
VEN = 2V 2.1
EN Input Current VEN = 0V 0.1 A
Supply Current (Shutdown) IS V
EN = 0V 0.1 1.0 A
Supply Current (Quiescent) IQ VEN = 2V, VFB = 1V 0.8 1.0 mA
Thermal Shutdown (4) 150 °C
Note:
4) Guaranteed by design.
MP29368 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
MP29368 Rev. 1.2 www.MonolithicPower.com 3
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© 2007 MPS. All Rights Reserved.
TM
PIN FUNCTIONS
Pin # Name Description
1 BST
Bootstrap. A capacitor is connected between SW and BS pins to form a floating supply across
the power switch driver. This capacitor is needed to drive the power switch’s gate above the
supply voltage.
2 GND
Ground. This pin is the voltage reference for the regulated output voltage. For this reason care
must be taken in its layout. This node should be placed outside of the D1 to C1 ground path to
prevent switching current spikes from inducing voltage noise into the part.
3 FB
Feedback. An external resistor divider from the output to GND, tapped to the FB pin sets the
output voltage. To prevent current limit run away during a short circuit fault condition, the
frequency foldback comparator lowers the oscillator frequency when the FB voltage is below
250mV.
4 EN On/Off Control Input. Pull EN above 1.2V to turn the device on.
5 IN
Supply Voltage. The MP29368 operates from a +4.5V to +24V unregulated input. C1 is needed
to prevent large voltage spikes from appearing at the input.
6 SW Switch Output.
TYPICAL PERFORMANCE CHARACTERISTICS
VIN = 12V, VOUT = 3.3V, L = 4.7µH, C1 = 10µF, C2 = 22µF, TA = +25ºC, unless otherwise noted.
Efficiency vs
Load Current
V
IN
= 12V
V
IN
= 24V
V
OUT
= 5V
100
90
80
70
60
50
40
30
20
10
0
LOAD CURRENT (A)
EFFICIENCY (%)
100
90
80
70
60
50
40
30
20
10
0
EFFICIENCY (%)
0.01 0.1 110
LOAD CURRENT ( A)
Efficiency vs
Load Current
V
IN
= 18V
V
IN
= 24V
V
OUT
= 2.5V
Feedback Voltage vs
Die Temperature
FEEDBACK VOLTAGE (V)
0.820
0.818
0.816
0.814
0.812
0.810
0.808
0.806
0.804
0.802
0.800
DIE TEMPERATURE (
O
C)
-50 -25 0 25 75 100 12550 150
Switching Frequency vs
Die Temperature
SWITCHING FREQUENCY (MHz)
1.50
1.48
1.46
1.44
1.42
1.40
1.38
1.36
1.34
1.32
1.30
DIE TEMPERATURE (
O
C)
-50 -25 0 25 75 100 12550 150
0.01 0.1 110
MP29368 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
MP29368 Rev. 1.2 www.MonolithicPower.com 4
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© 2007 MPS. All Rights Reserved.
TM
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN = 12V, VOUT = 3.3V, L = 4.7µH, C1 = 10µF, C2 = 22µF, TA = +25ºC, unless otherwise noted.
V
SW
10V/div.
V
EN
5V/div.
V
OUT
1V/div.
I
L
1A/div.
Start-up through Enable
IOUT = 1A Resistive Load
V
OUT
1V/div.
V
SW
10V/div.
V
EN
5V/div.
Start-up through Enable
No Load
V
OUT
2V/div.
V
SW
10V/div.
V
EN
5V/div.
I
L
1A/div.
Shut-down through Enable
No Load
Shut-down through Enable
IOUT = 1A Resistive Load
I
L
500mA/div.
V
OUT
2V/div.
V
SW
10V/div.
V
EN
5V/div.
I
L
1A/div.
V
OUT
1V/div.
I
L
1A/div.
Short Circuit Entry Short Circuit Recovery
V
OUT
1V/div.
I
L
1A/div.
400ns/div.
Steady State Test
IOUT = 0.5A
V
OUT
AC Coupled
50mV/div.
I
L
1A/div.
I
LOAD
1A/div.
V
OUT
20mV/div.
V
SW
10V/div.
I
L
500mA/div.
Current Limit vs
Duty Cycle
CURRENT LIMIT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
DUTY CYCLE (%)
0 20 40 60 80 100
MP29368 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
MP29368 Rev. 1.2 www.MonolithicPower.com 5
12/13/2007 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2007 MPS. All Rights Reserved.
TM
OPERATION
The MP29368 is a current mode buck regulator.
That is, the EA output voltage is proportional to
the peak inductor current.
At the beginning of a cycle, M1 is off. The EA
output voltage is higher than the current sense
amplifier output, and the current comparator’s
output is low. The rising edge of the 1.4MHz
CLK signal sets the RS Flip-Flop. Its output
turns on M1 thus connecting the SW pin and
inductor to the input supply.
The increasing inductor current is sensed and
amplified by the Current Sense Amplifier. Ramp
compensation is summed to the Current Sense
Amplifier output and compared to the Error
Amplifier output by the PWM Comparator.
When the sum of the Current Sense Amplifier
output and the Slope Compensation signal
exceeds the EA output voltage, the RS Flip-
Flop is reset and M1 is turned off. The external
Schottky rectifier diode (D1) conducts the
inductor current.
If the sum of the Current Sense Amplifier output
and the Slope Compensation signal does not
exceed the EA output for a whole cycle, then
the falling edge of the CLK resets the Flip-Flop.
The output of the Error Amplifier integrates the
voltage difference between the feedback and
the 0.81V bandgap reference. The polarity is
such that a FB pin voltage lower than 0.81V
increases the EA output voltage. Since the EA
output voltage is proportional to the peak
inductor current, an increase in its voltage also
increases current delivered to the output.
5IN
EN
FB
GND
SW
BST
4
3
2
1
6
REGULATOR OSCILLATOR
1.4MHz/460KHz
RAMP
GENERATOR
DRIVER
CURRENT SENSE
AMPLIFIER
ERROR
AMPLIFIER
CURRENT
LIMIT
COMPARATOR
PWM
COMPARATOR
D
x20
M1
REGULATOR
REFERENCE
EA
--
+--
+
--
+
S
R
R
Q
--
+
27pF
1pF
Figure 1—Functional Block Diagram
MP29368 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
MP29368 Rev. 1.2 www.MonolithicPower.com 6
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© 2007 MPS. All Rights Reserved.
TM
APPLICATION INFORMATION
Setting Output Voltage
The external resistor divider is used to set the
output voltage (see the schematic on front
page). Table 1 shows a list of resistor selection
for common output voltages. The feedback
resistor R1 also sets the feedback loop
bandwidth with the internal compensation
capacitor (see Figure 1). R2 can be determined
by:
1
V81.0
V
1R
2R
OUT −
=
Table 1—Resistor Selection for Common
Output Voltages
VOUT (V) R1 (k) R2 (k)
1.8 80.6 (1%) 64.9 (1%)
2.5 49.9 (1%) 23.7 (1%)
3.3 49.9 (1%) 16.2 (1%)
5 49.9 (1%) 9.53 (1%)
Selecting the Inductor
A 1µH to 10µH inductor with a DC current rating
of at least 25% percent higher than the
maximum load current is recommended for
most applications. For highest efficiency, the
inductor’s DC resistance should be less than
200m. Refer to Table 2 for suggested surface
mount inductors. For most designs, the required
inductance value can be derived from the
following equation.
SWLIN
OUTINOUT
fIV
)VV(V
L×Δ×
−×
=
Where IL is the inductor ripple current.
Choose the inductor ripple current to be 30% of
the maximum load current. The maximum
inductor peak current is calculated from:
2
I
II L
LOAD)MAX(L
Δ
+=
Under light load conditions below 100mA, a
larger inductance is recommended for improved
efficiency. See Table 2 for suggested inductors.
Also note that the maximum recommended load
current is 1A if the duty cycle exceeds 35%.
Selecting the Input Capacitor
The input capacitor reduces the surge current
drawn from the input supply and the switching
noise from the device. The input capacitor
impedance at the switching frequency should be
less than the input source impedance to prevent
high frequency switching current from passing
through the input. Ceramic capacitors with X5R or
X7R dielectrics are highly recommended because
of their low ESR and small temperature
coefficients. For most applications, a 4.7µF
capacitor is sufficient.
Selecting the Output Capacitor
The output capacitor keeps the output voltage
ripple small and ensures feedback loop stability.
The output capacitor impedance should be low
at the switching frequency. Ceramic capacitors
with X5R or X7R dielectrics are recommended
for their low ESR characteristics. For most
applications, a 22µF ceramic capacitor will be
sufficient.
PC Board Layout
The high current paths (GND, IN and SW) should
be placed very close to the device with short,
direct and wide traces. The input capacitor needs
to be as close as possible to the IN and GND pins.
The external feedback resistors should be placed
next to the FB pin. Keep the switch node traces
short and away from the feedback network.
External Bootstrap Diode
It is recommended that an external bootstrap
diode be added when the input voltage is no
greater than 5V or the 5V rail is available in the
system. This helps improve the efficiency of the
regulator. The bootstrap diode can be a low
cost one such as IN4148 or BAT54.
MP29368
SW
BS
10nF
5V (External) or
VIN (4.5V to 5V)
Figure 2—External Bootstrap Diode
This diode is also recommended for high duty cycle
operation (when
IN
OUT
V
V>65%) applications.
MP29368 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
MP29368 Rev. 1.2 www.MonolithicPower.com 7
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© 2007 MPS. All Rights Reserved.
TM
Table 2—Suggested Surface Mount Inductors
Manufacturer Part Number Inductance(µH) Max DCR()Current
Rating (A)
Dimensions
L x W x H (mm3)
Toko A921CY-4R7M 4.7 0.027 1.66 6 x 6.3 x 3
Sumida CDRH4D28C/LD 4.7 0.036 1.5 5.1 x 5.1 x 3
Wurth Electronics 7440530047 4.7 0.038 2.0 5.8 x 5.8 x 2.8
TYPICAL APPLICATION CIRCUITS
VIN
Notes:
1) D3 can be installed if VIN < 5V
2) D2 can be installed if VOUT < 5V
3) No need for both D2 and D3
VOUT
3.3V
C3
22nF
D1
B230A-13-F
U1
D2
1N4148
(Optional)
D3
1N4148
(Optional)
MP29368
FB
SW
BS
GND
EN
IN 15
46
3
OFF ON
Figure 3—1.4MHz, 3.3V Output at 1A Step-Down Converter
MP29368 SW
FB
BST
GND
IN
EN
1
6
3
2
4
5
C3
10nF
LED1
LED2
LED3
D1
1N5819HW-7
VIN
6V-12V
OFF ON
-VOUT
-VOUT
-VOUT
-VOUT
Figure 4—White LED Driver Application
MP29368 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
NOTICE: The information in this document is subject to change without notice. Please contact MPS for current specifications.
Users should warrant and guarantee that third party Intellectual Property rights are not infringed upon when integrating MPS
products into any application. MPS will not assume any legal responsibility for any said applications.
MP29368 Rev. 1.2 www.MonolithicPower.com 8
12/13/2007 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2007 MPS. All Rights Reserved.
TM
PACKAGE INFORMATION
TSOT23-6
0.30
0.50
SEATING PLANE
0.95 BSC
0.84
0.90 1.00 MAX
0.00
0.10
TOP VIEW
FRONT VIEW SIDE VIEW
RECOMMENDED LAND PATTERN
2.80
3.00
1.50
1.70
2.60
3.00
13
46
0.09
0.20
NOTE:
1) ALL DIMENSIONS ARE IN MILLIMETERS.
2) PACKAGE LENGTH DOES NOT INCLUDE MOLD FLASH,
PROTRUSION OR GATE BURR.
3) PACKAGE WIDTH DOES NOT INCLUDE INTERLEAD FLASH
OR PROTRUSION.
4) LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING)
SHALL BE 0.10 MILLIMETERS MAX.
5) DRAWING CONFORMS TO JEDEC MO-193, VARIATION AB.
6) DRAWING IS NOT TO SCALE.
7) PIN 1 IS LOWER LEFT PIN WHEN READING TOP MARK FROM
LEFT TO RIGHT, (SEE EXAMPLE TOP MARK)
0.30
0.50
0o-8o
0.25 BSC
GAUGE PLANE
2.60
TYP
1.20
TYP
0.95
BSC
0.60
TYP
SEE DETAIL "A"
DETAIL “A”
AAAA
PIN 1
See Note 7
EXAMPLE
TOP MARK
