MAX1651CSA+ - Maxim Integrated

_______________General Description

The MAX1649/MAX1651 BiCMOS, step-down, DC-DC

switching controllers provide high efficiency over loads

ranging from 1mA to more than 2.5A. A unique, current-

limited pulse-frequency-modulated (PFM) control scheme

gives these devices the benefits of pulse-width-modula-

tion (PWM) converters (high efficiency at heavy loads),

while using only 100µA of supply current (vs. 2mA to

10mA for PWM converters). Dropout performance down

to 300mV is provided by a high switch duty cycle (96.5%)

and a low current-sense threshold (110mV).

A high switching frequency (up to 300kHz) allows these

devices to use miniature external components.

The MAX1649/MAX1651 have dropout voltages less

than 0.3V at 500mA and accept input voltages up to

16V. Output voltages are preset at 5V (MAX1649), or

3.3V (MAX1651). They can also be adjusted to any

voltage from 1.5V to the input voltage by using two

resistors.

These step-down controllers drive external P-channel

MOSFETs at loads greater than 12.5W. If less power is

required, use the MAX639/MAX640/MAX653 step-down

converters with on-chip FETs, which allow up to a

225mA load current.

________________________Applications

PDAs

High-Efficiency Step-Down Regulation

5V-to-3.3V Green PC Applications

Battery-Powered Applications

____________________________Features

♦More than 90% Efficiency (10mA to 1.5A Loads)

♦More than 12.5W Output Power

♦Less than 0.3V Dropout Voltage at 500mA

♦100µA Max Quiescent Supply Current

♦5µA Max Shutdown Supply Current

♦16V Max Input Voltage

♦5V (MAX1649), 3.3V (MAX1651), or Adjustable

Output Voltage

♦Current-Limited Control Scheme

♦Up to 300kHz Switching Frequency

♦Up to 96.5% Duty Cycle

______________Ordering Information

*Dice are tested at TA= +25°C.

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

________________________________________________________________ Maxim Integrated Products 1

GND

EXT

V+

REF

SHDN

OUT

DIP/SO

TOP VIEW

MAX1649

MAX1651

__________________Pin Configuration

__________Typical Operating Circuit

19-0305; Rev 3; 3/09

PART TEMP RANGE PIN-PACKAGE

MAX1649CPA 0°C to +70°C 8 Plastic DIP

MAX1649CSA 0°C to +70°C 8 SO

MAX1649C/D 0°C to +70°C Dice*

MAX1649EPA -40°C to +85°C 8 Plastic DIP

MAX1649ESA -40°C to +85°C 8 SO

MAX1651

V+

CSSHDN

FB GND

ON/OFF

EXT

REF OUT

OUTPUT

3.3V

INPUT

3.6V TO 16V

MAX1651CPA 0°C to +70°C 8 Plastic DIP

MAX1651CSA 0°C to +70°C 8 SO

MAX1651C/D 0°C to +70°C Dice*

MAX1651EPA -40°C to +85°C 8 Plastic DIP

MAX1651ESA -40°C to +85°C 8 SO

EVALUATION KIT

AVAILABLE

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,

or visit Maxim's website at www.maxim-ic.com.

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(V+ = 5V, TA= TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)

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.

PARAMETER SYMBOL MIN TYP MAX UNITS

1.4625 1.5 1.5375

FB Trip Point 1.470 1.5 1.530 V

I+

FB Input Current IFB

±50 nA

±70

Output Voltage VOUT

4.80 5.0 5.20 V

Supply Current

V+ Input Voltage Range V+ 3.0 16 V

78 100

µA

3.17 3.3 3.43

Reference Voltage VREF

1.470 1.5 1.530 V

1.4625 1.5 1.5375

REF Load Regulation mV410

CONDITIONS

MAX1649E, MAX1651E

MAX1649, V+ = 5.5V to 16V

MAX1649C, MAX1651C

V+ = 10V, SHDN ≥1.6V (shutdown)

MAX1649C, MAX1651C

MAX1649C, MAX1651C, IREF = 0μA

MAX1649E, MAX1651E

MAX1649E, MAX1651E, IREF = 0μA

VOUT < V+

V+ = 16V, SHDN ≤0.4V (operating, switch off)

V+ = 16V, SHDN ≥1.6V (shutdown)

Supply Voltage, V+ to GND.......................................-0.3V, +17V

REF, SHDN, FB, CS, EXT, OUT .......................-0.3V, (V+ + 0.3V)

Continuous Power Dissipation (TA= +70°C)

Plastic DIP (derate 9.09mW/°C above +70°C) .............727mW

SO (derate 5.88mW/°C above +70°C)..........................471mW

Operating Temperature Ranges

MAX1649C_A, MAX1651C_A ..............................0°C to +70°C

MAX1649E_A, MAX1651E_A............................-40°C to +85°C

Storage Temperature Range .............................-65°C to +160°C

Lead Temperature (soldering, 10sec) .............................+300°C

0µA ≤IREF ≤100µA, sourcing only

3V ≤V+ ≤16VREF Line Regulation 40 100 µV/V

2.6

mV/V

1.7

Output Voltage

Line Regulation

Circuit of

Figure 1

MAX1649, 5.5V ≤V+ ≤16V,

ILOAD = 1A

MAX1651, 3.6V ≤V+ ≤16V,

ILOAD = 1A

-47

mV/A

MAX1649, 0A ≤ILOAD ≤1.5A,

VIN = 10V

MAX1651, 0A ≤ILOAD ≤1.5A,

VIN = 5V -45

Output Voltage

Load Regulation

Circuit of

Figure 1

MAX1649, V+ = 10V,

ILOAD = 1A

MAX1651, V+ = 5V,

ILOAD = 1A 90

Efficiency Circuit of

Figure 1

SHDN Input Current V+ = 16V, SHDN = 0V or V+ 1µA

SHDN Input Voltage High VIH 3V ≤V+ ≤16V 1.6 V

SHDN Input Voltage Low VIL 3V ≤V+ ≤16V 0.4 V

MAX1651, V+ = 3.6V to 16V

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 5V, TA= TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)

PARAMETER SYMBOL MIN TYP MAX UNITSCONDITIONS

3V ≤V+ ≤16VCS Input Current ±1 µA

CEXT = 0.001µF, V+ = 12VEXT Rise Time 25 ns

CEXT = 0.001µF, V+ = 12VEXT Fall Time 25 ns

Current-Limit Trip Level

(V+ to CS) mVVCS 3V ≤V+ ≤16V 80 110 140

tON

tON + tOFF

x 100%

Maximum Duty Cycle 95 96.5 %

SUPPLY CURRENT vs. TEMPERATURE

I+ (μA)

-60 -20 60 140

MAX1649-TOC06

TEMPERATURE (°C)

20 100-40 0 8040 120

V+ = 10V

V+ = 16V

V+ = 4V

4.0

SHUTDOWN CURRENT

vs. TEMPERATURE

0.5

3.5

I+ (μA)

3.0

2.5

-60 -20 60 140

MAX1649-TOC05

TEMPERATURE (°C)

20 100-40 0 8040 120

2.0

1.5

V+ = 8V

V+ = 16V

V+ = 4V

1.0

V+ = 12VSwitch Maximum On-Time 24 32 40 µstON (max)

V+ = 12VSwitch Minimum Off-Time 0.8 1.1 1.8 µstOFF (min)

-60 -20 60 140

EXT RISE AND FALL TIMES

vs. TEMPERATURE (1nF)

MAX1649/51-01

TEMPERATURE (°C)

tRISE & tFALL (ns)

20 100-40 0 8040 120

V+ = 15V, tFALL

V+ = 15V, tRISE

V+ = 5V, tFALL

V+ = 5V, tRISE

CEXT = 1nF

1 10 10k

LOAD CURRENT (mA)

EFFICIENCY (%)

100

100 1k

0.1

MAX1649/51-A1

EFFICIENCY

vs. LOAD CURRENT (VOUT = 5V)

VOUT = 5V

CIRCUIT OF

FIGURE 1

TOP TO

BOTTOM:

VIN = 6V

VIN = 8V

VIN = 10V

VIN = 12V

VIN = 15V

__________________________________________Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

-60 -20 60 140

EXT RISE AND FALL TIMES

vs. TEMPERATURE (5nF)

MAX1649/51-02

TEMPERATURE (°C)

tRISE & tFALL (ns)

20 100-40 0 8040 120

100

140

120

160

200

180

240

220

V+ = 15V, tFALL

V+ = 15V, tRISE

V+ = 5V, tFALL

V+ = 5V, tRISE

CEXT = 5nF

1 10 10k

LOAD CURRENT (mA)

EFFICIENCY (%)

100

100 1k

0.1

MAX1649/51-A2

EFFICIENCY

vs. LOAD CURRENT (VOUT = 3.3V)

VOUT = 3.3V

CIRCUIT OF

FIGURE 1

TOP TO

BOTTOM:

VIN = 4.3V

VIN = 5V

VIN = 8V

VIN = 10V

VIN = 12V

VIN = 15V

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

4 _______________________________________________________________________________________

-60 -20 60 140

SWITCH ON-TIME

vs. TEMPERATURE

MAX1649/51-03

TEMPERATURE (°C)

tON (μs)

20 100-40 0 8040 120

34.0

33.5

33.0

32.5

32.0

31.5

31.0

30.0

30.5

-60 -20 60 140

SWITCH OFF-TIME

vs. TEMPERATURE

MAX1649/51-04

TEMPERATURE (°C)

tOFF (μs)

20 100-40 0 8040 120

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

-60 -20 60 140

MAXIMUM DUTY CYCLE

vs. TEMPERATURE

MAX1649/51-05

TEMPERATURE (°C)

DUTY CYCLE (%)

20 100-40 0 8040 120

100

-60 -20 60 140

CS TRIP LEVEL

vs. TEMPERATURE

MAX1649/51-06

TEMPERATURE (°C)

CS TRIP LEVEL (mV)

20 100-40 0 8040 120

120

115

110

100

105

0 0.5 1.0 1.5 2.0

DROPOUT VOLTAGE

vs. LOAD CURRENT

MAX1649/51-A3

LOAD CURRENT (A)

DROPOUT VOLTAGE (mV)

600

500

400

300

200

100

VOUT = 4.80V

VOUT = 3.17V

CIRCUIT OF

FIGURE 1

250

REFERENCE OUTPUT RESISTANCE

vs. TEMPERATURE

200

REFERENCE OUTPUT RESISTANCE (Ω)

150

100

-60 -20 60 140

MAX1649-TOC07

TEMPERATURE (°C)

20 100-40 0 8040 120

IREF = 10μA

IREF = 50μA

IREF = 100μA

1.506

1.492

REFERENCE OUTPUT VOLTAGE

vs. TEMPERATURE

1.494

1.504

REFERENCE OUTPUT VOLTAGE (V)

1.502

1.500

-60 -20 60 140

MAX1649-TOC01

TEMPERATURE (°C)

20 100-40 0 8040 120

1.498

1.496

IREF = 10μA

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

_______________________________________________________________________________________ 5

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

CIRCUIT OF FIGURE 1, ILOAD = 1A

A: VOUT = 5V, 100mV/div, AC-COUPLED

B: V+ = 6V TO 16V, 5V/div

MAX1649

LINE-TRANSIENT RESPONSE

5ms/div

16V

CIRCUIT OF FIGURE 1, V+ = 10V

A: VOUT = 5V, 100mV/div, AC-COUPLED

B: ILOAD = 30mA TO 1.6A, 1A/div

MAX1649

LOAD-TRANSIENT RESPONSE

1.6A

200μs/div

CIRCUIT OF FIGURE 1, V+ = 10V, ILOAD = 1A

MAX1649

SHDN RESPONSE TIME

OUTPUT

SHDN

INPUT

1ms/div

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

6 _______________________________________________________________________________________

_______________Detailed Description

The MAX1649/MAX1651 are BiCMOS, step-down,

switch-mode power-supply controllers that provide

adjustable and fixed outputs of 5V and 3.3V, respec-

tively. Their unique control scheme combines the

advantages of pulse-frequency-modulation (low supply

current) and pulse-width-modulation (high efficiency at

high loads). An external P-channel power MOSFET

allows peak currents in excess of 3A, increasing the

output current capability over previous PFM devices.

Figure 2 is the block diagram.

The MAX1649/MAX1651 offer four main improvements

over prior solutions:

1) The converters operate with miniature surface-mount

inductors, due to their 300kHz switching frequency.

2) The current-limited PFM control scheme allows

greater than 90% efficiencies over a wide range of

load currents (10mA to 1.5A).

3) Dropout voltage has been reduced to less than

300mV for many applications.

4) The quiescent supply current is only 100µA.

PFM Control Scheme

The MAX1649/MAX1651 use a proprietary, current-limit-

ed PFM control scheme. As with traditional PFM con-

verters, the external power MOSFET is turned on when

the voltage comparator senses that the output is out of

regulation. However, unlike traditional PFM converters,

switching is accomplished through the combination of a

peak current limit and a pair of one-shots that set the

maximum switch on-time (32µs) and minimum switch

off-time (1.1µs). Once off, the off-time one-shot holds

the switch off for 1.1µs. After this minimum time, the

switch either 1) stays off if the output is in regulation, or

2) turns on again if the output is out of regulation.

The MAX1649/MAX1651 also limit the peak inductor cur-

rent, which allows them to run in continuous-conduction

mode and maintain high efficiency with heavy loads

(Figure 3). This current-limiting feature is a key compo-

nent of the control circuitry. Once turned on, the switch

stays on until either 1) the maximum on-time one-shot

turns it off (32µs later), or 2) the current limit is reached.

EXT swings from V+ to GND and provides the drive out-

put for an external P-channel power MOSFET.

MAX1649

MAX1651

V+

FB GND

VIN

330μF

EXT

OUT

SHDN

0.1μF

100μF

0.05Ω

NSQ03A02L

47μH**

Si9430*OUTPUT

@ 1.5A

*SILICONIX SURFACE-MOUNT MOSFET

**SUMIDA CDRH125-470

REF

Figure 1. Typical Application Circuit

______________________________________________________________Pin Description

NAME FUNCTION

1OUT

Sense Input for fixed 5V or 3.3V output operation. OUT is internally connected to the on-chip voltage divider.

Although it is connected to the output of the circuit, OUT does not supply current. Leave OUT unconnected for

adjustable-output operation.

2FB Feedback Input. Connect to GND for fixed-output operation. Connect a resistor divider between OUT, FB, and

GND for adjustable-output operation. See Setting the Output Voltage section.

3SHDN Active-High Shutdown Input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the refer-

ence, output, and external MOSFET are turned off. Connect to GND for normal operation.

4REF 1.5V Reference Output that can source 100µA. Bypass with 0.1µF.

8GND Ground

7EXT Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.

6CS Current-Sense Input. Connect current-sense resistor between V+ and CS. When the voltage across the resistor

equals the current-limit trip level, the external MOSFET is turned off.

5V+ Positive Power-Supply Input

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

_______________________________________________________________________________________ 7

Shutdown Mode

When SHDN is high, the MAX1649/MAX1651 enter shut-

down mode. In this mode, the internal biasing circuitry is

turned off (including the reference) and the supply cur-

rent drops to less than 5µA. EXT goes high, turning off the

external MOSFET. SHDN is a logic-level input. Connect

SHDN to GND for normal operation.

Quiescent Current

In normal operation, the device's typical quiescent cur-

rent is 78µA. In an actual application, even with no load,

additional current is drawn to supply external feedback

resistors (if used) and the diode and capacitor leakage

currents. In the circuit of Figure 1, with V+ at 5V and

VOUT at 3.3V, typical no-load supply current for the

entire circuit is 90µA.

MAX1649

MAX1651

MINIMUM

OFF-TIME

ONE-SHOT

TRIG

MAXIMUM

ON-TIME

ONE-SHOT

TRIG

DUAL-MODE™

COMPARATOR

ERROR

COMPARATOR

CURRENT

COMPARATOR

110mV

FROM V+

EXT

OUT

GND

REF

SHDN

FBV+

1.5V

REFERENCE

F/F

50mV

Figure 2. Block Diagram

Dual Mode is a trademark of Maxim Integrated Products, Inc.

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

8 _______________________________________________________________________________________

Modes of Operation

When delivering high output currents, the MAX1649/

MAX1651 operate in continuous-conduction mode. In

this mode, current always flows in the inductor, and

the control circuit adjusts the switch duty cycle to main-

tain regulation without exceeding the switch current

capability (Figure 3). This provides excellent load-tran-

sient response and high efficiency.

In discontinuous-conduction mode, current through the

inductor starts at zero, rises to a peak value, then

ramps down to zero. Although efficiency is still excel-

lent, the output ripple increases slightly, and the switch

waveform exhibits ringing (at the inductor's self-reso-

nant frequency). This ringing is to be expected and

poses no operational problems.

Dropout

The MAX1649/MAX1651 are in dropout when the input

voltage (V+) is low enough that the output drops below

the minimum output voltage specification (see

Electrical Characteristics). The dropout voltage is the

difference between the input and output voltage when

dropout occurs. See the Typical Operating

Characteristics for the Dropout Voltage vs. Load

Current and Dropout Voltage vs. Temperature graphs.

__________________Design Procedure

Setting the Output Voltage

The MAX1649/MAX1651 are preset for 5V and 3.3V out-

put voltages, respectively; tie FB to GND for fixed-output

operation. They may also be adjusted from 1.5V (the

reference voltage) to the input voltage, using external

resistors R2 and R3 configured as shown in Figure 4. For

adjustable-output operation, 150kΩis recommended for

resistor R3—high enough to avoid wasting energy, yet

low enough to avoid RC delays caused by parasitic

capacitance at FB. R2 is given by:

VOUT

R2 = R3 x

(

——— -1

)

VREF

where VREF = 1.5V.

When using external resistors, it does no harm to con-

nect OUT and the output together, or to leave OUT

unconnected.

Current-Sense Resistor Selection

The current-sense resistor limits the peak switch cur-

rent to 110mV/RSENSE, where RSENSE is the value of

the current-sense resistor, and 110mV is the current-

limit trip level (see Electrical Characteristics).

V+ = 10V, ILOAD = 1.3A

CIRCUIT OF FIGURE 1, R1 = 75mΩ

1.5A

2μs/div

Figure 3. MAX1649 Continuous-Conduction Mode, Heavy

Load-Current Waveform (500mA/div)

(

)

MAX1649

MAX1651

V+

GND

VIN

330μF

EXT

OUT

SHDN

0.1μF

100μF

0.05Ω

1N5820

47μH

Si9430 OUTPUT

@ 1.5A

REF

FB R2

150k

R2 = R3 VOUT

VREF

– 1

VREF = 1.5V

Figure 4. Adjustable-Output Operation

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

_______________________________________________________________________________________ 9

To maximize efficiency and reduce the size and cost

of external components, minimize the peak current.

However, since the available output current is a func-

tion of the peak current, the peak current must not be

too low.

To choose the proper current-sense resistor for a par-

ticular output voltage, determine the minimum input

voltage and the maximum load current. Next, refer-

ring to Figures 5a or 5b, using the minimum input volt-

age, find the curve with the largest sense resistor that

provides sufficient output current. It is not necessary

to perform worst-case calculations. These curves take

into account the sense-resistor (±5%) and inductor

(47µH ±10%) values, the diode drop (0.4), and the

IC’s current-sense trip level (85mV); an external MOS-

FET on-resistance of 0.07Ωis assumed for VGS = -5V.

Standard wire-wound and metal-film resistors have an

inductance high enough to degrade performance.

Surface-mount (chip) resistors have very little inductance

and are well suited for use as current-sense resistors.

A U-shaped wire resistor made by IRC works well in

through-hole applications. Because this resistor is a

band of metal shaped as a “U”, its inductance is less

than 10nH (an order of magnitude less than metal film

resistors). Resistance values between 5mΩand 0.1Ω

are available (see Table 1).

Inductor Selection

The MAX1649/MAX1651 operate with a wide range of

inductor values, although for most applications coils

between 10µH and 68µH take best advantage of the con-

trollers’ high switching frequency. With a high inductor

value, the MAX1649/MAX1651 will begin continuous-cur-

rent operation (see Detailed Description) at a lower frac-

tion of full-load current. In general, smaller values pro-

duce higher ripple (see below) while larger values require

larger size for a given current rating.

In both the continuous and discontinuous modes, the

lower limit of the inductor is important. With a too-small

inductor value, the current rises faster and overshoots the

desired peak current limit because the current-limit com-

parator has a finite response time (300ns). This reduces

efficiency and, more importantly, could cause the current

rating of the external components to be exceeded.

Calculate the minimum inductor value as follows:

(V+(max) - VOUT) x 0.3µs

L(min) = ——————————––——

ΔI x ILIM

where ΔI is the inductor-current overshoot factor,

ILIM = VCS/RSENSE, and 0.3µs is the time it takes the com-

parator to switch. Set ΔI= 0.1 for an overshoot of 10%.

For highest efficiency, use a coil with low DC resis-

tance; a value smaller than 0.1V/ILIM works best. To

minimize radiated noise, use a toroid, pot core, or

shielded-bobbin inductor. Inductors with a ferrite core

or equivalent are recommended. Make sure the induc-

tor’s saturation-current rating is greater than ILIM(max).

However, it is generally acceptable to bias the inductor

into saturation by about 20% (the point where the

inductance is 20% below its nominal value).

5.0 5.4 5.8 6.2 6.6 16.0

1649 Fig05a

INPUT VOLTAGE (V)

MAXIMUM OUTPUT CURRENT (A)

3.0

2.5

2.0

1.5

1.0

0.5

rs = 0.030

VOUT = 5V

rs = 0.040

rs = 0.050

rs = 0.060

rs = 0.080

rs = 0.100

Figure 5a. MAX1649 Current-Sense Resistor Graph

3.0 3.4 3.8 4.2 4.6 16.0

1651 Fig05b

INPUT VOLTAGE (V)

MAXIMUM OUTPUT CURRENT (A)

3.0

2.5

2.0

1.5

1.0

0.5

rs = 0.030

rs = 0.040

rs = 0.050

rs = 0.060

rs = 0.080

rs = 0.100

VOUT = 3.3V

Figure 5b. MAX1651 Current-Sense Resistor Graph

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

10 ______________________________________________________________________________________

Table 1. Component Selection Guide

The peak current of Figure 1 is 2.35A for a 1.5A output.

The inductor used in this circuit is specified to drop by

10% at 2.2A (worst case); a curve provided by the

manufacturer shows that the inductance typically drops

by 20% at 2.7A. Using a slightly underrated inductor

can sometimes reduce size and cost, with only a minor

impact on efficiency.

Table 1 lists inductor types and suppliers for various

applications. The efficiencies of the listed surface-

mount inductors are nearly equivalent to those of the

larger size through-hole versions.

Diode Selection

The MAX1649/MAX1651’s high switching frequency

demands a high-speed rectifier. Schottky diodes, such

as the 1N5817 through 1N5823 (and their surface-

mount equivalents), are recommended. Choose a

diode with an average current rating equal to or greater

than ILIM(max) and a voltage rating higher than

V+(max).

External Switching Transistor

The MAX1649/MAX1651 drive P-channel enhancement-

mode MOSFET transistors only. The choice of power

transistor is primarily dictated by the input voltage and

the peak current. The transistor’s on-resistance, gate-

source threshold, and gate charge must also be appro-

priately chosen. The drain-to-source and gate-to-

source breakdown voltage ratings must be greater than

V+. The total gate-charge specification is normally not

critical, but values should be less than 100nC for best

efficiency. The MOSFET should be capable of handling

the peak current and, for maximum efficiency, have a

very low on-resistance at that current. Also, the on-

resistance must be low for the minimum available VGS,

which equals V+(min). Select a transistor with an on-

resistance between 50% and 100% of the current-

sense resistor. The Si9430 transistor chosen for the

Typical Operating Circuit has a drain-to-source rating

of -20V and a typical on-resistance of 0.070Ωat 2A with

VGS = -4.5V. Tables 1 and 2 list suppliers of switching

transistors suitable for use with these devices.

Capacitor Selection

Output Filter Capacitor

The primary criterion for selecting the output filter

capacitor is low equivalent series resistance (ESR),

rather than high capacitance. An electrolytic capacitor

with low enough ESR will automatically have high

enough capacitance. The product of the inductor-cur-

rent variation and the output filter capacitor’s ESR

determines the amplitude of the high-frequency ripple

seen on the output voltage. When a 330µF, 10V

Sprague surface-mount capacitor (595D series) with

ESR = 0.15Ωis used, 40mV of output ripple is typically

observed when stepping down from 10V to 5V at 1A.

The output filter capacitor's ESR also affects efficiency.

Again, low-ESR capacitors perform best. Table 1 lists

some suppliers of low-ESR capacitors.

PRODUCTION

METHOD INDUCTORS CAPACITORS DIODES CURRENT-SENSE

RESISTORS MOSFETS

Surface Mount

AVX

TPS series

Sprague

595D series

Motorola

MBRS340T3

Nihon

NSQ series

Dale

WSL Series

IRC

LRC series

Miniature

Through-Hole

Sumida

RCH875-470M (1.3A)

Sanyo

OS-CON series

low-ESR organic

semiconductor

IRC

OAR series Motorola

Low-Cost

Through-Hole

Coilcraft

PCH-45-473 (3.4A)

Motorola

1N5817 to

1N5823

Motorola

TMOS power MOSFETs

Sumida

CDRH125-470 (1.8A)

CDRH125-220 (2.2A)

Coilcraft

DO3316-473 (1.6A)

DO3340-473 (3.8A)

Siliconix

Little Foot series

Motorola

medium-power

surface-mount products

Nichicon

PL series

low-ESR electrolytics

United Chemi-Con

LXF series

Input Bypass Capacitor

The input bypass capacitor reduces peak currents

drawn from the voltage source, and also reduces the

amount of noise at the voltage source caused by the

switching action of the MAX1649/MAX1651. The input

voltage source impedance determines the size of the

capacitor required at the V+ input. As with the output fil-

ter capacitor, a low-ESR capacitor is recommended.

Bypass the IC separately with a 0.1µF ceramic capac-

itor placed close to the V+ and GND pins.

Reference Capacitor

Bypass REF with a 0.1µF or larger capacitor.

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

______________________________________________________________________________________ 11

___________________Chip Topography

TRANSISTOR COUNT: 428

SUBSTRATE CONNECTED TO V+

0.106"

(2.692mm)

0.081"

(2.057mm)

OUT GND

EXT

V+

SHDN

REF

Table 2. Component Suppliers

COMPANY PHONE FAX

(207) 282-5111

AVX USA or (207) 283-1941

(800) 282-4975

Coiltronics USA (516) 241-7876 (516) 241-9339

Coilcraft USA (708) 639-6400 (708) 639-1469

Dale USA (402) 564-3131 (402) 563-1841

International USA (310) 322-3331 (310) 322-3332

Rectifier

IRC USA (512) 992-7900 (512) 992-3377

(602) 244-3576

Motorola USA or (602) 244-4015

(602) 244-5303

Nichicon USA (708) 843-7500 (708) 843-2798

Japan 81-7-5231-8461 81-7-5256-4158

Nihon USA (805) 867-2555 (805) 867-2556

Japan 81-3-3494-7411 81-3-3494-7414

Sanyo USA (619) 661-6835 (619) 661-1055

Japan 81-7-2070-6306 81-7-2070-1174

(408) 988-8000

Siliconix USA or (408) 970-3950

(800) 554-5565

Sprague USA (603) 224-1961 (603) 224-1430

Sumida USA (708) 956-0666 (708) 956-0702

Japan 81-3-3607-5111 81-3-3607-5144

United USA (714) 255-9500 (714) 255-9400

Chemi-Con

Layout Considerations

Proper PC board layout is essential because of high

current levels and fast switching waveforms that radi-

ate noise. Minimize ground noise by connecting the

anode of the rectifier, the input bypass capacitor

ground lead, and the output filter capacitor ground

lead to a single point (“star” ground configuration). A

ground plane is recommended. Also minimize lead

lengths to reduce stray capacitance, trace resistance,

and radiated noise. In particular, the traces connected

to FB (if an external resistor divider is used) and EXT

must be short. Place the 0.1µF ceramic bypass capac-

itor as close as possible to the V+ and GND pins.

MAX1649/MAX1651 vs. MAX649/MAX651

The MAX1649 and MAX1651 are pin compatible with

the MAX649 and MAX651, but have been optimized for

improved dropout performance and efficiency—partic-

ularly with low input voltages. The MAX1649/MAX1651

feature increased maximum switch duty cycle (96.5%)

and reduced current-limit sense voltage (110mV).

Their predecessors, the MAX649/MAX651, use a high-

er two-step (210mV/110mV) current-limit sense voltage

to provide tighter current-sense accuracy and reduced

inductor peak current at light loads.

Package Information

For the latest package outline information and land patterns, go

to www.maxim-ic.com/packages.

PACKAGE TYPE PACKAGE CODE DOCUMENT NO.

8 PDIP P8-2 21-0041

8 SO S8-4 21-0043

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600

MAX1649/MAX1651

5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

Revision History

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

3 3/09 Corrected Output Voltage conditions and Figure 1 title 2, 6

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