Rainbow Electronics MAX1655 User Manual

19-0305; Rev 2; 9/95

Low-Dropout, Step-Down DC-DC Controllers

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

_______________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, current­limited pulse-frequency-modulated (PFM) control scheme
gives these devices the benefits of pulse-width-modula­tion (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

PART TEMP. RANGE PIN-PACKAGE

MAX1649CPA

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
MAX1651CPA
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

* Dice are tested at TA= +25°C.

0°C to +70°C 8 Plastic DIP

0°C to +70°C 8 Plastic DIP

MAX1649/MAX1651

__________Typical Operating Circuit

INPUT

3.6V TO 16V

V+

ON/OFF

REF

MAX1651

FB GND

CSSHDN

EXT

OUT

________________________________________________________________

P

OUTPUT

3.3V

__________________Pin Configuration

TOP VIEW

OUT

1

FB

2

SHDN

REF

MAX1649

3

MAX1651

4

DIP/SO

Maxim Integrated Products

Call toll free 1-800-998-8800 for free samples or literature.

8

GND
EXT

7

CS

6
5

V+

1

5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers

ABSOLUTE MAXIMUM RATINGS

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

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

Continuous Power Dissipation (T

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

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

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.

= +70°C)

A

ELECTRICAL CHARACTERISTICS

(V+ = 5V, TA= T

PARAMETER SYMBOL MIN TYP MAX UNITS

V+ Input Voltage Range V+ 3.0 16 V

Supply Current

MAX1649/MAX1651

FB Trip Point

FB Input Current I

Output Voltage V

Reference Voltage V
REF Load Regulation mV410

Output Voltage
Line Regulation

Output Voltage
Load Regulation

Efficiency

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

MIN

to T

, unless otherwise noted. Typical values are at TA= +25°C.)

MAX

CONDITIONS

V

< V+

OUT

V+ = 16V, SHDN ≤ 0.4V (operating, switch off)
V+ = 16V, SHDN ≥ 1.6V (shutdown)

I+

V+ = 10V, SHDN ≥ 1.6V (shutdown)
MAX1649C, MAX1651C
MAX1649E, MAX1651E
MAX1649C, MAX1651C

FB

MAX1649E, MAX1651E
Circuit of

OUT

Figure 1
MAX1649C, MAX1651C, I

REF

MAX1649E, MAX1651E, I
0µA ≤ I
3V ≤ V+ ≤ 16VREF Line Regulation 40 100 µV/V

Circuit of
Figure 1

Circuit of
Figure 1

Circuit of
Figure 1

3V ≤ V+ ≤ 16V 1.6 V

IH

3V ≤ V+ ≤ 16V 0.4 V

IL

≤ 100µA, sourcing only

REF

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

78 100

2
15

1.470 1.5 1.530

1.4625 1.5 1.5375
±50
±70

MAX1649, V+ = 5.5V to 16V
MAX1651, V+ = 3.6V to 16V

= 0µA

REF

= 0µA

REF

MAX1649, 5.5V ≤ V+ ≤ 16V,
I

= 1A

LOAD

MAX1651, 3.6V ≤ V+ ≤ 16V,
I

= 1A

LOAD

MAX1649, 0A ≤ I
VIN= 10V

MAX1651, 0A ≤ I
VIN= 5V

MAX1649, V+ = 10V,
I

= 1A

LOAD

MAX1651, V+ = 5V,
I

= 1A

LOAD

LOAD

LOAD

≤ 1.5A,

≤ 1.5A,

4.80 5.0 5.20

3.17 3.3 3.43

1.470 1.5 1.530

1.4625 1.5 1.5375

2.6

1.7

-47

-45

90

90

µA

V

nA

V

V

mV/V

mV/A

%

2 _______________________________________________________________________________________

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

Low-Dropout, Step-Down DC-DC Controllers

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 5V, TA= T

PARAMETER

Current-Limit Trip Level
(V+ to CS)

Maximum Duty Cycle 95 96.5 %

MIN

to T

, unless otherwise noted. Typical values are at TA= +25°C.)

MAX

SYMBOL MIN TYP MAX UNITSCONDITIONS

3V ≤ V+ ≤ 16V 80 110 140

CS

3V ≤ V+ ≤ 16VCS Input Current ±1 µA
V+ = 12VSwitch Maximum On-Time 24 32 40 µstON(max)
V+ = 12VSwitch Minimum Off-Time 0.8 1.1 1.8 µst

(min)

OFF

C

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

EXT

C

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

EXT

t

ON

OFF

x 100%

tON+ t

mVV

__________________________________________Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

SUPPLY CURRENT vs. TEMPERATURE

80

78

76

74

I+ (µA)

72

70

68

66

-60 -20 60 140

20 100-40 0 8040 120

TEMPERATURE (°C)

V+ = 16V

V+ = 10V

V+ = 4V

MAX1649-TOC06

SHUTDOWN CURRENT 

4.0

3.5

3.0

2.5

2.0

I+ (µA)

1.5

1.0

0.5
0

vs. TEMPERATURE

V+ = 16V

V+ = 8V

V+ = 4V

-60 -20 60 140

20 100-40 0 8040 120

TEMPERATURE (°C)

MAX1649-TOC05

(ns)

FALL

& t

RISE

t

EXT RISE AND FALL TIMES

60
55

50
45

40
35

30
25
20
15

vs. TEMPERATURE (1nF)

C

= 1nF

EXT

V+ = 5V, t

RISE

V+ = 5V, t

FALL

V+ = 15V, t

RISE

V+ = 15V, t

FALL

-60 -20 60 140

20 100-40 0 8040 120

TEMPERATURE (°C)

MAX1649/MAX1651

MAX1649/51-01

EXT RISE AND FALL TIMES

vs. TEMPERATURE (5nF)

240

C

= 5nF

EXT

220
200
180

V+ = 5V, t

(ns)

160

FALL

140

& t

120

RISE

t

100

80
60
40

-60 -20 60 140

V+ = 5V, t

RISE

FALL

V+ = 15V, t

V+ = 15V, t

20 100-40 0 8040 120

TEMPERATURE (°C)

_______________________________________________________________________________________ 3

RISE

FALL

MAX1649/51-02

vs. LOAD CURRENT (V

100

V

OUT

CIRCUIT OF 
FIGURE 1

90

80

70

EFFICIENCY (%)

60

50

40

0.1

EFFICIENCY

= 5V)

OUT

= 5V

TOP TO 
BOTTOM:

= 6V

V

IN

= 8V

V

IN

= 10V

V

IN

= 12V

V

IN

= 15V

V

IN

1 10 10k

LOAD CURRENT (mA)

100 1k

MAX1649/51-A1

vs. LOAD CURRENT (V

100

V

OUT

CIRCUIT OF 
FIGURE 1

90

80

70

EFFICIENCY (%)

60

50

40

0.1

EFFICIENCY

= 3.3V)

OUT

= 3.3V

TOP TO 
BOTTOM:

= 4.3V

V

IN

= 5V

V

IN

= 8V

V

IN

= 10V

V

IN

= 12V

V

IN

= 15V

V

IN

1 10 10k

LOAD CURRENT (mA)

100 1k

MAX1649/51-A2

5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

SWITCH ON-TIME

vs. TEMPERATURE

34.0

33.5

33.0

32.5

(µs)

32.0

ON

t

31.5

31.0

30.5

30.0

-60 -20 60 140

MAX1649/MAX1651

20 100-40 0 8040 120

TEMPERATURE (°C)

MAX1649/51-03

(µs)

OFF

t

CS TRIP LEVEL

vs. TEMPERATURE

120

115

110

105

CS TRIP LEVEL (mV)

100

95

-60 -20 60 140

20 100-40 0 8040 120

TEMPERATURE (°C)

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

MAX1649/51-06

SWITCH OFF-TIME
vs. TEMPERATURE

20 100-40 0 8040 120

TEMPERATURE (°C)

MAXIMUM DUTY CYCLE

100

99

MAX1649/51-04

98

97
96

DUTY CYCLE (%)

95

94

93

-60 -20 60 140

DROPOUT VOLTAGE

vs. LOAD CURRENT

600

CIRCUIT OF 
FIGURE 1

500

400

V

= 4.80V

OUT

300

200

DROPOUT VOLTAGE (mV)

100

0

0 0.5 1.0 1.5 2.0

V

= 3.17V

OUT

LOAD CURRENT (A)

vs. TEMPERATURE

20 100-40 0 8040 120

TEMPERATURE (°C)

MAX1649/51-05

MAX1649/51-A3

REFERENCE OUTPUT RESISTANCE

250

200

150

100

50

REFERENCE OUTPUT RESISTANCE (Ω)

0

vs. TEMPERATURE

I

REF

I

REF

-60 -20 60 140

20 100-40 0 8040 120

TEMPERATURE (°C)

= 10µA

I

REF

= 100µA

MAX1649-TOC07

= 50µA

REFERENCE OUTPUT VOLTAGE (V)

REFERENCE OUTPUT VOLTAGE

1.506

1.504

1.502

1.500

1.498

1.496

1.494

1.492

vs. TEMPERATURE

I

REF

-60 -20 60 140

20 100-40 0 8040 120

TEMPERATURE (°C)

= 10µA

4 _______________________________________________________________________________________

MAX1649-TOC01

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

Low-Dropout, Step-Down DC-DC Controllers

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

LINE-TRANSIENT RESPONSE

CIRCUIT OF FIGURE 1, I
A: V

= 5V, 100mV/div, AC-COUPLED

OUT

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

MAX1649

5ms/div

= 1A

LOAD

A

16V

B

6V

SHDN RESPONSE TIME

MAX1649

CIRCUIT OF FIGURE 1, V+ = 10V
A: V

= 5V, 100mV/div, AC-COUPLED

OUT

B: I

= 30mA TO 1.6A, 1A/div

LOAD

5V

OUTPUT

0V

4V

SHDN
INPUT

0V

MAX1649

LOAD-TRANSIENT RESPONSE

1.6A

0A

200µs/div

MAX1649/MAX1651

A

B

CIRCUIT OF FIGURE 1, V+ = 10V, I

1ms/div

LOAD

= 1A

_______________________________________________________________________________________

5

5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers

______________________________________________________________Pin Description

NAME FUNCTION

PIN

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

OUT

1

2 FB

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

5 V+ Positive Power-Supply Input
6 CS
7 EXT Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.

8 GND Ground

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

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

Active-High Shutdown Input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the refer­ence, output, and external MOSFET are turned off. Connect to GND for normal operation.

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.

Setting the Output Voltage

section.

MAX1649/MAX1651

V

IN

C4

C1

0.1µF

*

47µH

L1

**

330µF

100µF

OUTPUT

@ 1.5A

C2

MAX1649
MAX1651

3

SHDN

4

REF

FB GND

28

C3

0.1µF

Figure 1. Test Circuit

5

V+

R1

0.05Ω

6

CS

EXT

OUT

NSQ03A02L

*SILICONIX SURFACE-MOUNT MOSFET
**SUMIDA CDRH125-470

P1

7

Si9430

1

D1

_______________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, respec­tively. 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-limit­ed PFM control scheme. As with traditional PFM con­verters, 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 cur­rent, 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 compo­nent 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 out­put for an external P-channel power MOSFET.

6 _______________________________________________________________________________________

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

Low-Dropout, Step-Down DC-DC Controllers

FBV+

DUAL-MODE™
COMPARATOR

MAX1649/MAX1651

MAX1649

SHDN

REF

™ Dual-Mode is a trademark of Maxim Integrated Products

MAX1651

MINIMUM

Q

OFF-TIME

ONE-SHOT

TRIG

TRIG

MAXIMUM

ON-TIME

ONE-SHOT

Q

ERROR

COMPARATOR

1.5V

REFERENCE

COMPARATOR

CURRENT

GND

50mV

N

FROM V+

QS

F/F

R

110mV

FROM V+

OUT

EXT

CS

Figure 2. Block Diagram

Shutdown Mode

When SHDN is high, the MAX1649/MAX1651 enter shut­down mode. In this mode, the internal biasing circuitry is
turned off (including the reference) and the supply cur­rent 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.

_______________________________________________________________________________________ 7

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

Quiescent Current

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
V

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

OUT

entire circuit is 90µA.

5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers

V



V+ = 10V, I

LOAD

MAX1649/MAX1651

Figure 3. MAX1649 Continuous-Conduction Mode, Heavy
Load-Current Waveform (500mA/div)

CIRCUIT OF FIGURE 1, R1 = 75mΩ

2µs/div

= 1.3A

1.5A
1A

0A

IN

5

V+

EXT

OUT

R1

0.05Ω

6

CS

7

1
2

FB

D1

1N5820

MAX1649
MAX1651

3

SHDN

4

REF

GND

C3

0.1µF

R2 = R3

= 1.5V

V

REF

Figure 4. Adjustable-Output Operation

8

V

OUT

– 1

(

)

V

REF

P1
Si9430

C4

0.1µF

47µH

L1

R2

330µF

R3
150k

C1

100µF

OUTPUT

@ 1.5A

C2

When delivering high output currents, the MAX1649/

Modes of Operation

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 main­tain regulation without exceeding the switch current
capability (Figure 3). This provides excellent load-tran­sient 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 excel­lent, the output ripple increases slightly, and the switch
waveform exhibits ringing (at the inductor's self-reso­nant 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

Characteristics

for the Dropout Voltage vs. Load

Typical Operating

Current and Dropout Voltage vs. Temperature graphs.

8 _______________________________________________________________________________________

__________________Design Procedure

The MAX1649/MAX1651 are preset for 5V and 3.3V out­put 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, 150kΩ is 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:

where V

REF

= 1.5V.

When using external resistors, it does no harm to con­nect OUT and the output together, or to leave OUT
unconnected.

The current-sense resistor limits the peak switch cur­rent to 110mV/R
the current-sense resistor, and 110mV is the current­limit trip level (see

Setting the Output Voltage

V

OUT

R2 = R3 x

——— -1

(

V

REF

Current-Sense Resistor Selection

, where R

SENSE

SENSE

Electrical Characteristics

)

is the value of

).

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

Low-Dropout, Step-Down DC-DC Controllers

To maximize efficiency and reduce the size and cost
of external components, minimize the peak current.
However, since the available output current is a func­tion of the peak current, the peak current must not be
too low.

To choose the proper current-sense resistor for a par­ticular output voltage, determine the minimum input
voltage and the maximum load current. Next, refer­ring to Figures 5a or 5b, using the minimum input volt­age, 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 MOS­FET on-resistance of 0.07Ω is 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 5mΩ and 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-cur­rent operation (see

Detailed Description

) at a lower frac­tion of full-load current. In general, smaller values pro­duce 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 com­parator 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:

L(min) = ——————————––——

(V+(max) - V

∆I x I

OUT

LIM

) x 0.3µs

where ∆I is the inductor-current overshoot factor,
I

LIM

= VCS/R

, and 0.3µs is the time it takes the com-

SENSE

parator to switch. Set ∆I = 0.1 for an overshoot of 10%.
For highest efficiency, use a coil with low DC resis-

tance; a value smaller than 0.1V/I

works best. To

LIM

minimize radiated noise, use a toroid, pot core, or
shielded-bobbin inductor. Inductors with a ferrite core
or equivalent are recommended. Make sure the induc­tor’s saturation-current rating is greater than I

LIM

(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).

MAX1649/MAX1651

3.0
V

= 5V

OUT

2.5

2.0

1.5

1.0

0.5

MAXIMUM OUTPUT CURRENT (A)

0

5.0 5.4 5.8 6.2 6.6 16.0

Figure 5a. MAX1649 Current-Sense Resistor Graph

_______________________________________________________________________________________ 9



rs = 0.030

rs = 0.040

rs = 0.050
rs = 0.060

rs = 0.080

rs = 0.100

INPUT VOLTAGE (V)

1649 Fig05a

3.0
V

= 3.3V

OUT

2.5

2.0

1.5

1.0

0.5

MAXIMUM OUTPUT CURRENT (A)

0

3.0 3.4 3.8 4.2 4.6 16.0

Figure 5b. MAX1651 Current-Sense Resistor Graph

INPUT VOLTAGE (V)

rs = 0.030

rs = 0.040

rs = 0.050
rs = 0.060

rs = 0.080

rs = 0.100

1651 Fig05b

5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers

Table 1. Component Selection Guide

PRODUCTION

METHOD

Surface Mount

Miniature
Through-Hole

Low-Cost

MAX1649/MAX1651

Through-Hole

INDUCTORS CAPACITORS DIODES

Sumida
CDRH125-470 (1.8A)
CDRH125-220 (2.2A)

CoilCraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)

Sumida
RCH875-470M (1.3A)

CoilCraft
PCH-45-473 (3.4A)

AVX
TPS series

Sprague
595D series

Sanyo
OS-CON series
low-ESR organic
semiconductor

Nichicon
PL series
low-ESR electrolytics

United Chemi-Con
LXF series

Motorola
MBRS340T3

Nihon
NSQ series

Motorola
1N5817 to
1N5823

CURRENT-SENSE

RESISTORS

Dale
WSL Series

IRC
LRC series

IRC
OAR series

Siliconix
Little Foot series

Motorola
medium-power
surface-mount products

Motorola

Motorola
TMOS power MOSFETs

MOSFETS

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 surface­mount 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 surface­mount equivalents), are recommended. Choose a
diode with an average current rating equal to or greater
than I

(max) and a voltage rating higher than

LIM

V+(max).

External Switching Transistor

The MAX1649/MAX1651 drive P-channel enhancement­mode MOSFET transistors only. The choice of power
transistor is primarily dictated by the input voltage and
the peak current. The transistor’s on-resistance, gate­source threshold, and gate charge must also be appro­priately chosen. The drain-to-source and gate-to­source breakdown voltage ratings must be greater than
V+. The total gate-charge specification is normally not

10 ______________________________________________________________________________________

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 on­resistance must be low for the minimum available VGS,
which equals V+(min). Select a transistor with an on­resistance between 50% and 100% of the current­sense resistor. The Si9430 transistor chosen for the

Typical Operating Circuit

has a drain-to-source rating
of -20V and a typical on-resistance of 0.070Ω at 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-cur­rent 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 = 0.15Ω is 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.

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

Low-Dropout, Step-Down DC-DC Controllers

Table 2. Component Suppliers

COMPANY PHONE FAX

AVX USA or [phone]

Coiltronics USA [phone] [phone]
CoilCraft USA [phone] [phone]
Dale USA [phone] [phone]
International

Rectifier
IRC USA [phone] [phone]

Motorola USA or [phone]

Nichicon

Nihon

Sanyo

Siliconix USA or [phone]

Sprague USA [phone] [phone]
Sumida
United

Chemi-Con

USA [phone] [phone]

USA [phone] [phone]
Japan 81-7-5231-8461 81-7-5256-4158

USA [phone] [phone]
Japan 81-3-3494-7411 81-3-3494-7414

USA [phone] [phone]
Japan 81-7-2070-6306 81-7-2070-1174

USA [phone] [phone]
Japan 81-3-3607-5111 81-3-3607-5144

USA [phone] [phone]

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 fil­ter capacitor, a low-ESR capacitor is recommended.
Bypass the IC separately with a 0.1µF ceramic capac­itor placed close to the V+ and GND pins.

Bypass REF with a 0.1µF or larger capacitor.

[phone]
[phone]

[phone]
[phone]

[phone]
[phone]

Input Bypass Capacitor

Reference Capacitor

Proper PC board layout is essential because of high

Layout Considerations

current levels and fast switching waveforms that radi­ate 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 capac­itor 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—partic­ularly 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 high­er two-step (210mV/110mV) current-limit sense voltage
to provide tighter current-sense accuracy and reduced
inductor peak current at light loads.

___________________Chip Topography

OUT

FB

SHDN

GND

EXT

0.106"

(2.692mm)

CS

MAX1649/MAX1651

REF

0.081"

(2.057mm)

V+

TRANSISTOR COUNT: 428
SUBSTRATE CONNECTED TO V+

______________________________________________________________________________________ 11

5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers

________________________________________________________Package Information

INCHES MILLIMETERS

DIM

A2

A

A1

L

D

e

B

D1

MAX1649/MAX1651

D

A

0.101mm

e

A1

B

0.004in.

B1

A3

DUAL-IN-LINE

C

E

E1

0°-15°

eA
eB

Plastic DIP

PLASTIC

PACKAGE

(0.600 in.)

L

0°-8°

C

PKG.



P
P
P

A1
A2
A3

B1

D1

E1

eA
eB

DIM

DIM



A

B

C

E

e

L

PINS



D
D
D



A

A1

B
C
E
e
H
L

MAX

MIN

–

0.015

0.125

0.055

0.016

0.045

0.008

0.050

0.600

0.525

0.100

0.600
–

0.120

INCHES MILLIMETERS

MIN

24

1.230

28

1.430

40

2.025

INCHES MILLIMETERS

MIN

0.053

0.004

0.014

0.007

0.150


0.228

0.016

0.200
–

0.175

0.080

0.020

0.065

0.012

0.090

0.625

0.575
–
–

0.700

0.150

MAX

0.069

0.010

0.019

0.010

0.157

0.244

0.050

MAX

1.270

1.470

2.075



MIN

–

0.38

3.18

1.40

0.41

1.14

0.20

1.27

15.24

13.34

2.54

15.24
–

3.05

MIN

31.24

36.32

51.44

MIN

1.35

0.10

0.35

0.19

3.80


5.80

0.40

1.270.050

MAX

5.08
–

4.45

2.03

0.51

1.65

0.30

2.29

15.88

14.61
–
–

17.78

3.81

MAX

32.26

37.34

52.71

21-0044A

MAX

1.75

0.25

0.49

0.25

4.00


6.20

1.27

Narrow SO

HE

SMALL-OUTLINE

PACKAGE

(0.150 in.)

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 [phone]

DIM

D
D
D

MIN

MAX

MIN



8

0.189

0.337

0.386

0.197

0.344

0.394

14
16

4.80

8.55

9.80

MAX

5.00

8.75

10.00

21-0041A

INCHES MILLIMETERS

PINS

© 1995 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.