single 100µH (MAX7__AC) or 33µH (MAX7__AE/AM)

inductor. The inductor’s incremental saturation current

rating should be greater than 1A, and its DC resistance

should be less than 0.8

Ω. Table 2 lists inductor types

and suppliers for various applications. The surface-

mount inductors have nearly equivalent efficiencies to

the larger through-hole inductors.

Output Filter Capacitor Selection

The primary criterion for selecting the output filter

capacitor is low equivalent series resistance (ESR).

The product of the inductor current variation and the

output capacitor’s ESR determines the amplitude of the

sawtooth ripple seen on the output voltage. Also, mini-

mize the output filter capacitor’s ESR to maintain AC

stability. The capacitor’s ESR should be less than

0.25

Ω to keep the output ripple less than 50mVp-p over

the entire current range (using a 100µH inductor).

Capacitor ESR rises as the temperature falls, and

excessive ESR is the most likely cause of trouble at

temperatures below 0°C. Sanyo OS-CON series

through-hole and surface-mount tantalum capacitors

exhibit low ESR at temperatures below 0°C. Refer to

Table 2 for recommended capacitor values and sug-

gested capacitor suppliers.

Other Components

The catch diode should be a Schottky or high-speed

silicon rectifier with a peak current rating of at least

1.5A for full-load (750mA) operation. The 1N5817 is a

good choice. The 330pF outer-loop compensation

capacitor provides the widest input voltage range and

best transient characteristics. For low-current applica-

tions, the 510k

Ω resistor may be omitted (see the

Maximum Output Current vs. Supply Voltage graph (R1

removed) in the

Typical Operating Characteristics).

5V, Step-Down,

Current-Mode PWM DC-DC Converters

10

______________________________________________________________________________________

C1

0.1

µF

SS

REF

CC

OUT

C5

330pF

R1

510k

C6

0.01

µF

1M

±35%

GND

SS CLAMP

PWM

COMPARATOR

BIAS

GEN

SHDN

Σ

RAMP

GEN

SLOPE COMPENSATION

OVERCURRENT COMPARATOR

CURRENT

SENSE AMP

C2

1

µF

V+

C3

*

+5V

C4

*

D1

1N5817

LX

VUVLO

UNDERVOLTAGE

LOCKOUT

OSC

F/F

S

RQ

1.23V

BANDGAP

ERROR AMP

*SEE TABLE 2 FOR COMPONENT VALUES AND SUPPLIERS

L1

MAX730A

MAX738A

MAX744A

Figure 1. Detailed Block Diagram with External Components