5V, Step-Down,
Current-Mode PWM DC-DC Converters
MAX730A/MAX738A/MAX744A
V
IN
= +6.0V TO +16.0V
SHDN
OVERCURRENT COMPARATOR
SLOPE COMPENSATION
BIAS
GEN
OUT
C5
330pF
RAMP
GEN
R
SENSE
F/F
R
1.23V
BANDGAP
REF
C6
0.01µF
SS
C1
0.1µF
OSC
1M
±35%
S
PWM
COMPARATOR
Q
LX
L1
D1
1N5817
V
OUT
=
+5V
C4
*
CURRENT
SENSE AMP
V+
C2
C3
*
1µF
Σ
ERROR AMP
CC
R1
510k
SS CLAMP
MAX730A
MAX738A
MAX744A
UNDERVOLTAGE
LOCKOUT
VUVLO
GND
*SEE TABLE 2 FOR COMPONENT VALUES AND SUPPLIERS
Figure 1. Detailed Block Diagram with External Components
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 0.8Ω. Table 2 lists inductor types
and suppliers for various applications. The surface-
mount inductors have nearly equivalent efficiencies to
the larger through-hole inductors.
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.
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
0.25Ω to keep the output ripple less than 50mVp-p over
the entire current range (using a 100µH inductor).
10
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 510kΩ resistor may be omitted (see the
Maximum Output Current vs. Supply Voltage graph (R1
removed) in the
Typical Operating Characteristics).
______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
