MAX730 Maxim, MAX730 Datasheet - Page 10

• Manufacturer Part Number: MAX730
• Description: 5V / Step-Down / Current-Mode PWM DC-DC Converters
• Manufacturer: Maxim

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.
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).
5V, Step-Down,
Current-Mode PWM DC-DC Converters
Figure 1. Detailed Block Diagram with External Components
10
*SEE TABLE 2 FOR COMPONENT VALUES AND SUPPLIERS
______________________________________________________________________________________
0.01 F
330pF
R1
510k
C6
C5
C1
0.1 F
OUT
REF
CC
SS
Output Filter Capacitor Selection
1M
±35%
BANDGAP
1.23V
BIAS
GEN
SHDN
ERROR AMP
GND
COMPARATOR
SS CLAMP
PWM
SLOPE COMPENSATION
R
RAMP
OSC
GEN
F/F
S
Q
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.
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 ).
SENSE AMP
CURRENT
OVERCURRENT COMPARATOR
UNDERVOLTAGE
LOCKOUT
MAX730A
MAX738A
MAX744A
R
SENSE
V UVLO
V
V+
IN
= +6.0V TO +16.0V
Other Components
LX
D1
1N5817
1 F
C2
L1
C3
*
C4
V
+5V
OUT
*
=