5 V, S t e p -Do w n ,
Cu rre n t -Mo d e P WM DC-DC Co n ve rt e rs
V = +6.0V TO +16.0V
IN
C2
1µF
SHDN
V+
C3*
OVERCURRENT COMPARATOR
SLOPE COMPENSATION
BIAS
GEN
RAMP
GEN
OUT
CC
Σ
R
SENSE
C5
330pF
CURRENT
SENSE AMP
ERROR AMP
F/F
R
Q
V
+5V
=
OUT
L1
S
LX
1.23V
BANDGAP
R1
510k
PWM
COMPARATOR
D1
1N5817
C4*
REF
OSC
MAX730A
MAX738A
MAX744A
C6
0.01µF
1M
±35%
SS CLAMP
SS
UNDERVOLTAGE
LOCKOUT
C1
0.1µF
V
UVLO
GND
*SEE TABLE 2 FOR COMPONENT VALUES AND SUPPLIERS
038/MX74A
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.
Ca p a c itor ESR ris e s a s the te mp e ra ture fa lls , a nd
excessive ESR is the most likely cause of trouble at
te mp e ra ture s b e low 0°C. Sa nyo OS-CON s e rie s
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.
Ou t p u t Filt e r Ca p a c it o r S e le c t io n
The p rima ry c rite rion for s e le c ting the outp ut filte r
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
s ta b ility. The c a p a c itor’s ESR s hould b e le s s tha n
0.25Ω to keep the output ripple less than 50mVp-p over
the e ntire c urre nt ra ng e (us ing a 100µH ind uc tor).
Ot h e r Co m p o n e n t s
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
g ood c hoic e . The 330p F oute r-loop c omp e ns a tion
capacitor provides the widest input voltage range and
best transient characteristics. For low-current applica-
tions , the 510kΩ re s is tor ma y b e omitte d (s e e the
Maximum Output Current vs. Supply Voltage graph (R1
removed) in the Typical Operating Characteristics).
10 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
