Liteon Semiconductor Corporation
LSP5502
2A Synchronous Rectified Step Down DC/DC Converter
(3)
where I
OUTMAX
is the maximum output current, K
RIPPLE
is the ripple factor, R
ESR
is the ESR of the output capacitor, f
SW
is the switching frequency, L is the inductor value, and C
OUT
is the output capacitance. In the case of ceramic output
capacitors, R
ESR
is very small and does not contribute to the ripple. Therefore, a lower capacitance value can be
used for ceramic capacitors. In the case of tantalum or electrolytic capacitors, the ripple is dominated by R
ESR
multiplied by the ripple current. In that case, the output capacitor is chosen to have sufficiently low ESR.
For ceramic output capacitors, typically choose a capacitance of about 22µF. For tantalum or electrolytic capacitors,
choose a capacitor with less than 50mΩ ESR.
Optional Schottky Diode
During the transition between high-side switch and low-side switch, the body diode of the low side power MOSFET
conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky diode may be
paralleled between the SW pin and GND pin to improve overall efficiency. Table 2 lists example Schottky diodes and
their Manufacturers.
V
RIPPLE
=
I
OUTMAX
K
RIPPLE
R
ESR
+
V
IN
28
•
f
SW 2
LC
OUT
Stability Compensation
Figure 2. Stability Compensation
The feedback loop of the IC is stabilized by the components at the COMP pin, as shown in Figure 2. The DC loop
gain of the system is determined by the following equation:
(4)
The dominant pole P1 is due to C
COMP
:
G
EA
f
P1
=
2
π
A
VEA
C
COMP
(5)
The second pole P2 is the output pole:
I
OUT
f
P 2
=
2
π
V
OUT
C
OUT
(6)
The first zero Z1 is due to R
COMP
and C
COMP
:
1
f
Z1
=
2
π
R
COMP
C
COMP
(7)
And finally, the third pole is due to R
COMP
and C
COMP2
(if C
COMP2
is used):
1
f
P 3
=
2
π
R
COMP
C
COMP 2
(8)
The following steps should be used to compensate the IC:
STEP1. Set the crossover frequency at 1/10 of the switching frequency via RCOMP:
C
COMP2
is needed only for high ESR output capacitor
5/11
Rev1.1
LITEON [ LITE-ON TECHNOLOGY CORPORATION ]
