LTC3832 Datasheet, PDF (16/24 Pages) Linear Technology – High Power Step-Down Synchronous DC/DC Controllers for Low Voltage Operation

LTC3832/LTC3832-1
APPLICATIO S I FOR ATIO
LTC3832 application might exhibit 5A input ripple cur-
rent. Sanyo OS-CON capacitors, part number 10SA220M
(220µF/10V), feature 2.3A allowable ripple current at
85°C; three in parallel at the input (to withstand the input
ripple current) meet the above requirements. Similarly,
Sanyo POSCAP 4TPB470M (470µF/4V) capacitors have a
maximum rated ESR of 0.04Ω; three in parallel lower the
net output capacitor ESR to 0.013Ω.
Feedback Loop Compensation
The LTC3832 voltage feedback loop is compensated at the
COMP pin, which is the output node of the error amplifier.
The feedback loop is generally compensated with an RC +
C network from COMP to GND as shown in Figure 10a.
LTC3832
COMP
10
ERR
R_C
C_C
C1
V_REF
7 SENSE⁺
C2
R2
V_FB
6
R1
SENSE^–
5
3832 F10a
Figure 10a. Compensation Pin Hook-Up
Loop stability is affected by the values of the inductor, the
output capacitor, the output capacitor ESR, the error
amplifier transconductance and the error amplifier com-
pensation network. The inductor and the output capacitor
create a double pole at the frequency:
[ ] ^fLC ^{= 1/ 2π (L}O^)(COUT⁾
The ESR of the output capacitor and the output capacitor
value form a zero at the frequency:
[ ] ^fESR ^{= 1/ 2π(ESR)(C}OUT⁾
The compensation network used with the error amplifier
must provide enough phase margin at the 0dB crossover
frequency for the overall open-loop transfer function. The
zero and pole from the compensation network are:
f_Z = 1/[2π(R_C)(C_C)] and
f_P = 1/[2π(R_C)(C1)] respectively
Figure 10b shows the Bode plot of the overall transfer
function.
When low ESR output capacitors (Sanyo OS-CON) are
used, the ESR zero can be high enough in frequency that
it provides little phase boost at the loop crossover fre-
quency. As a result, the phase margin becomes
inadequate and the load transient is not optimized. To
resolve this problem, a small capacitor can be connected
f_SW = LTC3832 SWITCHING
f_Z
FREQUENCY
f_CO = CLOSED-LOOP CROSSOVER
FREQUENCY
20dB/DECADE
f_LC
f_ESR
f_CO
f_P
FREQUENCY
3832 F10b
Figure 10b. Bode Plot of the LTC3832 Overall Transfer Function
16
f_SW = LTC3832 SWITCHING
FREQUENCY
f_CO = CLOSED-LOOP CROSSOVER
FREQUENCY
f_Z
f_LC
f_ZC2
20dB/DECADE
f_CO
f_P f_PC2
FREQUENCY
f_ESR
3832 F10c
Figure 10c. Bode Plot of the LTC3832 Overall
Transfer Function Using a Low ESR Output Capacitor
sn3832 3832fs