LTC3831-1 Linear Technology, LTC3831-1 Datasheet - Page 16

• Manufacturer Part Number: LTC3831-1
• Description: High Power Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology

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LTC3831-1
APPLICATIO S I FOR ATIO
LTC3831-1 applications. OS-CON electrolytic capacitors
from Sanyo and other manufacturers give excellent per-
formance and have a very high performance/size ratio for
electrolytic capacitors. Surface mount applications can
use either electrolytic or dry tantalum capacitors. Tanta-
lum capacitors must be surge tested and specified for use
in switching power supplies. Low cost, generic tantalums
are known to have very short lives followed by explosive
deaths in switching power supply applications. Other
capacitor series that can be used include Sanyo POSCAPs
and the Panasonic SP line.
A common way to lower ESR and raise ripple current
capability is to parallel several capacitors. A typical
LTC3831-1 application might exhibit 5A input ripple cur-
rent. Sanyo OS-CON capacitors, part number 10SA220M
(220 F/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 (470 F/4V) capacitors have a
maximum rated ESR of 0.04 , three in parallel lower the
net output capacitor ESR to 0.013 .
Feedback Loop Compensation
The LTC3831-1 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 7a.
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:
The ESR of the output capacitor and the output capacitor
value form a zero at the frequency:
The compensation network used with the error amplifier
must provide enough phase margin at the 0dB crossover
16
f
f
LC
ESR
1 2
/
1 2
/
(
( )(
ESR C
L
O
U
)(
C
OUT
OUT
U
)
)
W
U
frequency for the overall open-loop transfer function. The
zero and pole from the compensation network are:
Figure 7b shows the Bode plot of the overall transfer
function.
Although a mathematical approach to frequency compen-
sation can be used, the added complication of input and/or
output filters, unknown capacitor ESR, and gross operat-
ing point changes with input voltage, load current varia-
tions, all suggest a more practical empirical method. This
can be done by injecting a transient current at the load and
using an RC network box to iterate toward the final values,
or by obtaining the optimum loop response using a
network analyzer to find the actual loop poles and zeros.
Figure 7b. Bode Plot of the LTC3831-1 Overall Transfer Function
f
f
Z
P
= 1/[2 (R
= 1/[2 (R
R
C
C
Figure 7a. Compensation Pin Connections
f
Z
C
f
LC
COMP
C
C
10
)(C
)(C1)] respectively.
C1
C
)] and
f
ESR
LTC3831-1
f
f
SW
CO
ERR
= CLOSED-LOOP CROSSOVER
= LTC3831 SWITCHING
f
FREQUENCY
FREQUENCY
CO
–
+
20dB/DECADE
V
REF
38311 F07a
f
P
V
6
FB
FREQUENCY
38311 F07b
V
(V
TT
OUT
)
38311f