LTC1530 Datasheet, PDF (14/24 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller

LTC1530
APPLICATIO S I FOR ATIO
The output capacitor in a buck converter under steady
state conditions sees much less ripple current than the
input capacitor. Peak-to-peak current is equal to inductor
ripple current, usually 10% to 40% of the total load
current. Output capacitor duty places a premium not on
power dissipation but on ESR. During an output load
transient, the output capacitor must supply all of the
additional load current demanded by the load until the
LTC1530 adjusts the inductor current to the new value.
ESR in the output capacitor results in a step in the output
voltage equal to the ESR value multiplied by the change in
load current. An 11A load step with a 0.05Ω ESR output
capacitor results in a 550mV output voltage shift; this is
19.6% of the output voltage for a 2.8V supply! Because of
the strong relationship between output capacitor ESR and
output load transient response, choose the output capaci-
tor for ESR, not for capacitance value. A capacitor with
suitable ESR will usually have a larger capacitance value
than is needed to control steady-state output ripple.
Electrolytic capacitors rated for use in switching power
supplies with specified ripple current ratings and ESR can
be used effectively in LTC1530 applications. OS-CON
electrolytic capacitors from Sanyo and other manufactur-
ers give excellent performance and have a very high
performance/size ratio for electrolytic capacitors. Surface
mount applications can use either electrolytic or dry
tantalum capacitors. Tantalum 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. AVX TPS series surface mount
devices are popular surge tested tantalum capacitors that
work well in LTC1530 applications.
A common way to lower ESR and raise ripple current
capability is to parallel several capacitors. A typical LTC1530
application might exhibit 5A input ripple current. 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, AVX
TPSE337M006R0100 (330µF/6V) capacitors have a rated
maximum ESR of 0.1Ω; seven in parallel lower the net
output capacitor ESR to 0.014Ω. For low cost applica-
tions, the Sanyo MV-GX capacitor series can be used with
acceptable performance.
Feedback Loop Compensation
The LTC1530 voltage feedback loop is compensated at the
COMP pin, which is the output node of the g_m error
amplifier. The feedback loop is generally compensated
with an RC + C network from COMP to GND as shown in
Figure 8a.
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 C_OUT
The ESR of the output capacitor and the output capacitor
value form a zero at the frequency:
( )( )( ) ^{fESR =}
2π
1
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)(^RC)(^CC)
and
f_P
=
1
(^2π)(^RC)(^C1)
respectively. Figure 8b shows the Bode plot of the overall
transfer function.
The compensation values used in this design are based on
the following criteria, f_SW = 12f_CO, f_Z = f_LC, f_P = 5f_CO. At the
closed-loop frequency f_CO, the attenuation due to the LC
filter and the input resistor divider is compensated by the
gain of the PWM modulator and the gain of the error
amplifier (g_mERR)(R_C).
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