Datasheet

LT3509
17
3509fd
For more information www.linear.com/LT3509
and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes. Because
loop stability and transient response depend on the value
of C
OUT
, this loss may be unacceptable. Use X7R and X5R
types.
The value of the output capacitor greatly affects the
transient response to a load step. It has to supply extra
current demand or absorb excess current delivery until
the feedback loop can respond. The loop response is
dependent on the error amplifier transconductance, the
internal compensation capacitor and the feedback net
-
work. Higher output voltages necessarily require a larger
feedback divider ratio. This will also reduce the loop gain
and
slow
the response time. Fortunately this effect can be
mitigated by use of a feed-forward capacitor, C
PL
, across
the top feedback resistor. The small-signal model shown
in Figure 10 can be used to model this in a simulator or
to give insight to an empirical design. Figure 11 shows
some load step responses with differing output capacitors
and C
PL
combinations.
Input Capacitor
The input capacitor needs to supply the pulses of charge
demanded during the on time of the switches. Little total
capacitance is required as a few hundred millivolts of ripple
at the V
IN
pin will not cause any problems to the device.
When operating at 2MHz and 12V, 2µF will work well. At
the lowest operating frequency and/or at low input voltages
a larger capacitor such as 4.7µF is preferred.
applicaTions inForMaTion
I
LOAD
700mA
300mA
V
OUT
(AC)
50mV/DIV
I
LOAD
700mA
300mA
V
OUT
(AC)
50mV/DIV
TIME 20µs/DIV TIME 20µs/DIV
3509 F11
C
OUT
= 10µF
C
PL
= 0
C
OUT
= 10µF
C
PL
= 82pF
Figure 11. Transient Load Response with Different Combinations
of C
OUT
and C
PL
Load Current Step from 300mA to 700mA
R1 = 10k, R2 = 32.4k, V
IN
= 12V, V
OUT
= 3.3V, f
SW
= 2.0MHz
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