Datasheet

LT3507
17
3507fb
For more information www.linear.com/LT3507
current and that the capacitor on the V
C
pin (C
C
) integrates
the error amplifier output current, resulting in two poles
in the loop. In most cases, a zero is required and comes
either from the output capacitor ESR or from a resistor
in series with C
C
. This model works well as long as the
inductor current ripple is not too low (ΔI
RIPPLE
> 5% I
OUT
)
and the loop crossover frequency is less than f
SW
/5. A
phase lead capacitor (C
PL
) across the feedback divider
may improve the transient response.
SHUTDOWN
The RUN pins are used to place the individual switching
regulators and the internal bias circuits in shutdown mode.
When all three RUN pins are pulled low, the LT3507 is in
shutdown mode and draws less than 1µA from the input
supply. When any RUN pin is pulled high (>1.25V) the
internal reference, the LDO and selected channel are all
turned on.
The RUN pins draw a small amount of current to power
the reference. The current is less than 3µA at 1.8V, so the
RUN pin can be driven directly from 1.8V logic. The RUN
pins are rated up to 36V and can be connected directly to
the input voltage.
A RUN pin cannot be pulled up by logic powered by its
own output, i.e., RUN1 can’t be pulled up by logic powered
by OUT1.
POWER GOOD INDICATORS
The PGOOD pin is the open-collector output of an internal
comparator. PGOOD remains low until the FB pin is within
10% of the final regulation voltage. Tie the PGOOD to any
supply with a pull-up resistor that will supply less than
200µA. Note that this pin will be open when the LT3507
is in shutdown mode (all three RUN pins at ground) re
-
gardless of the voltage at the FB pin. PGOOD is valid when
the LT3507 is enabled (any RUN pin is high) and V
IN
is
greater than ~3.5V.
OUTPUT SEQUENCING
The LT3507 outputs can be sequenced in several ways.
The circuits in Figure 7 show some examples of these. In
each case channel 1 starts first, followed by channel 2,
then channel 3. The sequence shown is not a requirement;
the LT3507 can sequence the channels in any order. Note
that these circuits sequence the outputs during start-up.
When shut down the three channels turn off simultaneously.
applications inForMation
Figure 6. Loop Response Model
–
+
V
FB
800mV
V
SW
V
C
LT3507
GND
3507 F06
R1
OUTPUT
ESR
C
F
C
C
R
C
500k
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
mp
330µS
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
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