LT3507A
16
3507afa
For more information www.linear.com/LT3507A
Frequency Compensation
The LT3507A uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3507A does not depend on the ESR of the output
capacitor for stability so you are free to use ceramic ca
-
pacitors to achieve low output ripple and small circuit size.
The components tied to
the V
C
pin provide frequency
compensation. Generally, a capacitor and a resistor in
series to ground determine loop gain. In addition, there
is a lower value capacitor in parallel. This capacitor filters
noise at the switching frequency and is not part of the
loop compensation.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and the type of output capacitor. A practical approach is to
start with one of the circuits in this data sheet that is similar
to your application and tune the compensation network
to optimize the performance. Check stability across all
operating conditions, including load current, input voltage
and temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load. Application
Note 76 is an excellent source as well.
Figure 6 shows an equivalent circuit for the LT3507A
control loop. The error amp is a transconductance am
-
plifier with finite output impedance. The power section,
consisting of the modulator
, power switch and inductor
is modeled as a transconductance amplifier generating an
output current proportional to the voltage at the V
C
pin.
The gain of the power stage (g
mp
) is 6S for chanel 1 and
4.3S for chanels 2 and 3. Note that the output capacitor
integrates this 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 LT3507A 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 LT3507A
is in shutdown mode (all three RUN pins at ground)
regardless of the voltage at the FB pin. PGOOD is valid
when the LT3507A is enabled (any RUN pin is high) and
V
IN
is greater than ~3.5V.
applications inForMation
Figure 6. Loop Response Model
–
+
V
FB
800mV
V
SW
V
C
LT3507A
GND
3507A 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
