LT3507
16
3507fb
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and is largest when V
IN
=2V
OUT
(50% duty cycle). As
the second, lower power channel draws input current,
the input capacitor’s RMS current actually decreases as
the out-of-phase current cancels the current drawn by the
higher power channel. Considering that the maximum load
current from a single phase (if SW2 and SW3 are both at
maximum current) is ~3A, RMS ripple current will always
be less than 1.5A.
The high frequency of the LT3507 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is often less than 10µF. The combi
-
nation of small size and low impedance (low equivalent
series resistance or ESR) of ceramic capacitors makes
them the preferred choice. The low ESR results in ver
y
low
voltage ripple. Ceramic capacitors can handle larger
magnitudes of ripple current than other capacitor types
of the same value. Use X5R and X7R types.
An alternative to a high value ceramic capacitor is a lower
value along with a larger electrolytic capacitor, for example
a 1µF ceramic capacitor in parallel with a low ESR tantalum
capacitor. For the electrolytic capacitor, a value larger than
10µF will be required to meet the ESR and ripple current
requirements. Because the input capacitor is likely to see
high surge currents when the input source is applied, tan
-
talum capacitors should be surge rated. The manufacturer
may also recommend operation below the rated voltage
of the capacitor. Be sure to place the
1µF ceramic as close
as possible to the V
IN
and GND pins on the IC for optimal
noise immunity.
A final caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plugging
the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT3507. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details, see Application Note 88.
Frequency Compensation
The LT3507 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3507 does not depend on the ESR of the output capacitor
for stability so you are free to use ceramic capacitors 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 LT3507 control
loop. The error amp is a transconductance amplifier 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 5S for Channel 1 and 3.6S for Chan-
nels 2 and 3. Note that the output capacitor integrates this
applications inForMation
