LT3508
14
3508fd
The minimum operating voltage of an LT3508 applica-
tion is limited by the undervoltage lockout (≈3.7V) and
by the maximum duty cycle. The boost circuit also limits
the minimum input voltage for proper start-up. If the
input voltage ramps slowly, or the LT3508 turns on when
the output is already in regulation, the boost capacitor
may not be fully charged. Because the boost capacitor
charges with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load current generally
goes to zero once the circuit has started. Figure 5 shows
a plot of minimum load to start and to run as a function
of input voltage. Even without an output load current, in
many cases the discharged output capacitor will present
a load to the switcher that will allow it to start. The plots
show the worst case, where V
IN
is ramping very slowly.
Frequency Compensation
The LT3508 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT3508 does not require 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.
Frequency compensation is provided by the components
tied to the V
C
pin, as shown in Figure 1. Generally a capaci-
tor (C
C
) and a resistor (R
C
) in series to ground are used. In
addition, there may be a lower value capacitor in parallel.
This capacitor (C
F
) is not part of the loop compensation
but is used to fi lter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
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 in particular 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 com-
pensation network to optimize the performance. Stability
should then be checked 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.
Figure 6 shows an equivalent circuit for the LT3508 control
loop. The error amplifi er is a transconductance amplifi er
with fi nite output impedance. The power section, consisting
of the modulator, power switch and inductor, is modeled
as a transconductance amplifi er generating an output
current proportional to the voltage at the V
C
pin. Note that
the output capacitor integrates this current, and that the
capacitor on the V
C
pin (C
C
) integrates the error amplifi er
output current, resulting in two poles in the loop. In most
cases a zero is required and comes from either the output
capacitor ESR or from a resistor R
C
in series with C
C
.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
APPLICATIONS INFORMATION
Figure 5. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
LOAD CURRENT (mA)
1
5.0
5.5
6.5
1000
3508 F05a
4.5
4.0
10 100 10000
3.5
3.0
6.0
INPUT VOLTAGE (V)
T
A
= 25°C
V
OUT
= 3.3V
TO START
TO RUN
LOAD CURRENT (mA)
1
INPUT VOLTAGE (V)
6
7
10000
3508 G05b
5
4
10
100
1000
9
8
T
A
= 25°C
V
OUT
= 5V
TO START
TO RUN
Minimum Input Voltage, V
OUT
= 3.3V
Minimum Input Voltage, V
OUT
= 5V
