LT3508
APPLICATIONS INFORMATION
cases the discharged output capacitor will present a load
to the switcher that will allow it to start. The plots show
The minimum operating voltage of an LT3508 application
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
the worst case, where V is ramping very slowly.
IN
Frequency Compensation
The LT3508 uses current mode control to regulate the
output.Thissimplifiesloopcompensation.Inparticular,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
tiedtotheV pin,asshowninFigure1.Generallyacapacitor
C
(C ) and a resistor (R ) in series to ground are used. In
C
C
addition, there may be a lower value capacitor in parallel.
This capacitor (C ) is not part of the loop compensation
F
Minimum Input Voltage, VOUT = 3.3V
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
6.5
T
= 25°C
OUT
A
V
= 3.3V
6.0
5.5
Loop compensation determines the stability and transient
performance.Designingthecompensationnetworkisabit
complicatedandthebestvaluesdependontheapplication
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,
includingloadcurrent, inputvoltageandtemperature. The
LT1375datasheetcontainsamorethoroughdiscussionof
loop compensation and describes how to test the stability
using a transient load.
5.0
4.5
4.0
3.5
3.0
TO START
TO RUN
10
100
10000
1
1000
LOAD CURRENT (mA)
3508 F05a
Minimum Input Voltage, VOUT = 5V
9
8
7
6
5
4
T
V
= 25°C
= 5V
A
OUT
Figure6showsanequivalentcircuitfortheLT3508control
loop. The error amplifier is a transconductance amplifier
withfiniteoutputimpedance.Thepowersection,consisting
of the modulator, power switch and inductor, is modeled
as a transconductance amplifier generating an output
TO START
TO RUN
current proportional to the voltage at the V pin. Note that
C
the output capacitor integrates this current, and that the
capacitor on the V pin (C ) integrates the error amplifier
C
C
output current, resulting in two poles in the loop. In most
1
10
100
1000
10000
cases a zero is required and comes from either the output
LOAD CURRENT (mA)
capacitor ESR or from a resistor R in series with C .
C
C
3508 G05b
This simple model works well as long as the value of the
Figure 5. The Minimum Input Voltage Depends on Output
Voltage, Load Current and Boost Circuit
inductor is not too high and the loop crossover frequency
3508fb
14
Linear [ Linear ]
