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LT3508HFE-PBF 数据表(PDF) 14 Page - Linear Technology

部件名 LT3508HFE-PBF
功能描述  Dual Monolithic 1.4A Step-Down Switching Regulator
Download  24 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LT3508HFE-PBF 数据表(HTML) 14 Page - Linear Technology

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LT3508
14
3508fb
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
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 VIN is ramping very slowly.
Frequency Compensation
The LT3508 uses current mode control to regulate the
output. This simplifies 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 VCpin,asshowninFigure1.Generallyacapacitor
(CC) and a resistor (RC) in series to ground are used. In
addition, there may be a lower value capacitor in parallel.
This capacitor (CF) is not part of the loop compensation
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.
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 amplifier 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 VC pin. Note that
the output capacitor integrates this current, and that the
capacitor on the VC pin (CC) integrates the error amplifier
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 RC in series with CC.
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
TA = 25°C
VOUT = 3.3V
TO START
TO RUN
LOAD CURRENT (mA)
1
6
7
10000
3508 G05b
5
4
10
100
1000
9
8
TA = 25°C
VOUT = 5V
TO START
TO RUN
Minimum Input Voltage, VOUT = 3.3V
Minimum Input Voltage, VOUT = 5V


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