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LTC3114-1 Datenblatt(PDF) 24 Page - Analog Devices

Teilenummer LTC3114-1
Bauteilbeschribung  40V, 1A Synchronous Buck-Boost DC/DC Converter with Programmable Output Current
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LTC3114-1
24
Rev. D
For more information www.analog.com
APPLICATIONS INFORMATION
A simplified diagram of the average output current pro-
gramming circuitry is shown in the Block Diagram. An
internal sense resistor, RS, and low offset amplifier directly
measure current in the VOUT path and produce a small
fraction of this current out of the PROG pin. Accordingly,
a resistor and filtering capacitor connected from PROG to
ground produce a voltage proportional to average output
current on PROG. An internal transconductance amplifier
compares the PROG voltage to the fixed 1V internal refer-
ence. If the PROG voltage tries to exceed the 1V reference
level, this amplifier will pull down on VC and take com-
mand of the PWM. As described earlier, VC is the current
command voltage, so limiting VC in this manner will also
limit output current. The resulting average output current
is given by the following equation:
IOUT(AVG) ≅ 25,000 •
1V
RPROG
where: RPROG = 24.9k to 100k.
The largest recommended PROG pin resistor is 100k. Values
of RPROG larger than 100k may latch-off the LTC3114-1 if
VOUT is forced to less than 2V by an external load. This is
generally not an issue for battery charging applications, but
may prevent the charging of very large capacitors. In some
general purpose power supply applications, this latch-off
behavior may be desirable and in these cases, values of
RPROG > 100k are acceptable to use.
The gain of 25,000 is generated internal to the LTC3114-1
and is factory trimmed to provide the best accuracy
at 500mA of output current. The accuracy of the pro-
grammed output current is best at the high end of the
range as the residual internal current sense amplifier off-
set becomes a smaller percentage of the total current
sense signal amplitude with increasing current. The pro-
vided electrical specifications define the PROG pin current
accuracy over a range of output currents.
Selecting the capacitor, CPROG, to put in parallel with
RPROG is a trade-off between response time, output cur-
rent ripple and interaction with the normal output voltage
control loop. In general, if speed is not a concern as is the
case for most current sourcing applications, then CPROG
should be made at least 3 times higher than the voltage
error amplifier compensation capacitor, CP1, described
in the Compensation section of this data sheet. This will
ensure minimal to no interaction when the transition
occurs between voltage regulation mode and output cur-
rent regulation mode.
In current sourcing applications, the maximum output
compliance voltage of the LTC3114-1 is set by the voltage
error amplifier dividers resistors as it is for standard
voltage regulation applications. For LED drivIng appli-
cations, select the VOUT divider resistors for a clamping
level 1V to 2V higher than the expected forward voltage
drop of the LED string. The average output current cir-
cuitry can also be used to monitor, rather than control the
output current. To do this, select an RPROG value that will
limit the voltage on the PROG pin to 0.8V or less at the
highest output current expected in the application.
Connect a 20k resistor and 33nF capacitor from PROG to
ground if the function is not going to be used to provide a
higher level of protection against inadvertent short-circuit
conditions on VOUT.
Compensation of the Buck-Boost Converter
The LTC3114-1 utilizes average current mode control to
regulate the output voltage. Average current mode control
has two loops that require frequency compensation, the
inner average current loop and the outer voltage loop.
The compensation for the inner average current loop is
fixed within the LTC3114-1 in order to provide the highest
possible bandwidth over the wide operating range of the
LTC3114-1. Therefore, the only control loop that requires
compensation design is the outer voltage loop. As will be
shown, compensation design of the outer loop is similar
to the techniques used in well known peak current mode
control devices.
The LTC3114-1 utilizing average current mode control can
be conceptualized in its simplest form as a voltage-con-
trolled current source (VCCS), driving the output load
formed primarily by RLOAD and COUT, as shown in Figure 6.
The error amplifier output (VC), provides the command
input to the VCCS. The full-scale range of VC is 0.865V
(135mV to 1V). With a full-scale command on VC, the
LTC3114-1 buck-boost converter will generate an average
1.7A of inductor current (typical) from the converter for a


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