LT1432-3.3 Просмотр технического описания (PDF) - Linear Technology
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LT1432-3.3 Datasheet PDF : 8 Pages
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LT1432-3.3
APPLICATI S I FOR ATIO
More applications information on the LT1432-3.3 is available
in the LT1432 data sheet.
Basic Circuit Description
The LT1432-3.3 is a dedicated 3.3V buck converter driver
chip intended to be used with an IC switcher from the LT1171/
LT1271 family. This family of current mode switchers in-
cludes current ratings from 1.25A to 10A, and switching
frequencies from 40kHz to 100kHz as shown in the table
below.
DEVICE
LT1270A
LT1270
LT1170
LT1070
LT1269
LT1271
LT1171
LT1071
LT1172
LT1072
SWITCH
CURRENT
10A
8A
5A
5A
4A
4A
2.5A
2.5A
1.25A
1.25A
FREQUENCY
60kHz
60kHz
100kHz
40kHz
100kHz
60kHz
100kHz
40kHz
100kHz
40kHz
OUTPUT CURRENT IN
BUCK CONVERTER
7.5A
6A
3.75A
3.75A
3A
3A
1.8A
1.8A
0.9A
0.9A
The maximum load current which can be delivered by these
chips in a buck converter is approximately 75% of their
switch current rating. This is partly due to the fact that buck
converters must operate at very high duty cycles when input
voltage is low. The current mode nature of the LT1271 family
requires an internal reduction of peak current limit at high
duty cycles, so these devices are rated at only 80% of their full
current rating when duty cycle is 80%. A second factor is
inductor ripple current, half of which subtracts from maxi-
mum available load current. The LT1271 family was originally
intended for topologies which have the negative side of the
switch grounded, such as boost converters. It has an ex-
tremely efficient quasi-saturating NPN switch which mimics
the linear resistive nature of a MOSFET but consumes much
less die area. Driver losses are kept to a minimum with a
patented adaptive antisat drive that maintains a forced beta of
40 over a wide range of switch currents. This family is
attractive for high efficiency buck converters because of the
low switch loss, but to operate as a positive buck converter,
the GND pin of the IC must be floated to act as the switch
output node. This requires a floating power supply for the
chip and some means for level shifting the feedback signal.
The LT1432-3.3 performs these functions as well as adding
current limiting, micropower shutdown, and dual mode
operation for high conversion efficiency with both heavy and
very light loads.
The circuit in Figure 1 is a basic 3.3V positive buck
converter which can operate with input voltage from 4.5V
to 30V. The power switch is located between the VSW pin
and GND pin on the LT1271. Its current and duty cycle are
controlled by the voltage on the VC pin with respect to the
GND pin. This voltage ranges from 1V to 2V as switch
current increases from zero to full-scale. Correct output
voltage is maintained by the LT1432-3.3 which has an
internal reference and error amplifier (see Equivalent
Schematic in Figure 2). The amplifier output is level
shifted with an internal open collector NPN to drive the VC
pin of the switcher. The normal resistor divider feedback
to the switcher feedback pin cannot be used because the
feedback pin is referenced to the GND pin, which is
switching up and down. The Feedback pin (FB) is simply
bypassed with a capacitor. This forces the switcher VC pin
to swing high with about 200µA sourcing capability. The
LT1432-3.3 VC pin then sinks this current to control the
loop. Transconductance from the regulator output to the
VC pin current is controlled to approximately 3600µmhos
by local feedback around the LT1432-3.3 error amplifier
(S2 closed in Figure 2). This is done to simplify frequency
compensation of the overall loop. A word of caution about
the FB pin bypass capacitor (C6): this capacitor value is
very non-critical, but the capacitor must be connected
directly to the GND pin or tab of the switcher to avoid
differential spikes created by fast switch currents flow-
ing in the external PCB traces. This is also true for the
frequency compensation capacitor C5. C5 forms the
dominant loop pole.
A floating power supply for the switcher is generated by D2
and C3 which peak detect the input voltage during switch off
time. This is different than the 5V version of the LT1432 which
connects the anode of the diode to the output rather than the
input. The output connection is more efficient because the
floating voltage is a constant 5V (or 3.3V), independent of
input voltage, but in the case of the 3.3V circuit, minimum
required input voltage for starting is several volts higher (see
the Typical Performance Characteristics curves). When the
diode is connected to the input, the suggested type is a
6
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