LT1432CN8 Ver la hoja de datos (PDF) - Linear Technology
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LT1432CN8 Datasheet PDF : 28 Pages
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LT1432
APPLICATI S I FOR ATIO
Basic Circuit Description
The LT1432 is a dedicated 5V buck converter driver chip
intended to be used with an IC switcher from the LT1070
family. This family of current mode switchers includes
current ratings from 1.25A to 10A, and switching frequen-
cies from 40kHz to 100kHz as shown in the table below.
DEVICE
LT1270A
LT1270
LT1170
LT1070
LT1271
LT1171
LT1071
LT1172
LT1072
SWITCH
CURRENT
10A
8A
5A
5A
4A
2.5A
2.5A
1.25A
1.25A
FREQUENCY
60kHz
60kHz
100kHz
40kHz
60kHz
100kHz
40kHz
100kHz
40kHz
OUTPUT CURRENT IN
BUCK CONVERTER
7.5A
6A
3.75A
3.75A
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 LT1070 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 maximum available load current.
See Inductor Selection for details. The LT1070 family was
originally intended for topologies which have the negative
side of the switch grounded, such as boost converters. It
has an extremely 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 main-
tains 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 ground 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 performs
these functions as well as adding current limiting, mi-
cropower shutdown, and dual mode operation for high
conversion efficiency with both heavy and very light loads.
The circuit in Figure 1 is a basic 5V positive buck converter
which can operate with input voltage from 6V 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 main-
tained by the LT1432 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 VC pin then sinks
this current to control the loop. Transconductance from
the regulator output to the VC pin current is controlled to
approximately 2000µmhos by local feedback around the
LT1432 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 flowing in the external PCB traces. This is also
true for the frequency compensation capacitors C4 and
C5. C4 forms the dominant loop pole with a loop zero
added by R1. C5 forms a higher frequency loop pole to
control switching ripple at the VC pin.
A floating 5V power supply for the switcher is generated by
D2 and C3 which peak detect the output voltage during
switch “off” time. The diode used for D2 is a low capaci-
tance type to avoid spikes at the output. Do not substitute
a Schottky diode for D2 (they are high capacitance). This
is a very efficient way of powering the switcher because
power drain does not increase with regulator input volt-
age. However, the circuit is not self-starting, so some
means must be used to start the regulator. This is per-
formed by the internal current path of the LT1432 which
allows current to flow from the input supply to the V+ pin
during startup.
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