LT1432CS8 datasheet(6/28 Pages) LINER

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LT1432

APPLICATI

I FOR ATIO

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

200

µA 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 2000

µmhos 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.

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.

SWITCH

OUTPUT CURRENT IN

DEVICE

CURRENT

FREQUENCY

BUCK CONVERTER

LT1270A

10A

60kHz

7.5A

LT1270

60kHz

LT1170

100kHz

3.75A

LT1070

40kHz

3.75A

LT1271

60kHz

LT1171

2.5A

100kHz

1.8A

LT1071

2.5A

40kHz

1.8A

LT1172

1.25A

100kHz

0.9A

LT1072

1.25A

40kHz

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.

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