107312 View Datasheet(PDF) - Linear Technology

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107312

Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

Linear Technology 

LT1073

APPLICATIO S I FOR ATIO

In Figure 9, the input is negative while the output is

positive. In this configuration, the magnitude of the input

voltage can be higher or lower than the output voltage. A

level shift, provided by the PNP transistor, supplies proper

polarity feedback information to the regulator.

Using the ILIM Pin

The LT1073 switch can be programmed to turn off at a set

switch current, a feature not found on competing devices.

This enables the input to vary over a wide range without

exceeding the maximum switch rating or saturating the

inductor. Consider the case where analysis shows the

LT1073 must operate at an 800mA peak switch current

with a 2V input. If VIN rises to 4V, the peak switch current

will rise to 1.6A, exceeding the maximum switch current

rating. With the proper resistor (see the “Maximum Switch

Current vs RLIM” characteristic) selected, the switch cur-

rent will be limited to 800mA, even if the input voltage

increases. The LT1073 does this by sampling a small

fraction of the switch current and passing this current

through the external resistor. When the voltage on the ILIM

pin drops a VBE below VIN, the oscillator terminates the

cycle. Propagation delay through this loop is about 2µs.

Another situation where the ILIM feature is useful is when

the device goes into continuous mode operation. This

occurs in step-up mode when

IL

ON

SWITCH

OFF

1073 F10

Figure 10. No Current Limit Causes

Large Inductor Current Build-Up

VOUT + VDIODE < 1

VIN – VSW 1– DC

When the input and output voltages satisfy this relation-

ship, inductor current does not go to zero during the

switch OFF time. When the switch turns on again, the

current ramp starts from the nonzero current level in the

inductor just prior to switch turn-on. As shown in

Figure 10, the inductor current increases to a high level

before the comparator turns off the oscillator. This high

current can cause excessive output ripple and requires

oversizing the output capacitor and inductor. With the ILIM

feature, however, the switch current turns off at a pro-

grammed level as shown in Figure 11, keeping output

ripple to a minimum.

Using the Gain Block

The gain block (GB) on the LT1073 can be used as an error

amplifier, low-battery detector or linear post-regulator.

The gain block itself is a very simple PNP input op amp with

an open-collector NPN output. The (–) input of the gain

block is tied internally to the 212mV reference. The (+)

input comes out on the SET pin.

Arrangement of the gain block as a low battery detector is

straightforward. Figure 12 shows hookup. R1 and R2 need

only be low enough in value so that the bias current of the

SET input does not cause large errors. 100kΩ for R2 is

adequate.

Output ripple of the LT1073, normally 150mV at 5VOUT,

can be reduced sigPRnOiGfRiAcMaMnEDtlCyURbREyNTpLIlMaITcing the gain block in

front of the FB input as shown in Figure 13. This effectively

reduces the comILparator hysteresis by the gain of the gain

block. Output riOpNple can be reduced to just a few millivolts

SWITCH

using this techOnFFique. Ripple reduction works 1073F11 with step-

down or inverting modes as well.

Figure 11. Current Limit Keeps Inductor Current Under Control

10

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