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LT1103/LT1105

Bootstrap Start

It is inefficient as well as impractical to power a switching

regulator control IC from the rectified DC input as this

voltage is several hundred volts. Self-biased switching

regulator topologies take advantage of a lower voltage

auxiliary winding on the power transformer or inductor to

power the regulator, but require a start-up cycle to begin

regulation.

Start-up circuitry with hysteresis built into the LT1103/

LT1105 allows the input voltage to increase from 0V to

16V before the regulator tries to start. During this time the

start-up current of the switching regulator is typically

200

µA and all internal voltage regulators are off. The low

quiescent current allows the input voltage to be trickled up

with only 500

µA of current from the rectified DC line

voltage, thereby minimizing power dissipation in the start-

up resistor. At 16V, the internal voltage regulators are

turned on and switching begins. If enough power feeds

back through the auxiliary winding to keep the input

voltage to the switching regulator above 12V, then switching

continues and a bootstrap start is accomplished. If the

input voltage drops below 12V, then the FET drive detection

circuit locks out switching. The input voltage continues to

fall as the VIN bypass capacitor is discharged by the

normal quiescent current of the LT1103/LT1105. Once the

input voltage falls below 7V, the internal voltage regulators

are turned off and the switching regulator returns to the

low start-up current state. A continuous “burp start” mode

indicates a fault condition or an incomplete power loop.

The trickle current required to bootstrap the regulator

input voltage is typically generated with a resistor from the

rectified DC input voltage. When combined with the

regulator input bypass capacitor, the start-up resistor

creates a ramp whose slope governs the turn-on time of

the regulator as well as the period of the “burp start” mode.

The design trade-offs are power dissipated in the trickle

resistor, the turn-on time of the regulator, and the hold-up

time of the regulator input bypass capacitor. The value of

the start-up resistor is set by the minimum rectified DC

input voltage to guarantee sufficient start-up current. The

recommended minimum trickle current is 500

µA. The

power rating of the start-up resistor is set by the maximum

rectified DC input voltage. A final consideration for the

start-up resistor is to insure that the maximum voltage

rating of the resistor is not exceeded. Typical carbon film

resistors have a voltage rating of 250V. The most reliable

and economical solution for the start-up resistor is generally

provided by placing several 0.25W resistors in series.

The LT1103/LT1105 is designed to operate with supply

pin voltages up to 30V. However, the auxiliary bias winding

should be designed for a typical output voltage of 17V to

minimize IC power dissipation and efficiency loss.

Allowances must also be made for cross regulation of the

bias voltage due to variations in the rectified DC line

voltage and output load current.

Soft-Start

Soft-start refers to the controlled increase of switch

current from a start-up or shutdown state. This allows the

power supply to come up to voltage in a controlled

manner and charge the output capacitor without activating

current limit. In general, soft-start is not required on the

LT1105 due to the design of the sampling error amplifier

gm stage which generates asymmetrical slew capability

on the VC pin.

This feature exhibits itself as a typical 3mA sink current

capability on the VC pin whereas source current is only

275

µA. The low gm of the error amplifier allows small-

valued compensation capacitors to be used on VC. This

allows the sink current to slew the compensation capacitor

quickly. Therefore, overshoot of the output voltage on

start-up sequences and recovery from overload or short-

circuit conditions is prevented. However, if a longer start-

up period is required, the soft-start function can be used.

Soft-start is implemented with an internal 40

µA pull-up

and a transistor clamp on the VC pin so that a single

external capacitor from SS ground can define the linear

ramp function. The voltage at VC is limited to one VBE

above the soft-start pin (SS). The time to maximum switch

current is defined as the capacitance on SS multiplied by

the active range in volts of the VC pin divided by the pull-

up current:

C • (3.2V)

40 A

APPLICATIONS INFORMATION

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