LT1725IGN datasheet(17/28 Pages) LINER

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LT1725

1725f

VIN

(6a) “Standard” UVLO

Divider Topology

UVLO

VIN

(6b) Filter Capacitor

Directly On UVLO Node

UVLO

100pF

VIN

(6c) Recommended Topology to

Filter High Frequency Ripple

UVLO

R1/2

1725 F06

100pF

APPLICATIO S I FOR ATIO

Figure 6

speed supply ripple, while leaving the UVLO pin node

impedance relatively unchanged at high frequency.

INTERNAL WIDE HYSTERESIS

UNDERVOLTAGE LOCKOUT

The LT1725 is designed to implement isolated DC/DC

converters operating from input voltages of typically 48V

or more. The standard operating topology utilizes a third

transformer winding on the primary side that provides

both feedback information and local power for the LT1725

via its VCC pin. However, this arrangement is not inherently

self-starting. Start-up is effected by the use of an external

“trickle-charge” resistor and the presence of an internal

wide hysteresis undervoltage lockout circuit that monitors

VCC pin voltage (see Figure 7). Operation is as follows:

“Trickle charge” resistor R1 is connected to VIN and

supplies a small current, typically on the order of a single

mA, to charge C1. At first, the LT1725 is off and draws only

its start-up current. After some time, the voltage on C1

(VCC) reaches the VCC turn-on threshold. The LT1725 then

turns on abruptly and draws its normal supply current.

Switching action commences at the GATE pin and the

MOSFET begins to deliver power. The voltage on C1

begins to decline as the LT1725 draws its normal supply

current, which greatly exceeds that delivered by R1. After

some time, typically tens of milliseconds, the output

voltage approaches its desired value. By this time, the

third transformer winding is providing virtually all the

supply current required by the LT1725.

One potential design pitfall is undersizing the value of

capacitor C1. In this case, the normal supply current

IVCC

1725 F07

VIN

IVCC

VVCC

VON THRESHOLD

VGATE

VCC

LT1725

GATE

PGND

SGND

Figure 7

drawn by the LT1725 will discharge C1 too rapidly; before

the third winding drive becomes effective, the VCC turn-off

threshold will be reached. The LT1725 turns off, and the

VCC node begins to charge via R1 back up to the turn-on

threshold. Depending upon the particular situation, this

may result in either several on-off cycles before proper

operation is reached, or, permanent relaxation oscillation

at the VCC node.

Component selection is as follows:

Resistor R1 should be selected to yield a worst-case

minimum charging current greater than the maximum

rated LT1725 start-up current, and a worst-case maxi-

mum charging current less than the minimum rated

LT1725 supply current.

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