LT1425CS datasheet(15/20 Pages) LINER

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LT1425

APPLICATIONS INFORMATION

minimum switch ON time, irrespective of current trip

point. If the duty cycle exhibited by this minimum ON time

is greater than the ratio of secondary winding voltage

(referred-to-primary) divided by input voltage, then peak

current will not be controlled at the nominal value, and will

cycle-by-cycle ratchet up to some higher level. Expressed

mathematically, the requirement to maintain short-circuit

control is:

VF + (ISC)(RSEC)

(VIN)(NSP)

)

(tON)(f) <

where,

tON = Output switch minimum ON time

f = Switching frequency

ISC = Short-circuit output current

VF = Output diode forward voltage at ISC

RSEC = Resistance of transformer secondary

VIN = Input voltage

NSP = Secondary-to-primary turns ratio

(NSEC/NPRI)

Trouble will typically only be encountered in applications

with a relatively high product of input voltage times

secondary-to-primary turns ratio. Additionally, several

real world effects such as transformer leakage inductance,

AC winding losses and output switch voltage drop com-

bine to make this simple theoretical calculation a conser-

vative estimate. In cases where short-circuit protection is

mandatory and this theoretical calculation indicates cause

for concern, the prototype should be observed directly as

follows: short the output while observing the VSW signal

with an oscilloscope. The measured output switch ON

time can then be compared against the specifications for

minimum tON.

THERMAL CONSIDERATIONS

Care should be taken to ensure that the worst-case input

voltage and load current conditions do not cause exces-

sive die temperatures. The narrow 16-pin package is rated

at 75

°C/W.

)) f(VOUT)

LSEC

)

IOUT(MIN) =

where,

f = Switching frequency (nominally 285kHz)

LSEC = Transformer secondary side inductance

VOUT = Output voltage

tED = Enable delay time

tEN = Minimum enable time

(tED + tEN)2

Note that generally, depending on the particulars of input

and output voltages and transformer inductance, one of

the above constraints will prove more restrictive. In other

words, the minimum load current in a particular applica-

tion will be either “output switch minimum ON time”

constrained, or “minimum flyback pulse time” constrained.

(A final note—LPRI and LSEC refer to transformer induc-

tance as seen from the primary or secondary side respec-

tively. This general treatment allows these expressions to

be used when the transformer turns ratio is nonunity.)

MAXIMUM LOAD/SHORT-CIRCUIT CONSIDERATIONS

The LT1425 is a current mode controller. It uses the VC

node voltage as an input to a current comparator which

turns off the output switch on a cycle-by-cycle basis as

this peak current is reached. The internal clamp on the VC

node, nominally 1.9V, then acts as an output switch peak

current limit. This action becomes the switch current limit

specification. The maximum available output power is

then determined by the switch current limit, which is

somewhat duty cycle dependent due to internal slope

compensation action.

Short-circuit conditions are handled by the same mecha-

nism. The output switch turns on, peak current is quickly

reached and the switch is turned off. Because the output

switch is only on for a small fraction of the available period,

internal power dissipation is controlled. (The LT1425

contains an internal overtemperature shutdown circuit,

that disables switch action, just in case.)

While the majority of users will not experience a problem,

there is however, a possibility of loss of current limit under

certain conditions. Remember that the LT1425 exhibits a

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