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LT1424CS8-5

Isolated Flyback Switching Regulator with 5V Output

Linear Technology 

LT1424-5

U

OPERATION

time. Certain parameters of flyback amp behavior will then

be directly affected by the variable enable period. These

include effective transconductance and VC node slew rate.

LOAD COMPENSATION THEORY

The LT1424-5 uses the flyback pulse to obtain information

about the isolated output voltage. A potential error source

is caused by transformer secondary current flow through

the real life nonzero impedances of the output rectifier,

transformer secondary and output capacitor. This has

been represented previously by the expression (ISEC)(ESR).

However, it is generally more useful to convert this expres-

sion to an effective output impedance. Because the sec-

ondary current only flows during the off portion of the duty

cycle, the effective output impedance equals the lumped

secondary impedance times the inverse of the OFF duty

cycle. That is,

) ROUT =

ESR

1

DC OFF

where,

ROUT = Effective supply output impedance

ESR = Lumped secondary impedance

DC OFF = OFF duty cycle

Expressing this in terms of the ON duty cycle, remember-

ing DC OFF = 1 – DC,

) ROUT

=

ESR

1

1

– DC

DC = ON duty cycle

In less critical applications, or if output load current

remains relatively constant, this output impedance error

may be judged acceptable and the external RFB resistor

value adjusted to compensate for nominal expected error.

In more demanding applications, output impedance error

may be minimized by the use of the load compensation

function.

To implement the load compensation function, a voltage

is developed that is proportional to average output switch

current. This voltage is then impressed across the inter-

nal ROCOMP resistor and the resulting current is then

10

subtracted from the RFB node. As output loading in-

creases, average switch current increases to maintain

rough output voltage regulation. This causes an increase

in ROCOMP resistor current subtracted from the RFB node,

through which feedback loop action causes a corre-

sponding increase in target output voltage.

Assuming a relatively fixed power supply efficiency, Eff

Power Out = (Eff)(Power In)

(VOUT)(IOUT) = (Eff)(VIN)(IIN)

Average primary side current may be expressed in terms

of output current as follows:

) IIN =

VOUT

(VIN)(Eff)

IOUT

combining the efficiency and voltage terms in a single

variable,

IIN = K1(IOUT) where,

) K1 = VOUT

(VIN)(Eff)

Switch current is converted to voltage by a sense resistor

and amplified by the current sense amplifier with associ-

ated gain G. This voltage is then impressed across the

internal ROCOMP resistor to form a current that is

subtracted from the RFB node. So the effective change in

VOUT target is:

) ∆VOUT = K1(∆IOUT)

(RSENSE)(G)

ROCOMP

RFB and,

) ∆VOUT

∆IOUT

= K1(RSENSE)(G)

RFB

ROCOMP

Nominal output impedance cancellation is obtained by

equating this expression with ROUT.

For simplicity, the data sheet refers to ∆VREF/∆ISW. This is

given as:

) ∆VREF

∆ISW

= (RSENSE)(G)

RFB

ROCOMP

sn14245 14245fs

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