LT1424-9 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1424-9' PDF : 16 Pages View PDF
LT1424-9
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 LT1424-9 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 external
ROCOMP resistor and the resulting current is then sub-
10
tracted from the RFB node. As output loading increases,
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 corresponding
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
external 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
Expressing the product of RSENSE and G as the data sheet
value of ∆VRCCOMP/∆ISW,
) ) ROUT
=
K1
∆VRCCOMP
∆ISW
RFB
ROCOMP
and,
) ) ROCOMP
=
K1
∆VRCCOMP
∆ISW
RFB
ROUT
where,
K1 = Dimensionless variable related to VIN, VOUT and
efficiency as above
sn14249 14249fs
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