LT1425CS Linear Technology, LT1425CS Datasheet - Page 8

• Manufacturer Part Number: LT1425CS
• Description: IC SW REG ISOLATED FLYBCK 16SOIC
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT1425CS

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 200mA
• Frequency - Switching: 285kHz
• Voltage - Input: 2.8 ~ 20 V
• Operating Temperature: 0°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Power - Output: 6W
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -

OPERATION
LT1425
Within the dashed lines in the Block Diagram can be found
the R
resistors on the user-programmable LT1425. The capaci-
tor connected to the R
The LT1425 operates much the same as traditional current
mode switchers, the major difference being a different
type of error amplifier which derives its feedback informa-
tion from the flyback pulse. Due to space constraints, this
discussion will not reiterate the basics of current mode
switcher/controllers and isolated flyback converters. A
good source of information on these topics is LTC’s
Application Note 19.
ERROR AMPLIFIER—PSEUDO DC THEORY
Please refer to the simplified diagram of the Flyback Error
Amplifier. Operation is as follows: when output switch Q4
turns off, its collector voltage rises above the V
amplitude of this flyback pulse, i.e., the difference between
it and V
The flyback voltage is then converted to a current by the
action of R
through resistor R
This is then compared to the internal bandgap reference by
the differential transistor pair Q2/Q3. The collector current
from Q2 is mirrored around and subtracted from fixed
current source I
integrates this net current to provide the control voltage to
set the current mode trip point.
The relatively high gain in the overall loop will then cause
the voltage at the R
bandgap reference V
voltage setting equation. See Applications Information
section.) The relationship between V
then be expressed as:
8
V
V
I
ESR = Total impedance of secondary circuit
N
SEC
FLBK
F
SP
REF
= D1 forward voltage
= Transformer effective secondary-to-primary
IN
= Transformer secondary current
, R
=
, is given as:
turns ratio
V
FB
FB
OUT
and R
and Q1. Nearly all of this current flows
FXD
+ V
U
REF
REF
at the V
BG
F
OCOMP
N
to form a ground-referred voltage.
CCOMP
+ (I
. (V
SP
resistor to be nearly equal to the
SEC
BG
resistors. They are external
C
is not present in final output
pin is also external.
)(ESR)
pin. An external capacitor
FLBK
and V
IN
rail. The
BG
may
Combination with the previous V
expression for V
programming resistors, transformer turns ratio and diode
forward voltage drop:
Additionally, it includes the effect of nonzero secondary
output impedance. See Load Compensation for details.
The practical aspects of applying this equation for V
found in the Applications Information section.
So far, this has been a pseudo-DC treatment of flyback
error amplifier operation. But the flyback signal is a pulse,
not a DC level. Provision must be made to enable the
flyback amplifier only when the flyback pulse is present.
This is accomplished by the dashed line connections to the
block labeled “ENABLE.” Timing signals are then required
to enable and disable the flyback amplifier.
ERROR AMPLIFIER—DYNAMIC THEORY
There are several timing signals that are required for
proper LT1425 operation. Please refer to the Timing
Diagram.
Minimum Output Switch ON Time
The LT1425 effects output voltage regulation via flyback
pulse action. If the output switch is not turned on at all,
there will be no flyback pulse, and output voltage informa-
tion is no longer available. This would cause irregular loop
response and start-up/latchup problems. The solution
chosen is to require the output switch to be on for an
absolute minimum time per each oscillator cycle. This in
turn establishes a minimum load requirement to maintain
V
V
V
OUT
FLBK
BG
V
= Ratio of Q1 I
R
FLBK
FB
= Internal bandgap reference
= V
= V
BG
=
BG
R
V
REF
R
BG
OUT
R
R
REF
R
FB
REF
FB
, in terms of the internal reference,
or,
C
to I
N
1
E
SP
FLBK
– V
F
expression yields an
– I
SEC
(ESR)
OUT
are