LT1737CGN Linear Technology, LT1737CGN Datasheet - Page 11

• Manufacturer Part Number: LT1737CGN
• Description: IC CTRLR ISOLATED FLYBACK 16SSOP
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT1737CGN

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Voltage - Input: 4.1 ~ 20 V
• Operating Temperature: 0°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

OPERATIO
transformer secondary and output capacitor. This has
been represented previously by the expression “I
ESR.” However, it is generally more useful to convert this
expression to an effective output impedance. Because the
secondary 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:
Expressing this in terms of the ON duty cycle, remember-
ing DC
In less critical applications, or if output load current
remains relatively constant, this output impedance error
may be judged acceptable and the external FB resistor
divider adjusted to compensate for nominal expected
error. In more demanding applications, output impedance
error may be minimized by the use of the load compensa-
tion 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
R
the voltage at the FB pin. As output loading increases,
average switch current increases to maintain rough output
voltage regulation. This causes an increase in R
resistor current which effects a corresponding increase in
flyback voltage amplitude.
Assuming a relatively fixed power supply efficiency, Eff,
Average primary side current may be expressed in terms
of output current as follows:
OCMP
R
ESR = lumped secondary impedance
DC
DC = ON duty cycle
Power Out = Eff • Power In
V
R
R
OUT
OUT
OUT
OUT
OFF
OFF
resistor, and the resulting current acts to decrease
• I
= effective supply output impedance
=
=
= OFF duty cycle
= 1 – DC,
OUT
ESR
ESR
= Eff • V
⎛
⎜
⎝
⎛
⎜
⎝
1–
DC
U
1
1
DC
OFF
IN
⎞
⎟
⎠
⎞
⎟
⎠
• I
where
IN
SEC
OCMP
•
combining the efficiency and voltage terms in a single
variable:
Switch current is converted to voltage by the external
sense resistor and averaged/lowpass filtered by R3 and
the external capacitor on R
impressed across the external R
A1 and transistor Q3. This produces a current at the
collector of Q3 which is then mirrored around and then
subtracted from the FB node. This action effectively in-
creases the voltage required at the top of the R1/R2
feedback divider to achieve equilibrium. So the effective
change in V
Nominal output impedance cancellation is obtained by
equating this expression with R
The practical aspects of applying this equation to deter-
mine an appropriate value for the R
in the Applications Information section.
I
K1 = dimensionless variable related to V
R
R
(R1||R2) = impedance of R1 and R2 in parallel
I
K
R
R
∆
IN
∆
IN
∆
SENSE
OUT
1=
OUT
OCMP
V
V
I
=
= K1 • I
OUT
OUT
OUT
⎛
⎜
⎝
⎛
⎜
⎝
efficiency as above
= uncompensated output impedance
=
V
V
=
= external sense resistor
=
IN
IN
V
V
=
K
OUT
OUT
OUT
(
1
K
K
•
•
K
OUT
⎛
⎜
⎝
1
1
Eff
Eff
1
R
R
⎛
⎜
⎝
⎛
⎜
⎝
target is:
•
SENSE
R
R
OCMP
, where
R
⎞
⎟
⎠
⎞
⎟
⎠
∆
R
SENSE
SENSE
OCMP
I
OUT
•
OUT
I
OUT
⎞
⎟
⎠
)
⎞
⎟
⎠
• (
⎞
⎟
⎠
⎛
⎜
⎝
R
• (
• (
R
R
SENSE
OCMP
1
R
R
CMPC
||
1
1
R and
||
||
OUT
OCMP
2
R where
R
⎞
⎟
⎠
)
OCMP
. This voltage is then
2
2
:
)
• (
)
R
resistor by op amp
1
resistor are found
||
R or
IN
2
LT1737
)
, V
OUT
11
and
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