LT1737IGN#TRPBF Linear Technology, LT1737IGN#TRPBF Datasheet - Page 17

• Manufacturer Part Number: LT1737IGN#TRPBF
• Description: IC CNTRLR ISOLATD FLYBACK 16SSOP
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT1737IGN#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Voltage - Input: 4.1 ~ 20 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

APPLICATIO S I FOR ATIO
FREQUENCY COMPENSATION
Loop frequency compensation is performed by connect-
ing a capacitor from the output of the error amplifier (V
pin) to ground. An additional series resistor, often re-
quired in traditional current mode switcher controllers, is
usually not required and can even prove detrimental. The
phase margin improvement traditionally offered by this
extra resistor will usually be already accomplished by the
nonzero secondary circuit impedance, which adds a “zero”
to the loop response.
In further contrast to traditional current mode switchers,
V
dynamic nature of the clamped feedback amplifier forms
an effective track/hold type response, whereby the V
voltage changes during the flyback pulse, but is then “held”
during the subsequent “switch on” portion of the next
cycle. This action naturally holds the V
during the current comparator sense action (current mode
switching).
OUTPUT VOLTAGE ERROR SOURCES
Conventional nonisolated switching power supply ICs
typically have only two substantial sources of output
voltage error: the internal or external resistor divider
network that connects to V
ence. The LT1737, which senses the output voltage in both
a dynamic and an isolated manner, exhibits additional
potential error sources to contend with. Some of these
errors are proportional to output voltage, others are fixed
in an absolute millivolt sense. Here is a list of possible
error sources and their effective contribution.
C
pin ripple is generally not an issue with the LT1737. The
(6a) “Standard” UVLO
Divider Topology
V
IN
U
R1
R2
UVLO
U
OUT
and the internal IC refer-
W
C
voltage stable
(6b) Filter Capacitor
Directly on UVLO Note
U
C1
100pF
V
IN
Figure 6
R1
R2
C
C
UVLO
Internal Voltage Reference
The internal bandgap voltage reference is, of course,
imperfect. Its error, both at 25°C and over temperature is
already included in the specifications.
User Programming Resistors
Output voltage is controlled by the user-supplied feedback
resistor divider ratio. To the extent that the resistor ratio
differs from the ideal value, the output voltage will be
proportionally affected. Highest accuracy systems will
demand 1% components.
Schottky Diode Drop
The LT1737 senses the output voltage from the trans-
former primary side during the flyback portion of the
cycle. This sensed voltage therefore includes the forward
drop, V
nominal V
feedback resistor divider calculations. Lot to lot and
ambient temperature variations will show up as output
voltage shift/drift.
Secondary Leakage Inductance
Leakage inductance on the transformer secondary re-
duces the effective secondary-to-third winding turns ratio
(N
voltage target by a similar percentage. To the extent that
secondary leakage inductance is constant from part to
part, this can be accommodated by adjusting the feedback
resistor ratio.
S
/N
T
) from its ideal value. This will increase the output
F
, of the rectifier (usually a Schottky diode). The
F
of this diode should therefore be included in
(6c) Recommended Topology to
Filter High Frequency Ripple
C2
C1
100pF
V
IN
R1/2
R1/2
R2
UVLO
1737 F06
LT1737
17
1737fa