ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 28

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The fl yback voltage is scaled by an external resistive
divider R1/R2 and presented at the FB pin. The feedback
amplifi er compares the voltage to the internal bandgap
reference. The feedback amp is actually a transconductance
amplifi er whose output is connected to V
a period in the fl yback time. An external capacitor on
the V
provide the control voltage to set the current mode trip
LTC4268-1
28
CMP
pin integrates the net feedback amp current to
R1
R2
16
FB
–
MOSFET DRAIN
PRIMARY SIDE
PG VOLTAGE
SG VOLTAGE
1V
+
VOLTAGE
ENABLE
COLLAPSE
DETECT
LTC4268-1 FEEDBACK AMP
1.237V
Figure 12a. LTC4268-1 Switching Regulator Feedback Amplifi er
V
Figure 12b. LTC4268-1 Switching Regulator Timing Diagram
FB
V
IN
–
+
R
S
Q
CMP
only during
t
ON(MIN)
V
V
FLBK
CMP
ENABLE
DELAY
17
point. The regulation voltage at the FB pin is nearly equal
to the bandgap reference V
the overall loop. The relationship between V
is expressed as:
42681 F12a
V
FLBK
AMPLIFIER
C
FEEDBACK
ENABLED
MIN ENABLE
VC
V
=
FLBK
MP
R
PRIMARY
FLYBACK
+ 1 2
R
V
IN
2
R
•
•
T1
•
PG DELAY
0.8 • V
V
•
SECONDARY
FB
FLBK
MS
FB
42681 F12b
because of the high gain in
+
C
OUT
ISOLATED
OUTPUT
FLBK
and V
42681fa
FB