Datasheet

LT3799
8
3799fa
block DiagraM
MASTER
LATCH
CURRENT
COMPARATOR
V
OUT
+
V
OUT
–
V
IN
INTV
CC
D1
R8
C7
L1A
FAULT
DETECTION
LOW OUTPUT
CURRENT
OSCILLATOR
R
Q
S
S
+
–
A1
+
–
A2
A4
+
–
GATE
V
IN
V
IN_SENSE
L1C
D2
DCM
600mV
MULTIPLIER
R4
R5
S&H
C4
C6
FB
SENSE
GND
3799 BD
DRIVER
+
–
A8
R6
R7
C5
R2
R1
CT
COMP
+
FAULT
COMP
–
CTRL1
CTRL2
V
REF
C1
+
–
A5
1M
+
+
SW1
T1
N:1
L1B
C3 C2
R3
R10
A6
A7
ONE
SHOT
A3
1.22V
6
5
7
8
1
2
CTRL3
3
4
17
15
14
13
1116109
M1
•
operaTion
The LT3799 is a current mode switching controller IC
designed specifically for generating an average current
output in an isolated flyback topology. The special problem
normally encountered in such circuits is that information
relating to the output voltage and current on the isolated
secondary side of the transformer must be communicated
to the primary side in order to maintain regulation. Histori-
cally, this has been done with an opto-isolator. The LT3799
uses a novel method of using the external MOSFETs peak
current information from the sense resistor to calculate
the output current of a flyback converter without the need
of an opto-coupler. In addition, it also detects open LED
conditions by examining the third winding voltage when
the main power switch is off.
Power factor has become an important specification for
lighting. A power factor of one is achieved if the current
drawn is proportional to the input voltage. The LT3799
modulates the peak current limit with a scaled version of
the input voltage. This technique provides power factors
of 0.97 or greater.
The Block Diagram shows an overall view of the system.
The external components are in a flyback topology con-
figuration. The third winding senses the output voltage
and also supplies power to the part in steady-state opera-
tion. The V
IN
pin supplies power to an internal LDO that
generates 10V at the INTV
CC
pin. The novel control circuitry
consists of an error amplifier, a multiplier, a transmission