Datasheet

LT1950
8
1950fa
The LT1950 is a constant frequency, current mode con-
troller for DC/DC forward, boost, flyback and SEPIC con-
verter applications. The Block Diagram in Figure 1 shows
all of the key functions of the IC.
In normal operation, a V
IN
voltage as low as 3V allows an
internal switcher at the BOOST pin to generate a separate
BLOCK DIAGRA
W
OPERATIO
U
Figure 1. LT1950 Block Diagram
–
+
–
+
–
+
–
+
–
+
–
+
V
REF
=
INTERNAL +
EXTERNAL
SUPPLY
V
IN2
2.5V
(SOURCE 2.5mA
EXTERNALLY)
1.23V
(V
IN
) (2.6V)
U/V LOCKOUT
(V
IN2
) (8V)
U/V LOCKOUT
(100-500)khz
(105/V
SEC
)%
OSC
(TYPICAL 200kHz)
7
15 16 14
12
13
11
6
4
3
5
10
81 92
SHDN
SYNC
R
OSC
SLOPE
1.32V
1.23V
MAX DC
CLAMP
SLOPE COMP
RAMP
S
Q
R
BLANKING
FB COMP GND BLANK
V
REF
V
IN
V
SEC
BOOST
V
IN2
SWITCHING PREREGULATOR
FIXED OFF TIME
(125mA CURRENT LIMIT)
PGND
DISABLE
–
+
CURRENT
SENSE
CMP
BLANKING
OVERRIDE
CMP
GATE
PGND
I
SENSE
±1A
DRIVER
13V
ENABLE
11V
8V
1950 BD
3µA
–
+
ERROR AMPLIFIER
VOLTAGE GAIN = 85dB
125mV
0mV – >100mV
11V supply at V
IN2
using a small surface mount external
inductor, diode and capacitor. Since V
IN2
supplies the
output driver of the IC, this architecture achieves high
GATE drive for an external N-channel power MOSFET even
though V
IN
voltage is very low. High GATE drive capability
reduces MOSFET R
DS(ON)
for improved efficiency,