Datasheet
LT1945
4
1945fa
PIN FUNCTIONS
NFB1 (Pin 1): Feedback Pin for Switcher 1. Set the output
voltage by selecting values for R1 and R2.
SHDN1 (Pin 2): Shutdown Pin for Switcher 1. Tie this
pin to 0.9V or higher to enable device. Tie below 0.25V
to turn it off.
GND (Pin 3): Ground. Tie this pin directly to the local
ground plane.
SHDN2 (Pin 4): Shutdown Pin for Switcher 2. Tie this
pin to 0.9V or higher to enable device. Tie below 0.25V
to turn it off.
FB2 (Pin 5): Feedback Pin for Switcher 2. Set the output
voltage by selecting values for R1B and R2B.
SW2 (Pin 6): Switch Pin for Switcher 2. This is the
collector of the internal NPN power switch. Minimize the
metal trace area connected to the pin to minimize EMI.
PGND (Pins 7, 9): Power Ground. Tie these pins directly
to the local ground plane. Both pins must be tied.
V
IN
(Pin 8): Input Supply Pin. Bypass this pin with a
capacitor as close to the device as possible.
SW1 (Pin 10): Switch Pin for Switcher 1. This is the
collector of the internal NPN power switch. Minimize the
metal trace area connected to the pin to minimize EMI.
BLOCK DIAGRAM
–
+
–
+
8
400ns
ONE-SHOT
DRIVER
RESET RESET
ENABLE
42mV
0.12Ω
A2
A1
Q3
9
3
R4
280k
R3
60k
R6
80k
R5
80k
Q2
X10
Q1
1
V
IN
NFB1
2
SHDN1
10
SW1
PGND
GND
1945 BD
L1
C3
C2
V
OUT1
V
IN
D1
R2
(EXTERNAL)
R1
(EXTERNAL)
V
OUT1
C1
–
+
–
+
400ns
ONE-SHOT
ENABLE
42mV
0.12Ω
A2B
A1B
Q3B
7
R4B
140k
R3B
30k
R6B
40k
R5B
40k
Q2B
X10
Q1B
5
V
IN
FB2
4
SHDN2
6
SW2
PGND
L3L2
C4
V
OUT2
V
IN
D2
R2B
(EXTERNAL)
R1B
(EXTERNAL)
V
OUT2
SWITCHER 1 SWITCHER 2
DRIVER
Figure 1. LT1945 Block Diagram
OPERATION
The LT1945 uses a constant off-time control scheme
to provide high effi ciencies over a wide range of output
current. Operation can be best understood by referring
to the block diagram in Figure 1. Q1 and Q2 along with
R3 and R4 form a bandgap reference used to regulate
the output voltage. When the voltage at the NFB1 pin is
slightly below –1.23V, comparator A1 disables most of
the internal circuitry. Output current is then provided by
capacitor C2, which slowly discharges until the voltage
at the NFB1 pin goes above the hysteresis point of A1
(typical hysteresis at the NFB1 pin is 8mV). A1 then enables
the internal circuitry, turns on power switch Q3, and the