LT1945 Ver la hoja de datos (PDF) - Linear Technology
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LT1945 Datasheet PDF : 8 Pages
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LT1945
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.
VIN (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
VIN
C1
8 VIN
L1
SHDN1
2
VOUT1
R1
(EXTERNAL)
R2
(EXTERNAL)
NFB1
1
R5 R6
80k 80k
+
–
Q1 Q2
X10
R3
60k
R4
280k
A1
ENABLE
400ns
ONE-SHOT
RESET
A2
SWITCHER 1
GND
3
C3
L2
D2
L3
VOUT1
VOUT2
VIN
D1
C2
C4
SW1
10
SW2
6
SHDN2
4
VIN
R6B R5B
40k 40k
Q3
DRIVER
Q3B
DRIVER
42mV
0.12Ω
0.12Ω
42mV
PGND PGND
9
7
A1B
ENABLE
400ns
ONE-SHOT
RESET
A2B
SWITCHER 2
+
–
Q2B
X10
R3B
30k
R4B
140k
VOUT2
Q1B
FB2
5
R1B
(EXTERNAL)
R2B
(EXTERNAL)
1945 BD
Figure 1. LT1945 Block Diagram
OPERATION
The LT1945 uses a constant off-time control scheme
to provide high efficiencies 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
4
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
1945fa
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