Datasheet

LT1946
4
1946fb
Σ
–
+
–
+
A2
FBSHDN
SHUTDOWN
V
IN
0.5V
DRIVER
Q1
0.01Ω
120k
90pF
SW
GND
COMPARATOR
5
V
C
4µA
1
SS
8
COMP
4
S
RQ
RAMP
GENERATOR
1.2MHz
OSCILLATOR
÷3
–
+
–
+
23
1.250V
REFERENCE
6
7
R1 (EXTERNAL)
FB
V
OUT
R2 (EXTERNAL)
A3
A1
GND
(MSE8 ONLY)
9
1946 BD
UU
U
PI FU CTIO S
V
C
(Pin 1): Error Amplifier Output Pin. Tie external com-
pensation network to this pin, or use the internal compen-
sation network by shorting the V
C
pin to the COMP pin.
FB (Pin 2): Feedback Pin. Reference voltage is 1.250V.
Connect resistive divider tap here. Minimize trace area at
FB. Set V
OUT
according to V
OUT
= 1.250(1 + R1/R2).
SHDN (Pin 3): Shutdown Pin. Tie to 2.4V or more to enable
device. Ground to shut down. Do not float this pin.
GND (Pin 4): Ground. Tie directly to local ground plane.
SW (Pin 5): Switch Pin. This is the collector of the internal
NPN power switch. Minimize the metal trace area con-
nected to this pin to minimize EMI.
V
IN
(Pin 6): Input Supply Pin. Must be locally bypassed.
COMP (Pin 7): Internal Compensation Pin. Provides an
internal compensation network. Tie directly to the V
C
pin
for internal compensation. Tie to GND if not used.
SS (Pin 8): Soft-Start Pin. Place a soft-start capacitor
here. Upon start-up, 4µA of current charges the capacitor
to 1.5V. Use a larger capacitor for slower start-up. Leave
floating if not in use.
Exposed Pad (MS8E, Pin 9): Ground. Must be soldered to
PCB.
BLOCK DIAGRA
W
Figure 2. Block Diagram