LT1948
PI FU CTIO S
FB1 (Pin 1):
Feedback Pin for First Switcher. Connect
resistor divider tap here. Set AV
DD
according to AV
DD
=
1.26V(1 + R1/R2).
C
T
(Pin 2):
Timing Capacitor Pin. Connect a 22nF capacitor
from C
T
to ground to program a 3ms delay from FB1
reaching 1.26V to V
ON
turning on.
SW1 (Pin 3):
AV
DD
Switch Node. Connect inductor and D1
here (see Figure 1). Minimize trace area at this pin to keep
EMI down.
GND (Pin 4):
Ground. Connect directly to local ground
plane.
V
IN
(Pin 5):
Input Supply Pin. Must be bypassed with a
ceramic capacitor close to the pin.
SW2 (Pin 6):
VO2 Switch Node. Connect inductor and D2
here. Minimize trace area at this pin to keep EMI down.
VO2 (Pin 7):
Sense Pin for 23V Output. Connect to VO2
output capacitor. This node is also internally connected to
the emitter of Q3 (see Block Diagram), the high side switch
between VO2 and V
ON
.
V
ON
(Pin 8):
This is the Delayed 23V Output. V
ON
becomes
23V after the internal timer times out.
OPERATIO
To best understand operation of the LT1948, please refer
to the LT1948 Block Diagram. The device contains two
switching regulators, a timer and a high side switch. Three
outputs can be generated: an adjustable AV
DD
output, a
charge-pumped inversion of the AV
DD
output, called V
OFF
,
and a 23V/15mA output, called V
ON
. Q3 keeps V
ON
off for
an externally set time interval, set by a capacitor connected
to the C
T
pin.
The switching frequency of both switchers is 3MHz, set
internally. The switchers are current mode and are inter-
nally compensated. The main AV
DD
switcher is current
limited at 1.5A, while the second V
ON
switcher is limited to
800mA. They share the same 1.26V reference voltage.
AV
DD
R1
1
R2
D1
3
C2
4
2
U
U
U
U
When the input voltage is below approximately 2.4V, an
undervoltage lockout circuit disables switching.
When AV
DD
is less than its final voltage, Q4 is turned on,
holding the C
T
pin at ground. When AV
DD
reaches final
value, Q4 lets go of the C
T
pin, allowing the 5.5µA current
source to charge the external capacitor, C
T
. When the
voltage on the C
T
pin reaches 1.25V, Q3 turns on, connect-
ing VO2 to V
ON
. Capacitor value can be calculated using
the following formula:
C = (5.5µA • t
DELAY
)/1.25V
A 22nF capacitor results in approximately 3ms of delay.
V
ON
8
7
6
5
L1–L2
V
IN
C1
L3
D2
C3
GND
C6
D3A
D4B
V
OFF
C4
1948 F02
Figure 2. Recommended Component Placement
3
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
