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LT1948

PI FU CTIO S

FB1 (Pin 1): Feedback Pin for First Switcher. Connect

resistor divider tap here. Set AVDD according to AVDD =

1.26V(1 + R1/R2).

CT (Pin 2): Timing Capacitor Pin. Connect a 22nF capacitor

from CT to ground to program a 3ms delay from FB1

reaching 1.26V to VON turning on.

SW1 (Pin 3): AVDD 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.

VIN (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 VON.

VON (Pin 8): This is the Delayed 23V Output. VON becomes

23V after the internal timer times out.

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 AVDD output, a

charge-pumped inversion of the AVDD output, called VOFF,

and a 23V/15mA output, called VON. Q3 keeps VON off for

an externally set time interval, set by a capacitor connected

to the CT pin.

The switching frequency of both switchers is 3MHz, set

internally. The switchers are current mode and are inter-

nally compensated. The main AVDD switcher is current

limited at 1.5A, while the second VON switcher is limited to

800mA. They share the same 1.26V reference voltage.

1948 F02

VON

VIN

VOFF

D4B

D3A

L1–L2

GND

AVDD

OPERATIO

Figure 2. Recommended Component Placement

When the input voltage is below approximately 2.4V, an

undervoltage lockout circuit disables switching.

When AVDD is less than its final voltage, Q4 is turned on,

holding the CT pin at ground. When AVDD reaches final

value, Q4 lets go of the CT pin, allowing the 5.5µA current

source to charge the external capacitor, CT. When the

voltage on the CT pin reaches 1.25V, Q3 turns on, connect-

ing VO2 to VON. Capacitor value can be calculated using

the following formula:

C = (5.5

µA • tDELAY)/1.25V

A 22nF capacitor results in approximately 3ms of delay.

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