4

LT1615/LT1615-1

sn16151 16151fas

BLOCK DIAGRA

Figure 1. LT1615 Block Diagram

–

+

–

+

5

400ns

ONE-SHOT

DRIVER

RESET

ENABLE

42mV*

0.12

Ω

A2

A1

Q3

2

R4

140k

R3

30k

R6

40k

R5

40k

Q2

X10

Q1

3

FB

4

SHDN

1

SW

GND

1615/-1 BD

L1

C2

D1

R2

(EXTERNAL)

R1

(EXTERNAL)

C1

* 12mV FOR LT1615-1

OPERATIO

The LT1615 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 FB pin is slightly above

1.23V, comparator A1 disables most of the internal cir-

cuitry. Output current is then provided by capacitor C2,

which slowly discharges until the voltage at the FB pin

drops below the lower hysteresis point of A1 (typical

hysteresis at the FB pin is 8mV). A1 then enables the

internal circuitry, turns on power switch Q3, and the

current in inductor L1 begins ramping up. Once the switch

current reaches 350mA, comparator A2 resets the one-

shot, which turns off Q3 for 400ns. L1 then delivers

current to the output through diode D1 as the inductor

current ramps down. Q3 turns on again and the inductor

current ramps back up to 350mA, then A2 resets the one-

shot, again allowing L1 to deliver current to the output.

This switching action continues until the output voltage is

charged up (until the FB pin reaches 1.23V), then A1 turns

off the internal circuitry and the cycle repeats. The LT1615

contains additional circuitry to provide protection during

start-up and under short-circuit conditions. When the FB

pin voltage is less than approximately 600mV, the switch

off-time is increased to 1.5

µs and the current limit is

reduced to around 250mA (70% of its normal value). This

reduces the average inductor current and helps minimize

the power dissipation in the LT1615 power switch and in

the external inductor and diode. The LT1615-1 operates in

the same manner, except the switch current is limited to

100mA (the A2 reference voltage is 12mV instead of

42mV).