4

LT1617/LT1617-1

BLOCK DIAGRA

Figure 1. LT1617 Block Diagram

–

+

–

+

5

400ns

ONE-SHOT

DRIVER

RESET

ENABLE

42mV*

0.12

Ω

A2

A1

Q3

2

R4

280k

R3

60k

R6

80k

R5

80k

Q2

X10

Q1

3

NFB

4

SHDN

1

SW

GND

1617/-1 BD

L1

C2

D1

R2

(EXTERNAL)

R1

(EXTERNAL)

C1

L2

C3

* 12mV FOR LT1617-1

OPERATIO

The LT1617 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 NFB pin is 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 NFB pin

goes above the hysteresis point of A1 (typical hysteresis

at the NFB pin is 8mV). A1 then enables the internal

circuitry, turns on power switch Q3, and the current in

inductors L1 and L2 begins ramping up. Once the switch

current reaches 350mA, comparator A2 resets the one-

shot, which turns off Q3 for 400ns. L2 continues to deliver

current to the output while Q3 is off. Q3 turns on again and

the inductor currents ramp back up until the switch

current reaches 350mA, then A2 again resets the one-

shot. This switching action continues until the output

voltage is charged up (until the NFB pin reaches –1.23V),

then A1 turns off the internal circuitry and the cycle

repeats. The LT1617-1 operates in the same manner,

except the switch current is limited to 100mA (the A2

reference voltage is 12mV instead of 42mV).