8

LT1500/LT1501

–

+

–

+

–

+

BIAS

1.265V

REFERENCE

+

BURST

COMPARATOR

NEGATIVE

ERROR AMP

ERROR AMP

CURRENT

COMPARATOR

FIXED

HYSTERESIS

100k

150pF

FB

NFB

GND

100k

S1

VARIABLE

HYSTERESIS

SW

PGND

R2

Q1

R1

18mV

0.28

Ω

Rh

IN

SYNC

LBO

1.24V

0.75V

SHDN

LBI

OUTPUT

LTC1500/01 • BD

OPERATION (SEE BLOCK DIAGRAM)

The LT1500 uses a current mode architecture without the

need for an internal oscillator. Switching frequency is

determined by the value of the external inductor used. This

technique allows the selection of an operating frequency

best suited to each application and considerably simplifies

the internal circuitry needed. It also eliminates a

subharmonic oscillation problem common to all fixed

frequency (clocked) current mode switchers. In addition,

it allows for high efficiency micropower operation while

maintaining higher operating frequencies. Because the

power switch (Q1) is grounded, the basic topology used

will normally be a boost converter with output voltage

always higher than the input voltage. Special topologies

such as the SEPIC, flyback and Cuk converter can also be

used when the output voltage may not always be higher

than the input or when full shutdown of the output voltage

is needed. Operation as a boost converter is as follows.

Assume that inductor current is continuous, meaning that

it never drops to zero. When the switch is on, inductor

current will increase with voltage across the inductor

Switching frequency will be determined by the inductor