LT1500 데이터 시트보기 (PDF) - Linear Technology

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LT1500

Adaptive-Frequency Current Mode Switching Regulators

Linear Technology 

LT1500/LT1501

BLOCK DIAGRAM

LBI

+

–

LBO

SYNC

IN

Rh

RSENSE

0.28Ω

18mV

ISENSE

1.24V

SHDN

BIAS

BURST

COMPARATOR

0.75V

1.265V

REFERENCE

100k

–

NEGATIVE

100k E+RROR AMP

+

ERROR AMP

–

150pF

–+

CURRENT

COMPARATOR

FIXED

HYSTERESIS

I1

VARIABLE

HYSTERESIS

I2

SW

Q1

S1

OUTPUT

R1

R2

NFB

FB

VC

GND

PGND

LTC1500/01 • BD

APPLICATIONS INFORMATION

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

equal to VIN. When the switch is off inductor current will

decrease with inductor voltage equal to VOUT – VIN.

Switching frequency will be determined by the inductor

8

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