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LT1613

PIN FUNCTIONS

SW (Pin 1): Switch Pin. Connect inductor/diode here.

Minimize trace area at this pin to keep EMI down.

GND (Pin 2): Ground. Tie directly to local ground plane.

FB (Pin 3): Feedback Pin. Reference voltage is 1.23V.

Connect resistive divider tap here. Minimize trace area at

FB. Set VOUT according to VOUT = 1.23V(1 + R1/R2).

SHDN (Pin 4): Shutdown Pin. Tie to 1V or more to enable

device. Ground to shut down.

VIN (Pin 5): Input Supply Pin. Must be locally bypassed.

BLOCK DIAGRAM

–

0.15

Ω

DRIVER

COMPARATOR

SHUTDOWN

SHDN

–

RAMP

GENERATOR

1.4MHz

OSCILLATOR

GND

1613 • BD

40k

140k

30k

x10

(EXTERNAL)

40k

VOUT

VIN

VIN 5

The LT1613 is a current mode, internally compensated,

fixed frequency step-up switching regulator. Operation

can be best understood by referring to the Block Diagram.

Q1 and Q2 form a bandgap reference core whose loop is

closed around the output of the regulator. The voltage

drop across R5 and R6 is low enough such that Q1 and Q2

do not saturate, even when VIN is 1V. When there is no

load, FB rises slightly above 1.23V, causing VC (the error

amplifier’s output) to decrease. Comparator A2’s output

stays high, keeping switch Q3 in the off state. As increased

output loading causes the FB voltage to decrease, A1’s

output increases. Switch current is regulated directly on a

cycle-by-cycle basis by the VC node. The flip flop is set at

the beginning of each switch cycle, turning on the switch.

When the summation of a signal representing switch

current and a ramp generator (introduced to avoid

subharmonic oscillations at duty factors greater than

OPERATIO

50%) exceeds the VC signal, comparator A2 changes

state, resetting the flip flop and turning off the switch.

More power is delivered to the output as switch current is

increased. The output voltage, attenuated by external

resistor divider R1 and R2, appears at the FB pin, closing

the overall loop. Frequency compensation is provided

internally by RC and CC. Transient response can be opti-

mized by the addition of a phase lead capacitor CPL in

parallel with R1 in applications where large value or low

ESR output capacitors are used.

As the load current is decreased, the switch turns on for a

shorter period each cycle. If the load current is further

decreased, the converter will skip cycles to maintain

output voltage regulation. If the FB pin voltage is increased

significantly above 1.23V, the LT1613 will enter a low

power state where quiescent current falls to approxi-

mately 100

µA.

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