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LT1952

1952f

OPERATIO

Slope Compensation

The current mode architecture requires slope compensa-

tion to be added to the current sensing loop to prevent

subharmonic oscillations which can occur for duty cycles

above 50%. Unlike most current mode converters which

have a slope compensation ramp that is fixed internally,

placing a constraint on inductor value and operating

frequency, the LT1952 has externally adjustable slope

compensation. Slope compensation can be programmed

by inserting an external resistor (RSLOPE) in series with the

ISENSE pin. The LT1952 has a linear slope compensation

ramp which sources current out of the ISENSE pin of

approximately 8µA at 0% duty cycle to 35µA at 80% duty

cycle.

Over-Current Detection and Soft-Start (OC Pin)

An added feature to the LT1952 is a precise 100mV sense

threshold at the OC pin used to detect over-current condi-

tions in the converter and set a soft-start latch. The OC pin

is connected directly to the source of the primary side

MOSFET to monitor peak current in the MOSFET

(Figure 7). The 100mV threshold is constant over the

entire duty cycle range of the converter because it is

unaffected by the slope compensation added to the

ISENSE pin.

Synchronizing

A SYNC pin allows the LT1952 oscillator to be synchro-

nized to an external clock. The SYNC pin can be driven

from a logic level output, requiring less than 0.8V for a

logic level low and greater than 2.2V for a logic level high.

Duty cycle should run between 10% and 90%. To avoid

loss of slope compensation during synchronization, the

free running oscillator frequency (fOSC) should be pro-

grammed to 80% of the external clock frequency (fSYNC).

The RSLOPE resistor chosen for non-synchronized opera-

tion should be increased by 1.25x (= fSYNC/fOSC).

APPLICATIO S I FOR ATIO

Shutdown and Programming Undervoltage Lockout

The LT1952 has an accurate 1.32V shutdown threshold at

the SD_VSEC pin. This threshold can be used in conjunc-

tion with a resistor divider to define the undervoltage

lockout threshold (UVLO) of the system input voltage (VS)

to the power converter (Figure 3). A pin current hysteresis

(11µA before part turn on, 0µA after part turn on) allows

UVLO hysteresis to be programmed. Calculation of the

ON/OFF thresholds for the supply (SVIN) to the power

converter can be made as follows:

VS OFF Threshold = 1.32[1 + (R1/R2)]

VS ON Threshold = SVIN OFF + (11µA • R1)

A simple open drain transistor can be added to the resistor

divider network at the SD_VSEC pin to control the turn off

of the LT1952 (Figure 3).

The SD_VSEC pin must not be left open since there must

be an external source current >11µA to lift the pin past its

1.32V threshold for part turn on.

Micropower Start-Up: Selection of Start-Up Resistor

and Capacitor for VIN

The LT1952 uses turn-on voltage hysteresis at the VIN pin

and low start-up current to allow micro-power start-up

(Figure 4). The LT1952 monitors VIN pin voltage to allow

part turn on at 14.25V and part turn off at 8.75V. Low start-

up current (460µA) allows a large resistor to be connected

Figure 3. Programming Undervoltage Lockout (UVLO)

1.32V

SYSTEM

INPUT (VS)

OPTIONAL

SHUTDOWN

TRANSISTOR

1952 F03

SD_VSEC

11µA

LT1952

–

OFF

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