LT1952-1(RevD) Просмотр технического описания (PDF) - Linear Technology

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LT1952-1
(Rev.:RevD)

Single Switch Synchronous Forward Controller

Linear Technology 

LT1952/LT1952-1

OPERATION

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/LT1952-1 have externally adjust-

able slope compensation. Slope compensation can be

programmed by inserting an external resistor (RSLOPE)

in series with the ISENSE pin. The LT1952/LT1952-1 have

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.

Overcurrent Detection and Soft-Start (OC Pin)

An added feature to the LT1952/LT1952-1 is a precise

100mV sense threshold at the OC pin used to detect

overcurrent conditions 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 107mV 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/LT1952-1 oscillator to be

synchronized 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 synchroniza-

tion, the free running oscillator frequency (fOSC) should

be programmed to 80% of the external clock frequency

(fSYNC). The RSLOPE resistor chosen for non-synchronized

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

APPLICATIONS INFORMATION

Shutdown and Programming Undervoltage Lockout

The LT1952/LT1952-1 have an accurate 1.32V shutdown

threshold at the SD_VSEC pin. This threshold can be

used in conjunction 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 (10μA 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 + (10μA • 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/LT1952-1 (Figure 3).

The SD_VSEC pin must not be left open since there must

be an external source current >10μA to lift the pin past its

1.32V threshold for part turn on.

SYSTEM

INPUT (VS)

R1

OPTIONAL

SHUTDOWN

TRANSISTOR R2

SD_VSEC

–

11μA

1.32V +

ON OFF

LT1952/LT1952-1

1952 F03

Figure 3. Programming Undervoltage Lockout (UVLO)

Micropower Start-Up: Selection of Start-Up Resistor

and Capacitor for VIN

The LT1952/LT1952-1 use turn-on voltage hysteresis at

the VIN pin and low start-up current to allow micro-power

start-up (Figure 4). The LT1952/LT1952-1 monitor VIN pin

voltage to allow part turn on at 14.25V (7.75V LT1952-1)

and part turn off at 8.75V (6.5V LT1952-1). Low start-up

19521fd

12

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