LT3752 datasheet(21/52 Pages) AD

21 / 52 page

LT3752/LT3752-1

Rev. C

For more information www.analog.com

1.250V

3752 F05

OVLO PIN

SYSTEM

INPUT (VS)

UVLO_VSEC

LT3752/LT3752-1

5µA

Figure 5. Programming Undervoltage Lockout (UVLO)

APPLICATIONS INFORMATION

Programming System Input Undervoltage Lockout

(UVLO) Threshold and Hysteresis

The LT3752/LT3752-1 have an accurate 1.25V shutdown

threshold at the UVLO_VSEC pin. This threshold can be

used in conjunction with an external resistor divider to

definethefallingundervoltagelockoutthreshold(UVLO(–))

for the converter’s system input voltage (VS) (Figure 5).

A pin hysteresis current of 5µA allows programming of

the UVLO(+) threshold.

VS (UVLO(–)) [begin SOFT-STOP then shut down]

= 1.25 1+

R2 + R3

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢

⎤

⎦

⎥

VS (UVLO(+)) [begin SOFT-START]

= VS (UVLO(–)) + (5µA • R1)

It is important to note that the part enters soft-stop when

the UVLO_VSEC pin falls back below 1.25V. During soft-

stop the converter continues to switch as it folds back

switching frequency, volt-second clamp and COMP pin

voltage. See Soft-Stop in the Applications Information

section. When the SS2 pin is finally discharged below its

150mV reset threshold both the housekeeping supply and

forward converter are shut down.

used to pull down the UVLO_VSEC pin below 1.25V but not

below the micropower shutdown threshold of 0.6V(max).

Typical VIN quiescent current after soft-stop is 165µA.

Micropower Shutdown

If a micropower shutdown is required using an external

control signal, an open-drain transistor can be directly

connected to the UVLO_VSEC pin. The LT3752/LT3752-1

haveamicropowershutdownthresholdoftypically0.4Vat

the UVLO_VSEC pin. VIN quiescent current in micropower

shutdown is 20µA.

Programming System Input Overvoltage Lockout

(OVLO) Threshold

The LT3752/LT3752-1 have an accurate 1.25V overvoltage

shutdown threshold at the OVLO pin. This threshold can

be used in conjunction with an external resistor divider to

define the rising overvoltage lockout threshold (OVLO(+))

for the converter’s system input voltage (VS) (Figure 6).

When OVLO(+) is reached, the part stops switching im-

mediately and a hard stop discharges the SS1 and SS2

pins. The falling threshold OVLO(–) is fixed internally at

1.215V and allows the part to restart in soft-start mode.

A single resistor divider can be used from system input

supply (VS) to define both the undervoltage and overvolt-

age thresholds for the system. Minimum value for R3 is

1k. If OVLO is unused, place a 10k resistor from OVLO

pin to ground.

VS OVLO(+) [stop switching; HARD STOP]

= 1.25 1+

R1+ R2

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢

⎤

⎦

⎥

VS OVLO(–) [begin SOFT-START]

= VS OVLO +

(

) •

1.215

1.25

Soft-Stop Shutdown

Soft-stop shutdown (similar to system undervoltage) can

be commanded by an external control signal. A MOSFET

with a diode (or diodes) in series with the drain should be

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