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LT1952

1952f

APPLICATIO S I FOR ATIO

between system input supply and VIN. Once the part is

turned on, input current increases to drive the IC (4.5mA)

and the output drivers (IDRIVE). A large enough capacitor

is chosen at the VIN pin to prevent VIN falling below 8.5V

before an auxiliary winding in the converter takes over

supply to VIN. This technique allows a simple resistor/

capacitor for start-up which draws low power from the

system supply to the converter. The values for RSTART and

CSTART are given by:

RSTART(MAX) = (VS(MIN) – VIN ON(max))/ISTART(MAX)

CSTART(MIN) = (IQ(MAX) + IDRIVE(MAX)) • tSTART/

VIN HYST(MIN)

Example:

For VS(MIN) = 36V, VIN ON(MAX) = 15.75V,

ISTART(MAX) = 700µA, IQ(MAX) = 5.5mA,

IDRIVE(MAX) = 5mA, VIN HYST(MIN) = 3.75V

and tSTART = 100µs,

RSTART = (36 – 15.75)/700µA = 28.9k (choose 28.7k)

CSTART = (5.5mA + 5mA) • 100µs/3.75V = 0.28µF

(typically choose ≥ 1µF)

For system input voltages exceeding the absolute maxi-

mum rating of the LT1952 VIN pin, an external zener should

be connected from the VIN pin to ground. This covers the

condition where VIN charges past VIN ON (typically 14.25V)

but the part does not turn on because SD_VSEC < 1.32V. In

this condition VIN will continue to charge towards system

Figure 4. Low Power Start-Up

VIN, possibly exceeding the rating for the VIN pin. The zener

voltage should obey VIN ON(MAX) < VZ < 25V.

Programming Oscillator Frequency

The oscillator frequency (fOSC) of the LT1952 is pro-

grammed using an external resistor (ROSC) connected

between the ROSC pin and ground. Figure 5 shows typical

fOSC vs. ROSC resistor values. The LT1952 free-running

oscillator frequency is programmable in the range of

100kHz to 500kHz.

Stray capacitance and potential noise pickup on the ROSC

pin should be minimized by placing the ROSC resistor as

close as possible to the ROSC pin and keeping the area of

the ROSC node as small as possible. The ground side of the

ROSC resistor should be returned directly to the (analog

ground) GND pin. ROSC can be calculated by,

ROSC = 9.125k [(4100k/fOSC) – 1]

Figure 5. Oscillator Frequency (fOSC) vs ROSC

ROSC (kΩ)

400

1952 F05

100

150

250

200

300

350

500

450

400

350

300

250

200

150

100

1.32V

SYSTEM

INPUT (VS)

FROM AUXILIARY WINDING

*FOR VS > 25V, ZENER D1 RECOMMENDED

(VIN ON(MAX) < VZ < 25V)

1952 F04

VIN

LT1952

(14.25V ON, 8.75V OFF)

CSTART

D1*

RSTART

–

Programming Leading Edge Blank Time

For PWM controllers driving external MOSFETs, noise can

be generated at the source of the MOSFET during gate rise

time and some time thereafter. This noise can potentially

exceed the OC and ISENSE pin thresholds of the LT1952 to

cause premature turn off of SOUT and OUT in addition to

false trigger of soft-start. The LT1952 provides program-

mable leading edge blanking of the OC and ISENSE com-

parator outputs to avoid false current sensing during

MOSFET switching.

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