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

APPLICATIONS INFORMATION

current (460μA LT1952; 400μA LT1952-1) allows a large

resistor to be connected 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 its turn off threshold 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: (LT1952)

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 maximum

rating of the LT1952/LT1952-1 VIN pin, an external zener

should be connected from the VIN pin to ground. This

covers the condition where VIN charges past VIN ON but

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

condition VIN will continue to charge towards system VIN,

SYSTEM

INPUT (VS)

RSTART

FROM AUXILIARY WINDING

VIN (14.25V ON, 8.75V OFF) LT1952

(7.75V ON, 6.5V OFF) LT1952-1

D1*

–

CSTART

1.32V +

*FOR VS > 25V, ZENER D1 RECOMMENDED

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

1952 F04

Figure 4. Low Power Start-Up

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/LT1952-1 is

programmed using an external resistor (ROSC) connected

between the ROSC pin and ground. Figure 5 shows typical

fOSC vs ROSC resistor values. The LT1952/LT1952-1 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]

500

450

400

350

300

250

200

150

100

50

100 150 200 250 300 350 400

ROSC (kΩ)

1952 F05

Figure 5. Oscillator Frequency (fOSC) vs ROSC

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/LT1952-1 to cause premature turn off of SOUT and

OUT in addition to false trigger of soft-start. The LT1952/

LT1952-1 provide programmable leading edge blanking

of the OC and ISENSE comparator outputs to avoid false

current sensing during MOSFET switching.

19521fd

13

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