LT3752 datasheet(28/52 Pages) AD

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LT3752/LT3752-1

Rev. C

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VIN

INTVCC

VOUT

RSENSE

3752 F10

RISLP

• •

LTC3752/LT3752-1

INTVCC

COMP

OUT

ISENSEP

FROM

REGULATION

LOOP

ISENSEN

GND

Figure 10. Current Sensing and Programmable

Slope Compensation

APPLICATIONS INFORMATION

Adaptive leading edge blanking occurs from the start of

OUT rise and completes when OUT reaches within 1V of

its maximum level (INTVCC for LT3752, VIN for LT3752-1).

An extended blanking then occurs which is programmable

using the RTBLNK resistor given by:

tBLNK = 50ns+

2.2ns

•RTBLNK







,

7.32k < RTBLNK < 249k

Adaptive leading edge blanking minimizes the value re-

quiredforRTBLNK.IncreasingRTBLNKfurtherthanrequired

increases M1 minimum on time (Figure 10).

In addition, the critical volt-second clamp (DVSEC) is not

blanked. Therefore, if DVSEC decreases far enough (in soft

startfoldbackandatmaximuminputvoltage)M1mayturn

off before blanking has completed. Since OC and ISENSEP

signals are only seen when M1 is on (and after blanking

has completed), RTBLNK value should be limited by:

(2.2ns/k)RTBLNK < TVSEC(MIN) – tADAPTIVE – 50ns

where,

TVSEC(MIN) = 109(DVSEC (MAX) /(fold.fosc))

(Input(MIN)/Input(MAX))

fold = fOSC and DVSEC foldback ratio (for OUT pin)

( = 4 for LT3752 , = 2 for LT3752-1)

tADAPTIVE = OUT pin rise time to INTVCC – 1V

Example: For Figure 20 circuit, DVSEC(MAX) = 0.77,

Input(MIN)/(MAX) = 17.4V/74V, fold = 4, tADAPTIVE = 23ns

and fOSC = 240kHz,

TVSEC(MIN) = 109(0.77/(4 • 2.4 • 105)) • 17.4/74 = 188ns

(2.2ns/1k)RTBLNK < 188 – 23 – 50

RTBLNK < 52.5k (Actual Circuit Uses 34k)

Current Sensing and Programmable Slope

Compensation

The LT3752/LT3752-1 command cycle-by-cycle peak

current in the external switch and primary winding of the

forward transformer by sensing voltage across a resistor

connected in the source of the external n-channel MOSFET

(Figure 10).

The sense voltage across RSENSE is compared to a sense

threshold at the ISENSEP pin, controlled by COMP pin level.

Two sense inputs, ISENSEP and ISENSEN, are provided to

allow a Kelvin connection to RSENSE. For operation in con-

tinuous mode and above 50% duty cycle, required slope

compensation can be programmed by adding a resistor,

RISLP, in series with the ISENSEP pin. A ramped current

always flows out of the ISENSE pin. The current starts from

2µA at 0% duty cycle and linearly ramps to 33µA at 80%

duty cycle. A good starting value for RISLP is 1.5kΩ which

gives a 41mV total drop in current comparator threshold

at 65% duty cycle.

The COMP pin commands an ISENSEP threshold between

0mV and 220mV. The 220mV allows a large slope com-

pensation voltage drop to exist in RISLP without effecting

the programming of RSENSE to set maximum operational

currents in M1. An fSYNC/fOSC ratio of x (1.0 < x < 1.25)

will reduce the externally programmed slope compensa-

tion by a factor of 1.2x. If required, the external resistor

RISLP can be reprogrammed higher by a factor of 1.2x.

Overcurrent: Hiccup Mode

The LT3752/LT3752-1 use a precise 96mV sense thresh-

old at the OC pin to detect excessive peak switch current

(Figure 10). During an overload condition switching

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