LT3752 datasheet(29/52 Pages) AD

29 / 52 page

LT3752/LT3752-1

Rev. C

For more information www.analog.com

APPLICATIONS INFORMATION

stops immediately and the SS1/SS2 pins are rapidly

discharged. The absence of switching reduces the sense

voltage at the OC pin, allowing SS1/SS2 pins to recharge

and eventually attempt switching again. The part exists

in this hiccup mode as long as the overcurrent condition

exists. This protects the converter and reduces power

dissipation in the components (see Hard Stop in the

Applications Information section). The 96mV peak switch

current threshold is independent of the voltage drop in

RISLP used for slope compensation.

Output DC load current to trigger hiccup mode:

LOAD(OVERCURRENT)

•

96mV

RISENSE









 – 1/2 IRIPPLE(P-P)

(

)

where:

NP = forward transformer primary turns

NS = forward transformer secondary turns

IRIPPLE(P-P) = Output inductor peak-to-peak ripple

current

RISENSEshouldbeprogrammedtoallowmaximumDCload

current for the application plus enough margin during load

transients to avoid overcurrent hiccup mode.

Programming Maximum Duty Cycle Clamp: DVSEC

(Volt-Second Clamp)

Unlike other converters which only provide a fixed maxi-

mum duty cycle clamp, the LT3752/LT3752-1 provide an

accurate programmable maximum duty cycle clamp

(DVSEC) on the OUT pin which moves inversely with

system input. DVSEC provides a duty cycle guardrail to

limit the volt-seconds-on product over the entire natural

duty cycle range (Figures 11 and 12). This limits the

drain voltage required for complete transformer reset.

A resistor RIVSEC from the IVSEC pin to analog ground

(Pin 18) programs DVSEC.

DVSEC (OUT pin duty cycle clamp)

= 0.725 •

RIVSEC

51.1k

•

fOSC

300

•

1.25

UVLO_VSEC

where:

RIVSEC = programming resistor at IVSEC pin

fOSC = switching frequency (kHz)

UVLO_VSEC = resistor divided system input voltage

RIVSEC can program any DVSEC required at minimum

system input. DVSEC will then follow natural duty cycle

as VIN varies. Maximum programmable DVSEC is typi-

Figure 11. Volt-Second (DVSEC) Clamp

3752 F12

UVLO_VSEC

OVLO

RIVSEC

SYSTEM

INPUT

LT3752/LT3752-1

IVSEC

Figure 12. Programming DVSEC

tON_VSEC

tON

OUT

D = tON/t

3752 F11

(PROGRAMMED

BY RIVSEC)

DVSEC = tON_VSEC/t

DVSEC = “DUTY CYCLE GUARDRAIL”

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