LTC3726IGN datasheet(9/16 Pages) LINER

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LTC3726

3726fb

When the LTC3726 has adequate operating voltage, it will

begin the procedure of assuming control from the primary

side. To do this, it first measures the voltage on the power

supply’s main output and then automatically advances its

own soft-start voltage to correspond to the main output

voltage. This ensures that the output voltage increases

monotonically as the soft-start control is transferred from

primary to secondary. The LTC3726 then begins sending

PWM signals to the LTC3705/LTC3725 on the primary

side through a pulse transformer. When the LTC3705/

LTC3725 has detected a stable signal from the secondary

controller, it transfers control of the primary switches over

to the LTC3726, beginning the secondary-side soft-start

mode. The LTC3726 continues in this mode until the

output voltage has ramped up to its final value. If for any

reason, the LTC3726 either stops sending (or initially fails

to send) PWM information to the LTC3705/LTC3725, the

LTC3705/LTC3725 will detect a FAULT and initiate a soft-

start retry (see the LTC3705/LTC3725 data sheet).

Slope Compensation

Slope compensation is added at the input of the PWM

comparator to improve stability and noise margin of the

peak current control loop. The amount of slope compen-

sation can be selected from one of five preprogrammed

values using the SLP pin as shown in Table 1. Note that the

amount of slope compensation doubles when the duty

cycle exceeds 50%.

Table 1

SLP PIN

SLOPE (D < 0.5)

SLOPE (D > 0.5)

GND

0.05 • ISMAX • fOSC

0.1 • ISMAX • fOSC

VCC

None

400k

Ω to GND

0.1 • ISMAX • fOSC

0.2 • ISMAX • fOSC

200k

Ω to GND

0.15 • ISMAX •fOSC

0.3 • ISMAX •fOSC

100k

Ω to GND

0.25 • ISMAX • fOSC

0.5 • ISMAX • fOSC

50k

Ω to GND

0.5 • ISMAX • fOSC

1.0 • ISMAX • fOSC

In Table 1 above, ISMAX is the maximum current limit, and fOSC is the

switching frequency.

Current Sensing and Current Limit

For current sensing, the LTC3726 supports either a cur-

rent sense resistor or a current sense transformer. The

current sense resistor may either be placed in series with

the inductor (either high side or ground lead sensing), or

in the source of the “forward” switch. If a current sense

transformer is used, the IS– input should be tied to VCC and

the IS+ pin to the output of the current sense transformer.

This causes the gain of the internal current sense amplifier

to be reduced by a factor of 16, so that the maximum

current sense voltage (current limit) is increased from

78mV to 1.28V. An internal, adaptive leading edge blank-

ing circuit ensures clean operation for “switch” current

sensing applications.

Current limit is achieved in the LTC3726 by limiting the

maximum voltage excursion of the error signal (ITH volt-

age). Note that if slope compensation is used, the precise

value at which current limit occurs will be a function of

duty cycle (see Typical Performance Characteristics). If a

short circuit is applied, an independent overcurrent com-

parator may be tripped. In this case, the LTC3726 will enter

a “hiccup” mode using the soft-start circuitry.

Frequency Setting and Synchronization

The LTC3726 uses a single pin to set the operating

frequency, or to synchronize the internal oscillator to a

reference clock with an on-chip phase-locked loop (PLL).

The FS pin may be tied to GND, VCC or have a single resistor

to GND to set the switching frequency. If a clock signal

(>2V) is detected at the FS pin, the LTC3726 will automati-

cally synchronize to the rising edge of the reference clock.

Table 2 summarizes the operation of the FS pin.

For synchronization between multiple LTC3726s, the PT+

pin of one LTC3726 can be used as a master clock

reference and tied to the FS pin of the other LTC3726s.

Table 2

FS PIN

SWITCHING FREQUENCY

GND

200kHz

VCC

300kHz

RFS to GND

fOSC (Hz) = 4RFS – 200k

Reference Clock

fOSC = fREF (75kHz to 500kHz)

This will cause all LTC3726s to operate at the same

frequency and phase. The LTC3726 can also be used as a

“Slave” in a PolyPhase application. In this case, the phase

angle of each LTC3726 can be set by using the FB/PHASE

OPERATIO

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