LTC3706IGN datasheet(12/20 Pages) LINER

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LTC3706

3706fb

OPERATION

Note that in self-starting secondary-side control applica-

tions (with the LTC3705), the presence of the LT3706

bias voltage is dependent upon the regular switching of

the primary-side MOSFETs. Therefore, depending on the

details of the application circuit, the LTC3706 may lose

its bias voltage after a fault has been detected and before

completing a soft-start retry. In this case, the “hiccup-

mode” operation is actually governed by the LTC3705

soft-start circuitry. (See the LTC3705 data sheet.)

Drive Mode and Maximum Duty Cycle

Although the LTC3706 is primarily intended to be used with

the LTC3705 in 2-transistor forward applications, the MODE

pin provides the ﬂexibility to use the LTC3706 in a wide

variety of additional applications. This pin can be used to

defeat the gate drive encoding scheme, as well as change

the maximum duty cycle from its default value of 50%.

The use of the MODE pin is summarized in Table 4.

When the gate drive encoding scheme is defeated, a

standard PWM-style signal will be present at the PT+ pin

and a reference clock (in phase with the PWM signal) will

be present at the PT– pin. These outputs can be used in

“standalone” applications (without the LTC3705) to drive

the gates of MOSFETs in a conventional manner.

Table 4

MODE PIN

PT+/PT– Mode

(MAX DUTY CYCLE)

INTENDED

APPLICATION

GND

Encoded PWM

(DMAX = 50%)

2-Switch Forward

with LTC3705

VCC

Encoded PWM

(DMAX = 75%)

1-Switch Forward

200kΩ to GND

Standard PWM

(DMAX = 50%)

2-Switch Forward

Standalone

100kΩ to GND

Standard PWM

(DMAX = 75%)

1-Switch Forward

Standalone

Power Good/Overvoltage Protection

This circuit monitors the voltage on the FB input. The

open-drain PGOOD output will be logic high if the voltage

on the FB pin is within +17%/–7% of 0.6V. If the voltage on

the FB pin exceeds 117% of 0.6V (0.7V), an overvoltage

(OVP) is detected. For overvoltage protection, the sec-

ondary-side synchronous MOSFET is turned on while all

other MOSFETs are turned off. This protection mode is

not latched, so that the overvoltage detection is cleared if

the FB voltage falls below 115% of 0.6V (0.69V).

Linear Regulator Operation

The LTC3706 provides a linear regulator controller that

drives an external N-type pass device. This controller is used

to create a 7V DC bias from the peak-charged secondary

bias voltage (8V to 30V). Internal divider resistors are used

to establish a regulation voltage of 7V at the VCC pin. An

auxiliary bias supply with a regulated voltage greater than

7V may be applied to the VCC pin to bypass (bootstrap) the

linear regulator. This improves efﬁciency and also helps to

avoid overheating the linear regulator pass device.

Thermal protection for the linear regulator pass device is

also provided by means of the REGSD pin. A current is

sourced from this pin that is proportional to the voltage

across the linear regulator pass device (VIN – VCC). Since

the VCC load current is essentially constant for a given

switching frequency and choice of power MOSFETs, the

power dissipated in the external pass device will only vary

with the voltage across it. Thus, a single resistor may be

placed between the REGSD pin and GND to develop a volt-

age that is proportional to the power in the external pass

device. An additional parallel capacitor can also be used

to account for the thermal time constant associated with

the external pass device itself. When the voltage on the

REGSD pin exceeds 4V, an overtemperature fault occurs

and the LTC3706 attempts a soft-start retry.

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