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LTC3706

3706fb

APPLICATIONS INFORMATION

Bias Supply Generation

Figure 2 shows a commonly used method of developing

a VCC bias supply for the LTC3706. During start-up, bias

winding 1 uses a peak detection method to rapidly develop

a VIN voltage for the LTC3706, which in turn drives the

linear regulator that generates the VCC voltage (7V). When

the main output of the converter is in regulation, winding

2 (conﬁgured as a forward-style output) is designed to

produce a regulated auxiliary voltage of approximately

7.5V to 8.5V. Since the auxiliary voltage is greater than that

of the linear regulator, the linear regulator will effectively

be shut down. Note that the output inductor L1 must be

adequately large so that its ripple current is continuous

given the amount of VCC load current, thereby providing

a stable output voltage.

Figure 2. Typical Bias Supply Conﬁguration

VIN

LTC3706

4.7Ω

MBRO530

BAS21

FMMT491A

1mH

NDRV

REGSD

RREGSD

CREGSD

4.7μF

16V

WINDING 1

NB1

3706 F02

MAIN

TRANSFORMER

WINDING 2

NB2

•

BAS21

1μF

50V

VCC

•

The turns ratio (NB1) of the bias winding 1 should be cho-

sen to ensure that there is adequate voltage to operate the

LTC3706 over the entire range for the DC/DC converter’s

input bus voltage (VBUS). This may be calculated using:

CC MIN

BUS MIN

()

VCC(MIN) can be as low as 5V (if this provides adequate

gate drive voltage to maintain acceptable efﬁciency) or as

high as 7V. For VCC(MIN) = 6V and VBUS = 36V to 72V, this

would mean a turns ratio of NB1 ≈ 0.21 and a VIN voltage

range at the LTC3706 of 7.5V to 15V.

Using the bias circuit of Figure 2, the linear regulator

would normally operate only for a brief interval during the

initial soft-start ramp of the main output voltage. Under

some fault conditions (e.g., output overload), the auxiliary

voltage produced by bias winding 2 may decrease below

7V, causing the linear regulator to again supply the VCC

bias current. Since the amount of power dissipation in the

linear regulator pass device may be quite high, it can take

considerable board area when the linear regulator pass

device is sized to handle this power continuously. As an

alternative, the REGSD pin may be used to effectively detect

an overtemperature condition on the linear regulator pass

device and generate a shut down (soft-start retry) before

overheating occurs. This allows for the use of a small (e.g.,

SOT-23) package for the linear regulator pass device.

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