LTC3726IGN datasheet(12/16 Pages) LINER

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LTC3726

3726fb

The linear regulator of Figure 2 should be designed to

handle the total expected ICC current. For self-starting

applications with the LTC3705/LTC3725, this regulator

will supply the operating bias current for both primary and

secondary side control circuitry. This current may be

approximated using the following:

M f

S OSC G SEC

POSC G PRI

3726

705

()

CORE

SNUB CC OSC

where IQ,3705 and IQ,3726 are the operating supply currents

of the LTC3705/LTC3725 and LTC3706, MP and MS are the

number of power MOSFETs used on the primary and

secondary sides, QG,PRI and QG,SEC are the total gate

charge of the primary and secondary MOSFETs, ICORE is

the core loss current associated with the pulse trans-

former, and CSNUB is the snubber capacitor across the

pulse transformer. Note that the current used by the

primary side circuitry is doubled by the 2:1 turns ratio of

the pulse transformer. For the Typical Application circuit

of Figure 5, the total ICC delivered by the linear regulator

is 5mA + 3(50nC)(200kHz) + 2(2(38nC)(200kHz) + 2mA)

+ 3mA + 13mA = 85mA. To accommodate this current, Q1

should have a high Beta (>300), and R2 should be chosen

to supply adequate base current at low VIN (e.g., at 36V on

the converter input), while maintaining a reasonable power

dissipation in D2 at high VIN (72V).

The turns ratio (NB) of the bias winding should be chosen

to ensure that there is adequate voltage to operate the

LTC3726 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 the Figure 2 circuit if VCC(MIN) = 6V, ICC =

85mA, and VBUS = 36V-72V, this would mean a turns ratio

NB = 0.24, or a 9:2 transformer. Generally, if the output

voltage of the DC/DC converter is 3.3V or higher, then the

main output of the power transformer (tied to SW node on

one of two reasons: 1) the start-up time-out feature will be

activated since the LTC3726 never sends signals to the

primary side or 2) the primary-side overcurrent circuit will

be tripped because of current buildup in the output induc-

tor. In either case, the LTC3705/LTC3725 will initiate a

shutdown followed by a soft-start retry. See the LTC3705/

LTC3725 data sheets for further details.

Bias Supply Generation

Figure 2 shows a commonly used method of developing a

VCC bias supply for the LTC3726. During start-up, the

circuit of Figure 2 uses a peak detector followed by a

simple linear regulator to rapidly develop a VCC voltage for

the LTC3726. Note that this bias voltage must rise faster

than the open-loop soft-start that is initiated by the

LTC3705/LTC3725. This ensures that the LTC3726 be-

gins switching and assumes control of the soft-start

before the output voltage has risen substantially.

The value of R1 should be chosen to keep the peak

charging current below the maximum (non-repetitive peak)

rating of diode D1, but should otherwise be as small as

APPLICATIO S I FOR ATIO

LTC3726

1.2

Ω

PEAK CHARGER

REGULATOR

µF

16V

7.5V

FZT690B

BIAS

WINDING

3726 F02

MAIN

TRANSFORMER

•

CMPSH1-4

µF

25V

VCC

Figure 2. Typical Bias Supply Configuration

possible to provide a rapid charging of capacitor C1. This

capacitor serves as a reservoir to provide bias voltage as

the LTC3726 begins switching and assumes control of the

soft-start from the LTC3705/LTC3725. Care should be

taken to ensure that capacitor C1 is adequately large to

provide enough hold-up time for the LTC3726 to assume

control and establish a firm bias voltage at the main

transformer.

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