MAX5071 datasheet(13/22 Pages) MAXIM

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VCC and Startup

In normal operation, VCC is derived from a tertiary wind-

ing of the transformer. However, at startup there is no

energy delivered through the transformer, thus a resistor

must be connected from VCC to the input power source

(see RST and CST in Figures 5 to 8). During startup, CST

charges up through RST. The 5V reference generator,

comparator, error amplifier, oscillator, and drive circuit

remain off during UVLO to reduce startup current below

65µA. When VCC reaches the undervoltage-lockout

threshold of 16V, the output driver begins to switch and

the tertiary winding will supply power to VCC. VCC has an

internal 26.5V current-limited clamp at its input to protect

the device from overvoltage during startup.

Size the startup resistor, RST, to supply both the maxi-

mum startup bias (ISTART) of the device (65µA max)

and the charging current for CST. The startup capacitor

CST must charge to 16V within the desired time period

tST (for example, 500ms). The size of the startup

capacitor depends on:

1) IC operating supply current at a programmed oscilla-

tor frequency (fOSC).

2) The time required for the bias voltage, derived from

a bias winding, to go from 0 to 11V.

3) The MOSFET total gate charge.

4) The operating frequency of the converter (fSW).

High-Performance, Single-Ended, Current-Mode

PWM Controllers

______________________________________________________________________________________

UVLO

REFERENCE

2.5V

PREREGULATOR

VOLTAGE-

DIVIDER

THERMAL

SHUTDOWN

EN-REF

SNS

VDD

5V REGULATOR

VOLTAGE-

DIVIDER

26.5V

VCC

VREF

2.5V

REG_OK

DELAY

OSC

4 RT/CT

6 OUT

ILIM

CPWM

EN-DRV-BAR

GND

COMP

ADV_CLK

CLK

MAX5071C

2.5V

50% MAX DUTY CYCLE

16V/10V

Figure 3. MAX5071C Functional Diagram

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