Rev. B 062011

13

LCS700-708

www.powerint.com

If the input voltage is variable (e.g. no PFC pre-regulator), and

the variation is greater than 24%, the OV threshold should be

increased with external circuitry on the resistor divider. External

circuitry is also needed if V

the default ratio.

In the example in the left-hand side of Figure 14 the resistor

divider is set so that brown-in threshold is 376 V, just under the V

PFC

set-point of 385 V. The OV shutdown threshold is 495 V, which

gives adequate margin against the device max V

This minimizes required minimum LLC gain, and minimizes the

peak current at brown-out. In the example on the right of Figure

14, the OV restart threshold is set to 418 V, just above V

maximizes hold-up time for a given bulk capacitor value.

The OV/UV pin has an integrated 5 MW pull-down to detect

pin-open fault conditions.

The recommended pull-down resistor value for the OV/UV pin

divider is 20 kW - 22 kW. A very large resistor value will cause

the pin pull-down current to affect accuracy, and a small value

will increase power loss.

DT/BF Pin

The DT/BF pin senses the voltage divider ratio by entering into a

high-impedance mode for 500 ms after VCC is applied. It

senses the pin voltage, before the HiperLCS starts switching.

See Figure 15.

There are 3 discrete Burst Threshold settings that can be

selected. (This determines the burst start and stop switching

frequencies, see Table 3).

For proper selection, set the ratio of R

Table 3.

Burst Threshold Selection Table.

The Burst Threshold setting is stored until VCC is powered down.

After the Burst Threshold detection, the DT/BF pin operates in

normal mode, sinking current, resembling a diode to ground,

with a Thevenin equivalent circuit of nominally 0.66 V and

1.1 kW. The current from the resistor divider into the pin,

determines the dead-time and the maximum frequency f

The relationship between dead-time and f

approximated by:

Time

-

f

kHz

Dead

ns

27 000

0

MAX

=

^

^

h

h

The relationship between DT/BF pin current and f

switching frequency vs. FEEDBACK pin current (which has the

same characteristic), is show in Figure 16.

The burst mode start and stop frequency thresholds are fixed

fractions of f

as set by the resistor divider ratio on the DT/BF pin.

Table 4. Burst Start and Stop Frequencies as Ratios of f

Figure 14. OV/UV Pin Voltage Thresholds, at Minimum and Maximum Divider Ratios, for 385 V Nominal Input Voltage.

495 V

VOVH

376 V

VSDH

298 V

VSDL

475 V

VOVL

OV/UV pin resistor divider chosen

for minimum required LLC gain

200 V

Time

Time

385 V

436 V

VOVH

331 V

VSDH

262 V

VSDL

418 V

VOVL

OV/UV pin resistor divider chosen

for minimum hold-up capacitance

200 V

385 V

PI-6154-051811

GND

VREF

DT/BF

PI-6460-051811

Figure 15. DT/BF Pin Divider.

Burst Threshold

R

1

19

2

9

3

5.67

Burst Threshold

Setting

f

f

1

7/16

8/16

2

6/16

7/16

3

5/16

6/16