Rev. G 05/15

13

LCS700-708

www.power.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

BROWNOUT needs to be reduced below 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

DS rating of 530 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

PFC. This 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

BURST to RFMAX as per Table 3.

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

MAX. The relationship

between dead-time and f

MAX is fixed and approximated by:

Time

-

f

kHz

Dead

ns

27 000

0

MAX

=

^

^

h

h

The relationship between DT/BF pin current and f

MAX, and switching

frequency vs. FEEDBACK pin current (which has the same character-

istic), is show in Figure 16.

The burst mode start and stop frequency thresholds are fixed

fractions of f

MAX, which depend on the Burst Threshold setting, as set

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

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

Enable pin resistor divider chosen

for minimum required LLC gain

200 V

385 V

436 V

VOVH

331 V

VSDH

262 V

VSDL

418 V

VOVL

Enable pin resistor divider chosen

for minimum hold-up capacitance

200 V

385 V

PI-6154-113010

RFMAX

RBURST

GND

VREF

DT/BF

PI-6460-051811

Figure 15. DT/BF Pin Divider.

Table 3.

Burst Threshold Selection Table.

Table 4.

Nominal Burst Start and Stop Frequencies as Ratios of f

MAX.

Burst Threshold

R

BURST / RFMAX

1

19

2

9

3

5.67

Burst Threshold

Setting

f

START/fMAX

f

STOP/fMAX

1

7/16

8/16

2

6/16

7/16

3

5/16

6/16