RDK-239 Power Integrations, Inc., RDK-239 Datasheet - Page 13

• Manufacturer Part Number: RDK-239
• Description: Specifications: Family: Eval Boards - DC/DC & AC/DC (Off-Line) SMPS ; Series: HiperLCS™ ; Main Purpose: DC/DC, Step Down ; Outputs and Type: 1, Isolated ; Power - Output: 150W ; Voltage - Output: 24V ; Current - Output: 6.25A ; Voltage - Input:
• Manufacturer: Power Integrations, Inc.

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
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.
The Burst Threshold setting is stored until VCC is powered down.
Figure 14. OV/UV Pin Voltage Thresholds, at Minimum and Maximum Divider Ratios, for 385 V Nominal Input Voltage.
www.powerint.com
385 V
200 V
Burst Threshold
Burst Threshold Selection Table.
1
2
3
376 V
OV/UV pin resistor divider chosen
495 V
for minimum required LLC gain
BROWNOUT
VSDH
VOVH
needs to be reduced below
BURST
475 V
298 V
to R
R
BURST
FMAX
5.67
VOVL
DS
19
9
/ R
VSDL
as per Table 3.
rating of 530 V.
FMAX
PFC
Time
. This
PFC
Figure 15.
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:
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
385 V
200 V
Burst Threshold
Setting
DT/BF Pin Divider.
1
2
3
R
MAX
R
BURST
FMAX
, which depend on the Burst Threshold setting,
f
MAX
331 V
436 V
OV/UV pin resistor divider chosen
for minimum hold-up capacitance
^
kHz
h
VSDH
VOVH
=
f
START
DT/BF
VREF
7/16
6/16
5/16
GND
Dead
/f
MAX
LCS700-708
418 V
27 000
-
262 V
0
Time
PI-6460-051811
MAX
VOVL
^
MAX
PI-6154-051811
VSDL
ns
is fixed and
.
h
f
MAX
STOP
8/16
7/16
6/16
Time
, and
/f
MAX
Rev. B 062011
MAX
13
.