RDK-239 Power Integrations, Inc., RDK-239 Datasheet - Page 14

• 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.

Figure 17. R
For example, if BT2 is selected, and f
= 300 kHz, and f
load is reduced and the frequency rises to 350 kHz, the switching
will stop. This causes the output voltage to drop and the
feedback loop to decrease the FEEDBACK pin current. When
the current decreases to a value which corresponds to 300 kHz,
switching will commence, and the cycle will repeat. During
start-up mode, however, the outputs can switch at a frequency
between f
example). Start-up mode is exited once the switching frequency
drops below f
burst mode if the feedback loop attempts to produce a
switching frequency >f
f
HiperLCS is in the off-state of the auto-restart cycle, or in the
power-up delay before switching.
The minimum recommended dead-time is 275 ns, and thus the
maximum f
To simplify the selection of R
Rev. B 062011
MAX
Figure 16. FEEDBACK Pin and DT/BF Pin Current vs. Frequency.
14
13.0
12.0
10.0
11.0
450
400
350
300
250
200
150
100
is the frequency at which the internal counters run when the
9.0
8.0
7.0
6.0
5.0
50
0
250
0
STOP
FMAX
LCS700-708
MAX
vs. Dead-Time, for the 3 Different Burst Threshold Settings.
STOP
and f
setting is 1 MHz.
STOP
, and the HiperLCS will subsequently enter
200
300
MAX
BT1
BT2
BT3
= 350 kHz. If during normal operation the
(250 kHz and 800 kHz in the above
STOP
.
FMAX
Frequency (kHz)
400
Dead-Time (ns)
350
, see the selection curves in Figure 17.
MAX
600
400
is 800 kHz, then f
800
450
START
1000
500
Figure 18. f
The f
Threshold setting (see Table 5).
Table 5. Ratio of f
As a first approximation, during burst mode, the frequency
ramps from f
repeats.
FEEDBACK Pin
The FEEDBACK pin is the voltage regulation FEEDBACK pin. It
has a nominal Thevenin equivalent circuit of 0.65 V and 2.5 kW.
In normal operation, it sinks current. During the off-period of
auto-restart, and during the clocked delay before start-up, it pulls
up internally to V
The current entering the pin determines switching frequency.
Higher current yields higher frequency and thus reduces LLC
output voltage. In a typical application an optocoupler connected
to the VREF pin pulls up on the FEEDBACK pin, via a resistor
network. The optocoupler is configured to source increasing
FEEDBACK pin current, as the output rises. The resistor network
between the optocoupler, FEEDBACK pin, and VREF pin,
determine the minimum and maximum FEEDBACK pin current
(and thus the minimum and maximum operating frequency), that
the optocoupler can command as it goes from cutoff to saturation.
This network also contains the soft-start timing capacitor, C
(Figure 19).
The minimum frequency as set by this network must be lower
than the frequency required by the powertrain at minimum input
voltage. In Figure 19 this is determined by the sum of R
R
resistors when the optocoupler is cut off. C
during normal operation. Do not confuse R
START
500
450
400
350
300
250
200
150
STOP
. The FEEDBACK pin current is determined by these two
250
Burst Threshold
Different Burst Threshold Settings (BT1, BT2, BT3).
to f
START
START
Setting
START
(Lower Burst Threshold Frequency) vs. Dead-Time Setting for
STOP
1
2
3
REF
/f
to f
ratio is fixed, and dependent on the Burst
300
START
in order to discharge the soft-start capacitor.
STOP
vs. Burst Threshold Selection.
; then switching stops, and then the cycle
Dead-Time (ns)
350
400
START
START
f
STOP
, which determines
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1.20
1.14
1.17
/ f
can be ignored
START
450
BT1
BT2
BT3
FMIN
and
START
500