SI9122A_08 VISHAY [Vishay Siliconix], SI9122A_08 Datasheet - Page 11

• Manufacturer Part Number: SI9122A_08
• Description: 500-kHz Half-Bridge DC/DC Controller with Integrated Secondary Synchronous Rectification Drivers
• Manufacturer: VISHAY [Vishay Siliconix]

Current Limit
Current mode control providing constant current operation is
achieved by monitoring the differential voltage V
the CS1 and CS2 pins, which are connected to a current
sense resistor on the primary low-side MOSFET. In the
absence of an overcurrent condition, V
current limit threshold V
pulled up linearly via the 120 µA current source (I
both DL and DH switch at half the oscillator set frequency.
When a moderate overcurrent condition occurs (V
< V
that is proportional to V
source. Both driver outputs are in frequency fold-back mode
and the switching frequency becomes roughly 20 % of
normal switching frequency. When a severe overcurrent
condition occurs (V
C
C
and DH outputs.
Before V
of the C
drive being taken over by the current limit control loop
through C
switching duty cycle toward the minimum the chip can reach
(D
current, then the switching frequency will start to fold back to
minimum 1/5 of the nominal frequency. This prevents the
on-time of the primary drivers from being reduced to below
100 ns and avoids current tails. If V
will then stop.
With constant current mode control and frequency foldback
protection of the MOSFET switches is increased. The
converter reverts to voltage mode operation immediately
when the primary current falls below the limit level, and
C
soft-start function does not apply during current limit period,
as this would constitute hiccup mode operation.
Document Number: 73492
S-80038-Rev. D, 14-Jan-08
L_CONT
L_CONT
L_CONT
CS1
CS2
MIN
THCL
Blank
). If this duty cycle reduction still cannot lower the load
L_CONT
), the C
CS
capacitor is charged up and clamped to 6.5 V. The
voltage will be clamped to 1.2 V disabling both DL
capacitor immediately at 2 mA rate and the
L_CONT
reaches severe overcurrent condition, a lowering
voltage results in PWM control of the output
L_CONT
. Current control initially reduces the
-
+
AV
THCL
capacitor will be discharged at a rate
TLCL
CS
< V
- 100 mV by the I
Peak Detect
(typical 100 mV); C
CS
), the NMOS discharges
CS
> V
CS
THCL
is less than lower
A
A
V
V
, the switching
150 mV
100 mV
CS
Figure 6 . Current Limit Circuit
TLCL
PD
L_CONT
between
PU
current
< V
) and
CS
is
+
-
+
-
GM
GM
V
The chip provides a means of sensing the voltage of V
withholding operation of the output drivers until a minimum
voltage of V
is achieved by choosing an appropriate resistive tap between
the ground and V
reference voltage. When the applied voltage is greater than
V
provides the input to the voltage feed forward function.
However, if the divided voltage applied to the V
greater than V
switching until the voltage drops below V
resistive tap on the V
the normal V
Alternatively, a zener clamp diode from V
also be used.
Shutdown Mode
If V
will enter SHUTDOWN mode. This powers down all
unnecessary functions of the controller, ensures that the
primary switches are off, and results in a low level current
demand from the V
IN
REF
INDET
Voltage Monitor - V
, the output drivers are activated as normal. V
V
CC
Figure 5. High-Voltage Pre-Regulator Circuit
is forced below the lower V
I
120 µA (nom)
I
0 to 240 µA (nom)
REF
PU
PD
CC
CC
V
(3.3 V, 300 mV hysteresis), is achieved. This
REF
(Si9122A)
operating voltage to avoid this condition.
- 0.3 V, the high-side driver, D
IN
IN
, and comparing this voltage with the
IN
or V
C
2 nF
V
EXT
divider must be set to accommodate
INEXT
12 V
REG_COMP
INDET
V
CC
IN
HV
supplies.
V
OFFSET
R
14.5 V
DMOS
EXT
Vishay Siliconix
AV
SD
PNP Ext
threshold, the device
CC
C
INDET
L_CLAMP
V
- 0.3 V. Thus, the
CC
C
0.5 µF
Si9122A
www.vishay.com
VCC
Auxillary
V
GND
OSC
CC
to GND may
INDET
H
INDET
, will stop
C
R
C
L_CONT
EXT
EXT
IN
pin is
, and
also
11