SI9122 Datasheet, PDF (10/17 Pages) Vishay Siliconix – 500-kHz Half-Bridge DC-DC Converter With Integrated Secondary Synchronous Rectification Drivers

Si9122
Vishay Siliconix
Current Limit
Current mode control providing constant current operation is
achieved by monitoring the differential voltage between the
CS1and CS2 pins which are connected across a primary
low-side sense resistor. Once this differential voltage exceeds
the 100-mV trigger point, the voltage on the C_{L_CONT} pin is
pulled lower at a rate proportional to the excess voltage and the
value of the external capacitor connected between the
C_{L_CONT} pin and ground. If the voltage between CS1 and CS2
exceeds 150 mV the C_{L_CONT} capacitor is discharged rapidly
resulting in minimum duty cycle and frequency immediately.
Lowering the C_{L_CONT} voltage results in PWM control of the
output drive being taken over by the current limit control loop.
Current control works to initially reduce the switching duty
cycle down to D_MIN (12.5%). Further reduction in the duty
cycle is accompanied by a reduction in switching frequency at
a rate proportional to the duty cycle. This prevents the on time
of the primary drivers f_nom from reducing below 100 ns and
avoiding a current tail. Frequency reduction occurs to a
maximum of one fifth of the nominal frequency.
With constant current mode control of on time and with
reduced operating frequency, protection of the MOSFET
switches is increased during fault conditions. Minimum duty
cycle and reduced frequency switching continues for the
duration of the fault condition. The converter reverts to voltage
mode operation immediately whenever the primary current
fails to reach the limit level. C_{L_CONT} clamps to 6.5 V when not
in current limit.
The soft-start function does not apply under current limit as this
would constitute hiccup mode operation.
V_IN Voltage Monitor –V_INDET
The chip provides a means of sensing the voltage of V_IN, and
withholding operation of the output drivers until a minimum
voltage of V_REF (3.3 V, 300-mV hysteresis), is achieved. This
is achieved by choosing an appropriate resistive tap between
the ground and V_IN, and comparing this voltage with the
reference voltage. When the applied voltage is greater than
V_REF, the output drivers are activated as normal. V_INDET also
provides the input to the voltage feed forward function.
However, if the divided voltage applied to the V_INDET pin is
greater than V_CC −0.3 V, the high-side driver, D_H, will stop
switching until the voltage drops below V_CC −0.3 V. Thus, the
resistive tap on the V_IN divider must be set to accommodate
the normal V_CC operating voltage to avoid this condition.
Alternatively, a zener clamp diode from V_INDET to GND may
also be used.
Shutdown Mode
If V_INDET is forced below the lower threshold, a minimum of
350 mV(V_SD), the device 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 or V_CC supplies.
V_INEXT
V_IN
(Si9122)
R_EXT
HV_DMOS
V_REF
REG_COMP
C_EXT
2 nF
14.5 V
PNP Ext
Auxillary
V_CC
V_CC
C_VCC
0.5 mF
GND
Figure 5. High-Voltage Pre-Regulator Circuit
CS1
CS2
Blank
− _AV
+
Peak Detect
A_V 150 mV
+ _GM
−
V_CC
I_PU
120 mA (nom)
AV
OSC
V_OFFSET
C_{L_CLAMP}
C_{L_CONT}
R_EXT
A_V 100 mV
+ _GM
−
I_PD
0 − 240 mA (nom)
C_EXT
Figure 6 . Current Limit Circuit
www.vishay.com
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Document Number: 71815
S-41944—Rev. F, 18-Oct-04