SI9122 Datasheet, PDF (8/17 Pages) Vishay Siliconix – 500-kHz Half-Bridge DC-DC Converter With Integrated Secondary Synchronous Rectification Drivers

Si9122
Vishay Siliconix
V_REF
Bandgap
Reference
3.3 V
V_INDET
C_{L_CONT}
Voltage
Feedforward
Frequency
Foldback
R_OSC
EP
Oscillator
OSC
Clock
132 kW
60 kW
−
+
V_REF/2
Current
Control ^Gain
CS2
+
Loop
CS1
−
Control
100 mV
Blanking
+
−
V_REF
V_UV
+
V_SD
−
550 mV
Clock
−
+
PWM
Generator
V_CC
V_IN
Pre-Regulator
V_REG
9.1 V
+
−
9.1 V
V_UVLO
+
−
8.6 V
High-Side
Primary
Driver
BST
160_C Temp
Protection
High Voltage
Interface
D_H
V_SD V_UV V_UVLO
L_X
OTP
Logic
V_CC
Low-Side
Driver
D_L
Logic
PGND
Timer
V_{CC Synchronous}
Driver
(High)
SR_H
C_{L_CONT}
SS
V_CC
20 mA
8V
Soft-Start
GND
SS Enable
BBM
V_CC
Si9122
Synchronous
Driver
(Low)
SR_L
Figure 4. Detailed Functional Block
DETAILED OPERATION
Start-Up
When V_INEXT rises above 0 V, the internal pre-regulator begins
to charge up the Vcc capacitor. Current into the external V_CC
capacitor is limited to typically 40 mA by the internal DMOS
device. When Vcc exceeds the UVLO voltage of 8.8 V a
soft-start cycle of the switch mode supply is initiated. The V_CC
supply continues to be charged by the pre-regulator until V_CC
equals Vreg. During this period, between V_UVLO and V_REG,
excessive load current will result in V_CC falling below V_UVLO
and stopping switch mode operation. This situation is avoided
by the hysteresis between V_REG and V_UVLO and correct sizing
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of the V_CC capacitor, bootstrap capacitor and the soft-start
capacitor. The value of the V_CC capacitor should therefore be
chosen to be capable of maintaining switch mode operation
until the V_CC can be supplied from the external circuit (e.g via
a power transformer winding and zener regulator). Feedback
from the output of the switch mode supply charges V_CC above
V_REG and fully disconnects the pre-regulator, isolating V_CC
from V_IN. V_CC is then maintained above V_REG for the duration
of switch mode operation. In the event of an over voltage
condition on V_CC, an internal voltage clamp turns on at 14.5 V
to shunt excessive current to GND.
Document Number: 71815
S-41944—Rev. F, 18-Oct-04