SI9122E Datasheet, PDF (8/20 Pages) Vishay Siliconix – 500-kHz Half-Bridge DC/DC Controller with Integrated Secondary Synchronous Rectification Drivers

Si9122E
Vishay Siliconix
V_REF
V_INDET
R_OSC
EP
CS2
CS1
Bandgap
Reference
3.3 V
C_{L_CONT}
Frequency
Foldback
Voltage
Feedforward
Oscillator
OSC
Clock
132 k
60 k
–
+
V_REF/2
Current
Control
Gain
+
Loop
–
Control
100 mV
Blanking
V_CC
V_IN
V_REG
9.1 V
12 V
Pre-Regulator
+
–
9.1 V
+
–
V_UV
V_REF
+
V_SD
–
550 mV
V_UVLO
+
–
8.8 V
135 °C Temp
Protection
High Voltage
Interface
High-Side
Primary
Driver
BST
DH
Clock
V_SD V_UV V_UVLO
Logic
OTP
–
+
PWM
Generator
Logic
LX
V_CC
Low-Side
Primary
Driver
DL
PGND
Timer
V_{CC Synchronous}
Driver
(High)
SR_H
C_{L_CONT}
SS
V_CC
20 µA
8V
Soft-Start
SS Enable
V_{CC Synchronous}
Driver
(Low)
SR_L
Si9122E
BBM
GND
Figure 4. Detailed Si9122E Block Diagram
DETAILED OPTION
Start-Up
When V_INEXT rises above 0 V, the internal pre-regulator
begins to charge up the V_CC capacitor. Current into the
external V_CC capacitor is limited to typically 40 mA by the
internal DMOS device. When V_CC 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 V_REG. 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 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
required V_CC current 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.
Care needs to be taken if there is a delay prior to the external
circuit feeding back to the V_CC supply. To prevent excessive
power dissipation within the IC it is advisable to use an
external PNP device. A pin has been incorporated on the IC,
(REG_COMP) to provide compensation when employing the
external device. In this case the V_IN pin is connected to the
base of the PNP device and controls the current, while the
REG_COMP pin determines the frequency compensation of
the circuit. The value of the REG_COMP capacitor cannot be
too big, otherwise it will slow down the response of the
pre-regulator in the case that fault situations occur and
pre-regulator needs to be turned on again. To understand
the operation, please refer to figure 5.
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Document Number: 73866
S-80112-Rev. D, 21-Jan-08