with Integrated Secondary Synchronous Rectification Drivers
DESCRIPTION
Si9122 is a dedicated half-bridge IC ideally suited to fixed
telecom applications where efficiency is required at low
output voltages (e.g. < 3.3 V). Designed to operate within the
fixed telecom voltage range of 33 to 72 V, the IC is capable
of controlling and driving both the low and high-side
switching devices of a half bridge circuit and also controlling
the switching devices on the secondary side of the bridge.
Due to the very low on-resistance of the secondary
MOSFETs, a significant increase in the efficiency can be
achieved as compared with conventional Schottky diodes.
Control of the secondary devices is by means of a pulse
transformer and a pair of inverters. Such a system has
efficiencies well in excess of 90 % even for low output
voltages. On-chip control of the dead time delays between
the primary and secondary synchronous signals keep
efficiencies high and prevent accidental destruction of the
power transformer. An external resistor sets the switching
frequency from 200 kHz to 625 kHz.
Si9122 has advanced current monitoring and control
circuitry which allow the user to set the maximum current in
the primary circuit. Such a feature acts as protection against
output shorting and also provides constant current into large
capacitive loads during start-up or when paralleling power
supplies. Current sensing is by means of a sense resistor on
the low-side primary device.
FEATURES
• 12 V to 72 V input voltage range
• Integrated half-bridge primary drivers
(1 A drive capability)
RoHS
COMPLIANT
• Secondary synchronous signals with
programmable deadtime delay
• Voltage mode control
• Voltage feedforward compensation
• High voltage pre-regulator operates during start-up
• Current sensing on low-side primary device
• Frequency foldback eliminates constant current tail
• Advanced maximum current control during start-up and
shorted load
• Low input voltage detection
• Programmable soft-start function
• Over temperature protections
APPLICATIONS
• Network cards
• Power supply modules
FUNCTIONAL BLOCK DIAGRAM
12 V to 72 V
V
CC
V
IN
REG_COMP
BST
Synchronous
Rectifiers
DH
LX
1 V to 12 V Typ.
+
V
OUT
-
Si9122
V
IN_DET
DL
CS2
CS1
C
L_CONT
SR
H
SR
L
PGND
R
OSC
V
REF
GND
B BM
EP
V
CC
Error
Amplifier
+
-
V
REF
SS
Opto Isolator
Figure 1.
Document Number: 71815
S-80038-Rev. J, 14-Jan-08
www.vishay.com
1
Si9122
Vishay Siliconix
TECHNICAL DESCRIPTION
Si9122 is a voltage mode controller for the half-bridge
topology. With 100 V depletion mode MOSFET capability,
the Si9122 is capable of powering directly from the high
voltage bus to V
CC
through an external PNP pass transistor,
or may be powered through an external regulator directly
through the V
CC
pin. With PWM control, Si9122 provides
peak efficiency throughout the entire line and load range. In
order to simplify the traditional secondary synchronous
rectification, Si9122 provides intelligent gate drive signals to
control the secondary MOSFETs. With independent gate
drive signals from the controller, transformer design is no
longer limited by the gate to a source rating of the MOSFETs.
Si9122 provides constant V
GS
voltage, independent of line
voltage to minimize the gate charge loss as well as
conduction loss. A break-before-make function is included to
prevent shoot through current or transformer shorting.
Adjustable Break-Before-Make time is incorporated into the
IC and is programmable by an external resistor value.
Si9122 is packaged in TSSOP-20 and MLP65-20 packages.
Both TSSOP-20 and MLP65-20 packages are available in
lead (Pb)-free option. In order to satisfy the stringent ambient
temperature requirements, Si9122 is rated to handle the
industrial temperature range of - 40 °C to 85 °C. When a
situation arises which results in a rapid increase in primary
(or secondary current) such as output shorted or start-up
with a large output capacitor, control of the PWM generator
is handed over to the current loop. Monitoring of the load
current is by means of a sense resistor on the primary low-
side switch.
DETAILED BLOCK DIAGRAM
V
IN
V
CC
R
OSC
High-Side
Primary
Driver
Int
REG_COMP
Pre-Regulator
V
REF
9.1
V
+
-
BST
D
H
L
X
V
UVLO
8.8 V
Low-Side
Driver
OSC
Ramp
V
INDET
V
REF
+
-
+
-
V
UV
V
FF
V
CC
D
L
V
SD
550 mV
132 kΩ
60 kΩ
EP
Error Amplifier
-
+
V
REF
PGND
+
-
2
PWM
Comparator
Driver
Control
and
Timing
V
CC
SR
H
SYNC
Driver High
20 µA
SS
I
SS
8V
OTP
V
CC
+
-
Peak DET
Duty Cycle
Control
SR
L
CS2
CS1
Over Current Protection
GND
C
L_CONT
BBM
Si9122
SYNC
Driver Low
Figure 2.
www.vishay.com
2
Document Number: 71815
S-80038-Rev. J, 14-Jan-08
Si9122
Vishay Siliconix
ABSOLUTE MAXIMUM RATINGS
All voltages referenced to GND = 0 V
Parameter
V
IN
(Continuous)
V
IN
(100 ms)
V
CC
V
BST
V
LX
V
BST
- V
LX
V
REF
, R
OSC
Logic Inputs
Analog Inputs
HV Pre-Regulator Input Current (Continuous)
Storage Temperature
Operating Junction Temperature
Power Dissipation
a
Thermal Impedance (Θ
JA
)
TSSOP-20
MLP65-20
TSSOP-20
b
MLP65-20
c
Limit
75
100
14.5
Continuous
100 ms
90
115
75
15
- 0.3 to V
CC
+ 0.3
- 0.3 to V
CC
+ 0.3
- 0.3 to V
CC
+ 0.3
5
- 65 to 150
150
850
2500
75
38
Unit
V
mA
°C
mW
°C/W
Notes:
a. Device Mounted on JEDEC compliant 1S2P test board.
b. Derate - 14 mW/°C above 25 °C.
c. Derate - 26 mW/°C above 25 °C.
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation
of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum
rating conditions for extended periods may affect device reliability.
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