SI9121DY-3-T1 VISHAY [Vishay Siliconix], SI9121DY-3-T1 Datasheet - Page 6

• Manufacturer Part Number: SI9121DY-3-T1
• Description: High-Voltage, Non-Isolated Buck-Boost Converter for ISDN Digital Phones
• Manufacturer: VISHAY [Vishay Siliconix]

DETAILED DESCRIPTION
Start-Up
The UVLO circuit prevents the internal circuits from turning on
if VCC is less than 7.6 V (typical) above the negative supply
voltage at V
controller is continuously powered on until the V
drops below 7.0 V. This hysteresis prevents the converter
from oscillating during the start-up phase and unintentionally
locking up the system. Once (V
threshold the internal reference, oscillator, and soft-start
circuits are enabled.
Soft-Start
The Si9121 has an on-chip soft-start circuit which utilizes the
error amplifier external compensation network to ramp the
output NMOS transistor current limit which, in turn, allows the
output voltage to rise gradually without excessive overshoot.
The soft-start circuit is enable once the V
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Si9121
Vishay Siliconix
PIN CONFIGURATION AND ORDERING INFORMATION
PIN DESCRIPTION
Pin Number
1
2
3
4
5
6
7
8
NEG
BYPASS
COMP
V
NEG
pin. With a typical hysteresis of 0.6 V, the
CS
1
2
3
4
BYPASS
Name
COMP
V
V
GND
V
CS
NEG
OUT
L
CC
X
Top View
SOIC-8
Si9121
CC
− V
Current sense pin to detect the inductor current for current mode control and over current protection
Negative supply voltage (−10 V to −60 V)
+1.5-V bandgap reference. Decouple with 0.1 mF capacitor.
Compensation node to stabilize the converter
Output voltage feedback connected to the PWM summing comparator
Low impedance system ground
Internally generated supply voltage for the internal circuit and MOSFET drive circuit. Decouple with an external
bypass capacitor.
Inductor connection node
NEG
8
7
6
5
) exceeds the UVLO
CC
voltage exceeds
L
V
GND
V
X
CC
OUT
CC
voltage
the UVLO threshold.
compensation components (see Typical Application Circuit)
the soft-start time is approximately 10 ms.
Oscillator
The oscillator is designed to operate at a nominal frequency
of 95 kHz with no external components. The 95-kHz operating
frequency allows the converter to operate in PWM mode
during the full load condition even though the duty cycle is very
low. The 95kHz switching frequency also allows the converter
to operate at optimal efficiency without a large output inductor
and capacitor.
PWM Mode and Current Limit
The Si9121 is a current-mode converter designed to operate
in PWM mode. It features pulse-by-pulse peak current limiting
such that when the peak current sensed voltage on the CS pin
is greater than 0.67 V the switch is turned off for the remainder
of the clock cycle.
Si9121DY-5-T1
Si9121DY-5-T1—E3
Si9121DY-3-T1
Si9121DY-3-T1—E3
Si9121DY-5
Si9121DY-3
Part Number
Si9121DB-5
Si9121DB-3
Eval Kit
Function
ORDERING INFORMATION
Temperature Range
Temperature Range
For the recommended frequency
−40 to 85_C
−10 to 70_C
40 to 85_C
10 to 70_C
S-40708—Rev. C, 19-Apr-04
Document Number: 71112
Surface Mount and
Surface Mount and
Board Type
Tape and Reel
Tape and Reel
Package
Thru-Hole
Bulk
Bulk