Si9121
Vishay Siliconix
Document Number: 71112
S-40708—Rev. C, 19-Apr-04
www.vishay.com
7
DETAILED DESCRIPTION
Bypass
The bypass voltage of the Si9121 is set at a particular positive
reference relative to the V
NEG
pin. The bypass voltage is used
to set an accurate voltage and bias current for the on-chip
oscillator and soft-start circuits. The 0.1-mF ceramic
decoupling capacitor is recommended between the bypass
and V
NEG
. No other components should be connected to this
pin.
Error Amplifier
With a −48-V bus voltage, the converter is referenced to the
−48-V (V
NEG
) node as its system ground. However, the +5-V
or +3.3-V output is referenced to the GND pin, which is
normally connected to 0 V. To regulate this output voltage, the
Si9121 has an on-chip error amplifier which continuously
monitors the output voltage and compares it to a reference
voltage. This difference signal is level-shifted to the low side
circuit to control the power switch duty-cycle and hence the
regulation of the output voltage. Frequency compensation for
the error amplifier is achieved by connecting an external
network between the COMP pin and the V
NEG
pin.
V
CC
Regulator
V
CC
is an internally generated bias supply voltage which
should be externally bypassed with a 0.1-mF capacitor
connected to the negative supply voltage, V
NEG
. No load
current should be drawn from the V
CC
pin. V
CC
may be
supplied from an external source of 9.5-V to 12-V referenced
to V
NEG
. In this configuration, the internal V
CC
regulator is
disabled when the V
CC
receives 9.5-V, or greater, above V
NEG
.
Thermal Shutdown
The Si9121 also includes thermal shutdown which shuts down
the device when junction temperature exceeds 170_C due to
over heating. In thermal shutdown once the die temperature
cools to below 145_C the regulator is enabled. If the die
temperature is excessive due to high package power
dissipation the regulator’s thermal circuit will continue to pulse
the regulator on and off. This is called thermal cycling.
Hiccup Mode Short Circuit Protection
In addition to the thermal shutdown and the cycle-by-cycle
current limiting features already described, the Si9121 has a
built-in hiccup mode timer to handle a continuous output
short-circuit and to automatically restart the device when the
short-circuit is removed.
If an output short-circuit occurs, the device immediately enters
a cycle-by-cycle current limiting mode. As long as the thermal
shutdown is not activated then the Si9121 automatically
determines whether the internal hiccup timer should be started
by monitoring the COMP pin. If V
COMP
exceeds an upper
threshold voltage (approximately 3.5 V) the timer is started
and the external network at the COMP pin is discharged by an
internal 2-mA current sink until V
COMP
reaches approximately
1.5 V. (Note: all voltages are with respect to V
NEG
). At this
point, the circuit reverts to the normal soft-start mode, whereby
the COMP network is charged by its internal soft-start 10-mA
current source and the circuit will attempt to start up in the
normal manner. However, if the output short circuit is still
present, the converter will again enter the cycle-by-cycle
current limiting mode until the COMP pin voltage reaches
3.5 V, whereupon this cycle repeats until the short circuit is
removed.
The duty cycle imposed by the hiccup timer allows the Si9121
to handle continuous short-circuit without damage as long as
the recommended component values shown in the Typical
Application Circuit are used.
MOSFET Switch
The low-side n-channel MOSFET switch is integrated to
provide optimum performance and to minimize the overall
converter size. The typical 1.5-W r
DS(on)
of the MOSFET
allows the converter to deliver up to 2 W of output power.
