Si9105
Vishay Siliconix
www.vishay.com
6
Document Number: 70003
S-42030—Rev. H, 15-Nov-04
DETAILED DESCRIPTION
Pre-Regulator/Start-Up Section
Due to the low quiescent current requirement of the Si9105
control circuitry, bias power can be supplied from the
unregulated input power source, from an external regulated
low-voltage supply, or from an auxiliary “bootstrap” winding on
the output inductor or transformer.
When power is first applied during start-up, +V
IN
will draw a
constant current. The magnitude of this current is determined
by a high-voltage depletion MOSFET device which is
connected between +V
IN
and V
CC
. This start-up circuitry
provides initial power to the IC by charging an external bypass
capacitance connected to the V
CC
pin. The constant current is
disabled when V
CC
exceeds 9.3 V. If V
CC
is not forced to
exceed the 9.3-V threshold, then V
CC
will be regulated to a
nominal value of 9.3 V by the pre-regulator circuit.
As the supply voltage rises toward the normal operating
conditions, an internal undervoltage (UV) lockout circuit keeps
the output MOSFET disabled until V
CC
exceeds the
undervoltage lockout threshold (typically 8.7 V). This
guarantees that the control logic will be functioning properly
and that sufficient gate drive voltage is available before the
MOSFET turns on. The design of the IC is such that the
undervoltage lockout threshold will not exceed the
pre-regulator turn-off voltage. Power dissipation can be
minimized by providing an external power source to V
CC
such
that the constant current source is always disabled.
BIAS
To properly set the bias for the Si9105, a 820-k resistor
should be tied from BIAS to −V
IN
. This determines the
magnitude of bias current in all of the analog sections and the
pull-up current for the SHUTDOWN and RESET pins. The
current flowing in the bias resistor is nominally 7.5 A.
Reference Section
The reference section of the Si9105 consists of a temperature
compensated buried zener and trimmable divider network.
The output of the reference section is connected internally to
the non-inverting input of the error amplifier. Nominal reference
output voltage is 4 V. The trimming procedure that is used on
the Si9105 brings the output of the error amplifier (which is
configured for unity gain during trimming) to within "1% of 4 V.
This automatically compensates for the input offset voltage in
the error amplifier.
The output impedance of the reference section has been
purposely made high so that a low impedance external voltage
source can be used to override the internal voltage source, if
desired, without otherwise altering the performance of the
device.
Error Amplifier
Closed-loop regulation is provided by the error amplifier,
whose 1-k dynamic output impedance enables it to be used
with feedback compensation (unlike transconductance
amplifiers). A MOS differential input stage provides for low
input current. The noninverting input to the error amplifier
(V
REF
) is internally connected to the output of the reference
supply and should be bypassed with a small capacitor to
ground.
Oscillator Section
The oscillator consists of a ring of CMOS inverters, capacitors,
and a capacitor discharge switch. Frequency is set by an
external resistor between the OSC IN and OSC OUT pins.
(See Typical Characteristics graph of resistor value vs.
frequency.) The DISCHARGE pin should be tied to −V
IN
for
normal internal oscillator operation. A frequency divider in the
logic section limits switch duty cycle to a maximum of 50% by
locking the switching frequency to one half of the oscillator
frequency.
Remote synchronization can be accomplished by capacitive
coupling of a synchronization pulse into the OSC IN terminal.
For a 5-V pulse amplitude and 0.5-s pulse width, typical
values would be 100 pF in series with 3 k to OSC IN.
