HV9100/HV9102/HV9103
Technical Description
Preregulator
Reference
The preregulator/startup circuit for the HV910x consists of a high-
voltage N-channel depletion-mode DMOS transistor driven by an
error amplifier to form a controlled current path between the VIN
terminal and the VDD terminal. Maximum current (about 20 mA)
occurswhenVDD =0, withcurrentreducingasVDD rises. Thispath
shuts off altogether when VDD rises to somewhere between 7.8
and 9.4V, so that if VDD is held at 10 or 12V by an external source
(generally the supply the chip is controlling) no current other than
leakage is drawn through the high voltage transistor. This mini-
mizes dissipation.
The reference consists of a stable bandgap reference followed by
a buffer amplifier which scales the voltage up to approximately
4.0V. The scaling resistors of the reference buffer amplifier are
trimmed during manufacture so that the output of the error
amplifier when connected in a gain of -1 configuration is as close
to 4.000V as possible. This nulls out any input offset of the error
amplifier. As a consequence, even though the observed refer-
encevoltageofaspecificpartmaynotbeexactly4V,thefeedback
voltage required for proper regulation will be 4V.
A resistor of approximately 50KΩ is placed internally between the
output of the reference buffer amplifier and the circuitry it feeds
(reference output pin and NON-INVERTING input to the error
amplifier). Thisallowsoverridingtheinternalreferencewithalow-
impedance voltage source ≤6V. Using an external reference
reinstates the input offset voltage of the error amplifier, and its
effect of the exact value of feedback voltage required. In general,
because the reference voltage of the Supertex HV910x is not
noisy, as some previous devices have been, overriding the
reference should seldom be necessary.
An external capacitor between VDD and VSS is generally required
to store energy used by the chip during the time between shutoff
of the high voltage path and the VDD supply’s output rising enough
totakeoverthepoweringofthechip. Thiscapacitorgenerallyalso
servesastheoutputfiltercapacitorforthatoutputfromthesupply.
1µF is generally sufficient to assure against double-starting.
Capacitorsassmallas0.1µFcanworkwhenfasterresponsefrom
the VDD line is required. Whatever capacitor is chosen should
haveverygoodhighfrequencycharacteristics. Stackedpolyester
or ceramic capacitors work well. Electrolytic capacitors are gen-
erally not suitable.
Because the reference is a high impedance node, and usually
there will be significant electrical noise near it, a bypass capacitor
betweenthereferencepinandVSS isstronglyrecommended. The
reference buffer amplifier is intentionally compensated to be
stable with a capacitive load of 0.01 to 0.1µF.
A common resistor divider string is used to monitor VDD for both
the undervoltage lockout circuit and the shutoff circuit of the high
voltage FET. Setting the undervoltage sense point about 0.6V
lower on the string than the FET shutoff point guarantees that the
undervoltage lockout always releases before the FET shuts off.
Error Amplifier
The error amplifier is a true low-power differential input opera-
tional amplifier intended for around-the-amplifier compensation.
It is of mixed CMOS-bipolar construction: a PMOS input stage is
used so the common-mode range includes ground and the input
impedance is very high. This is followed by bipolar gain stages
which provide high gain without the electrical noise of all-MOS
amplifiers. The amplifier is unity-gain stable.
Bias Circuit
Anexternalbiasresistor, connectedbetweenthebiaspinandVSS
isrequiredtosetcurrentsinaseriesofcurrentmirrorsusedbythe
analogsectionsofthechip.Nominalexternalbiascurrentrequire-
ment is 15 to 20µA, which can be set by a 390KΩ to 510KΩ
resistorifa10VVDD isused, ora510KΩ to680KΩ resistorifa12V
VDD is used. A precision resistor is NOT required; 5% is fine.
Current Sense Comparators
For extremely low power operation, the value of bias current can
be reduced to as low as 5µA by further increases in the value of
the bias resistor. This will reduce quiescent current by about a
third, reduce bandwidth of the error amp by about half, and slow
the current sense comparator by about 30%.
The HV910x uses a true dual comparator system with indepen-
dent comparators for modulation and current limiting. This allows
thedesignergreaterlatitudeincompensationdesign,asthereare
noclamps(exceptESDprotection)onthecompensationpin. Like
the error amplifier, the comparators are of low-noise BiCMOS
construction.
Clock Oscillator
The clock oscillator of the HV910x consists of a ring of CMOS
inverters, timing capacitors, a capacitor discharge FET, and, in
the 50% maximum duty cycle versions, a frequency dividing flip-
flop. A single external resistor between the OSC In and OSC Out
pins is required to set oscillator frequency (see Fig. 4). For the
50%maximumdutycycleversionsthe‘Discharge’pinisinternally
connected to GND. For the 99% duty cycle version, ‘Discharge’
can either be connected to VSS directly or connected to VSS
through a resistor used to set a deadtime.
Remote Shutdown
The shutdown and reset pins can be used to perform either
latching or non-latching shutdown of a converter as required.
These pins have internal current source pull-ups so they can be
driven from open-drain logic. When not used, they should be left
open, or connected to VDD
.
Main Switch
One difference exists between the Supertex HV910x and com-
petitive parts. The oscillator of the HV910x is shut off when a
shutoff command is received. This saves about 150µA of quies-
cent current, which aids in situations where an absolute minimum
of quiescent power dissipation is required.
The main switch is a normal N-channel power MOSFET. Unlike
thesituationwithcompetitivedevices,thebodydiodecanbeused
if desired without destroying the chip.
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SUPERTEX [ Supertex, Inc ]
