HV9112 Supertex, Inc., HV9112 Datasheet - Page 7

• Manufacturer Part Number: HV9112
• Description: High-voltage Current-mode Pwm Controller
• Manufacturer: Supertex, Inc.

Reference
The Reference of the HV9112 consists of a stable bandgap
reference followed by a buffer amplifi er which scales the
voltage up to approximately 4.0V. The scaling resistors of
the reference buffer amplifi er are trimmed during manufac-
ture so that the output of the error amplifi er, when connected
in a gain of –1 confi guration, is as close to 4.0V as possible.
This nulls out any input offset of the error amplifi er. As a con-
sequence, even though the observed reference voltage of a
specifi c part may not be exactly 4.0V, the feedback voltage
required for proper regulation will be.
A ≈ 50KΩ resistor is placed internally between the output of
the reference buffer amplifi er and the circuitry it feeds (refer-
ence output pin and non-inverting input to the error ampli-
fi er). This allows overriding the internal reference with a low
impedance voltage source ≤6.0V. Using an external refer-
ence reinstates the input offset voltage of the error amplifi er,
and its effect of the exact value of feedback voltage required.
Because the reference of the HV9112 is a high impedance
node, and usually there will be signifi cant electrical noise
near it, a bypass capacitor between the reference pin and
VSS is strongly recommended. The reference buffer ampli-
fi er is intentionally compensated to be stable with a capaci-
tive load of 0.01 to 0.1µF.
Error Amplifi er
The error amplifi er in the HV9112 is a true low-power dif-
ferential input operational amplifi er intended for around the
amplifi er compensation. It is of mixed CMOS-bipolar con-
struction: 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 amplifi ers. The
amplifi er is unity gain stable.
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Current Sense Comparators
The HV9112 uses a true dual comparator system with in-
dependent comparators for modulation and current limiting.
This allows the designer greater latitude in compensation
design, as there are no clamps (except ESD protection) on
the compensation pin. Like the error amplifi er, the compara-
tors are of low-noise BiCMOS construction.
Remote Shutdown
The shutdown and reset pins of the 9110 can be used to
perform either latching or non-latching shutdown of a con-
verter 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.
Output Buffer
The output buffer of the HV9112 is of standard CMOS con-
struction (P-channel pull-up, N-channel pull-down). Thus the
body-drain diodes of the output stage can be used for spike
clipping if necessary, and external Schottky diode clamping
of the output is not required.
HV9112