HV9911 查看數據表（PDF） - Supertex Inc

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HV9911

Switch-mode LED Driver IC with High Current Accuracy

Supertex Inc 

HV9911

an output short circuit condition will not remove the fault (Q1

is not in the path of the fault current). The disconnect switch

will help to disconnect the shorted load from the input.

Over Voltage Protection

Over voltage protection is achieved by connecting the output

voltage to the OVP pin through a resistive divider. The voltage

at the OVP pin is constantly compared to the internal 1.25V.

When the voltage at this pin exceeds 1.25V, the IC is turned

off and FAULT goes low.

Output Short Circuit Protection

The output short circuit condition is indicated by FAULT. At

startup, a monoshot circuit, (triggered by the POR circuit),

resets an internal flip-flop, which causes FAULT to go high,

and remains high during normal operation. This also allows

the gate drive to function normally.

The steady state current is reflected in the reference

voltage connected to the transconductance amplifier.

The instantaneous output current is sensed from the INV

terminal of the amplifier. The short circuit threshold current is

internally set to 200% of the steady state current.

During short circuit condition, when the current exceeds the

internally set threshold, the SR flip-flop is set and FAULT

goes low. At the same time, the gate driver of the power FET

is inhibited, providing a latching protection. The system can

be reset by cycling the input voltage to the IC.

Note: The short circuit FET should be connected before the

current sense resistor as reversing RS and Q2 will affect the

accuracy of the output current (due to the additional voltage

drop across Q2 which will be sensed).

When synchronized in this manner, a permanent HIGH

or LOW condition on the SYNC pin will result in a loss of

synchronization, but the HV9911 based converters will

continue to operate at their individually set operating

frequency. Since loss of synchronization will not result in total

system failure, the SYNC pin is considered fault tolerant.

Internal 1MHz Transconductance Amplifier

HV9911 includes a built in 1MHz transconductance amplifier,

with tri-state output, which can be used to close the feedback

loop. The output current sense signal is connected to the

FDBK pin and the current reference is connected to the IREF

pin.

The output of the opamp is controlled by the signal applied

to the PWMD pin. When PWMD is high, the output of the

opamp is connected to the COMP pin. When PWMD is low,

the output is left open. This enables the integrating capacitor

to hold the charge when the PWMD signal has turned off

the gate drive. When the IC is enabled, the voltage on the

integrating capacitor will force the converter into steady state

almost instantaneously.

The output of the opamp is buffered and connected to the

current sense comparator using a 15:1 divider. The buffer

helps to prevent the integrator capacitor from discharging

during the PWM dimming state.

Linear Dimming

Linear dimming can be accomplished by varying the voltage

at the IREF pin, as the output current is proportional to the

voltage at the IREF pin. This can be done either by using a

potentiometer from the REF pin or by applying an external

voltage source at the IREF pin.

Synchronization

The SYNC pin is an input/output (I/O) port to a fault tolerant

peer-to-peer and/or master clock synchronization circuit.

For synchronization, the SYNC pins of multiple HV9911

based converters can be connected together and may also

be connected to the open drain output of a master clock.

When connected in this manner, the oscillators will lock

to the device with the highest operating frequency. When

synchronizing multiple ICs, it is recommended that the same

timing resistor be (corresponding to the switching frequency)

be used in all the HV9911 circuits.

In rare occasions, given the length of the connecting lines for

the SYNC pins, a resistor between SYNC and GND may be

required to damp any ringing due to parasitic capacitances.

It is recommended that the resistor chosen be greater than

300kΩ.

Note that due to the offset voltage of the transconductance

opamp, pulling the IREF pin very close to GND will cause the

internal short circuit comparator to trigger and shut down the

IC. This limits the linear dimming range of the IC. However,

a 1:10 linear dimming range can be easily obtained. It is

recommended that the PWMD pin be used to get zero output

current rather than pull the IREF pin to GND.

PWM Dimming

PWM dimming can be achieved by driving the PWMD pin

with a TTL compatible square wave source. The PWM

signal is connected internally to the three different nodes

– the transconductance amplifier, the FAULT output, and the

GATE output.

When the PWMD signal is high, the GATE and FAULT pins

are enabled, and the output of the transconductance opamp

is connected to the external compensation network. Thus,

NR011206

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