HV9963 データシートの表示（PDF） - Supertex Inc

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HV9963

Closed Loop LED Driver with Enhanced PWM Dimming

Supertex Inc 

HV9963

equal distribution between the rise and fall times, a 10nH

parasitic inductance causes a pre-charge voltage of:

VPRE-CHARGE

=

10nH

•

3A

50ns

=

600mV

As can be seen, a very conservative estimate of the pre-

charge voltage is already larger than the steady state peak

current sense voltage and will cause the converter to falsely

trip.

To prevent this behavior, a resistor (typically 500 – 800Ω)

can be added in series with the capacitor as shown in Figure

4. This resistor limits the charging current into the capacitor.

However, the resistor will also slow down the discharge of

the capacitor during the FET off time, so the maximum exter-

nal resistance will be limited by the switching frequency and

the slope compensation capacitor.

AVDD

GATE

Q1

-

ISC

+

GATE

Q2

GND

RT

CS

CSC

CDRAIN

REXT

RCS

- VLP +

+

VDRAIN

-

LP

ILP

Figure 4: Modified Slope compensation circuit

Rext,max =

1 • 0.07 •

3 fS

1

CSC

- 600Ω

FLT Output

The FLT pin is used to drive a disconnect FET when driving

boost and SEPIC converters. In the case of boost convert-

ers, when there is a short circuit fault at the output, there is a

direct path from the input source to ground which can cause

high currents to flow. The disconnect switch is used to inter-

rupt this path and prevent damage to the converter.

The disconnect switch also helps to disconnect the output

filter capacitors for the boost and SEPIC converters from

the LED load during PWM dimming and enables a very high

PWM dimming ratio.

Control of the LED Current (IREF, FDBK and

COMP)

The LED current in the HV9963 is controlled in a closed-

loop manner. The current reference which sets the three

LED currents at the IREF pin is set by using a resistor di-

vider from the AVDD pin (or can be set externally with a low

voltage source). This reference voltage is compared to the

voltage at the FDBK pin which senses the LED current by

using current sense resistors. HV9963 includes a 1.0MHz

transconductance amplifier with tri-state output, which is

used to close the feedback loops and provide accurate cur-

rent control. The compensation network is connected at the

COMP pin.

The output of the op-amp is buffered and connected to the

current sense comparator using a 11R:1R resistor divider.

The output of the op-amp is also controlled by the signal

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

of the op-amp 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 volt-

age on the integrating capacitor will force the converter into

a steady state almost instantaneously.

Note: The absolute maximum voltage rating of the IREF pin

is 3.5V and the voltage applied at this pin should not exceed

this rating.

Soft Start (SS)

Soft start of the LED current can be achieved by connecting

a capacitor at the SS pin. The rate of rise of SS pin limits the

LED current’s rate of rise.

Upon start-up, the capacitance at the COMP network

is being charged by the 200μA sourcing current of the

transconductance amplifier. Without the soft-start func-

tion, this larger current would cause the COMP voltage to

increase faster than the boost converter’s response time,

causing overshoots in the LED current during start-up.

The SS pin is used to prevent these LED current overshoots

by limiting the COMP pin’s rate of rise . A capacitor at the soft

start pin programs the voltage’s rate of rise at the pin. The

SS pin holds the COMP pin to 1.0V above the SS pin and

thereby controls the COMP pin’s rate of rise. The COMP pin

is released once the voltage reaches its steady state volt-

age.

● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com

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