HV9912NG-G 查看數據表（PDF） - Supertex Inc

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HV9912NG-G

Switch-mode LED Driver IC With High Current Accuracy and Hiccup Mode Protection

Supertex Inc 

HV9912

continuous and a very large output capacitor is required to

reduce the ripple in the LED current. Thus, this capacitor will

have a significant impact on the PWM dimming response.

By turning off the disconnect switch when PWMD goes low,

the output capacitor is prevented from being discharged and

thus the PWM dimming response of the boost converter is

greatly improved.

Note that in case of continuous conduction mode boost con-

verters, disconnecting the capacitor might cause a sudden

spike in the capacitor voltage as the energy in the inductor

is dumped into the capacitor. This increase in the capacitor

voltage might cause the OVP comparator to trip if the OVP

point is set too close to the maximum operating voltage.

Thus, either the capacitor has to larger to absorb this energy

without increasing the capacitor voltage significantly or the

OVP set point has to be increased.

False Triggering of the Short Circuit Comparator During

PWM Dimming

During PWM dimming, the parasitic capacitance of the LED

string causes a spike in the output current when the discon-

nect FET is turned on. With the HV9911, this parasitic spike

in the output current caused the IC to falsely detect an over

current condition and shut down. To prevent this false shut-

down, an R-C filter was used at the FDBK pin to filter this

spike.

the COMP pin is disconnected from the GM amplifier and the

GATE and FLT pins are pulled low disabling the LED driver.

When the fault has cleared, a 5μA current source is activated

which pulls the COMP network up to 5V. Once the voltage

at the COMP network reaches 5V, the 5μA sourcing current

is disconnected and a 5μA sinking current is activated which

pulls the COMP pin low. When the voltage at the COMP pin

reaches 1V, the sinking current is disconnected and the Gm

amplifier is reconnected to the COMP pin. The FLT pin goes

high and the GATE pin is now allowed to switch. The closed

loop control then takes over the control of the LED current.

Startup Condition

The startup waveforms are shown in Fig. 2.

Assuming a pole-zero R-C network at the COMP pin (series

combination of RZ and CZ in parallel with CC), the start-up

delay time can be approximately computed as

tdelay

≈

tPOR

+

(

CC

+

CZ)

x

9V

5µA

(Eqn. 8)

This equation assumes that the voltage drop across RZ can

be neglected compared to the voltage swing at the COMP

pin, which is true in most of the cases (RZ < 100kΩ). The

POR time (tPOR) for the HV9912 is 10μs.

To prevent these false triggering in the HV9912, there is a

built-in 500ns blanking network for the short circuit compara-

tor, which eliminates the need for the external R-C low pass

filter. This blanking network is activated when the PWMD

input goes high. Thus, the short circuit comparator will not

see the spike in the LED current during the PWM Dimming

turn-on transition. Once the blanking timer is completed, the

short circuit comparator will start monitoring the output cur-

rent. Thus, the total delay time for detecting a short circuit

will depend on the condition of the PWMD input.

If the output short circuit exists before the PWMD signal

goes high, the total detection time will be:

t = t + t ≈ detect1 blank,SC(max) delay(max) 900 + 250

≈ 1150ns(max)

(Eqn. 6)

If the short circuit occurs when the PWMD signal is already

high, the time to detect will be:

tdetect1 = tdelay(max) ≈ 250ns(max)

(Eqn. 7)

Hiccup Timer

The HV9912 reuses the compensation network on the

COMP pin to create a timer which is activated upon startup

or when a detected fault has been cleared. When a fault is

detected (either open circuit or short circuit) or upon startup,

VIN

POR

COMP

5.0V

Pull-up Pull-down

with 5.0µA with 5.0µA Gm control

1.0V

FLT

tPOR

tDELAY

Fig. 2 Waveforms During Startup

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

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