HV9912NG-G 데이터 시트보기 (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

FAULT Condition

In the case of a fault condition (either open circuit or short

circuit), the same sequence is repeated with the only differ-

ence being that the COMP pin voltage does not start from

zero, but rather from its steady-state condition.

Short Circuit Protection

When a short circuit condition is detected (output current be-

comes higher than twice the steady state current), the GATE

and FLT outputs are pulled low. As soon as the disconnect

FET is turned off, the output current goes to zero and the

short circuit condition disappears. At this time, the hiccup

timer is started (Fig. 3). Once the timing is complete, the

converter attempts to restart. If the fault condition still per-

sists, the converter shuts down and goes through the cycle

again. If the fault condition is cleared (due to a momentary

output short) the converter will start regulating the output

current normally. This allows the LED driver to recover from

accidental shorts without having to reset the IC.

The hiccup time will depend on the steady state voltage of

the COMP pin (VCOMP). This is typically in the range of 3 - 4V.

The hiccup time can be approximately computed as:

tHICCUP ≈ (CC + CZ) x

9V - VCOMP

5µA

(Eqn. 9)

When the load is disconnected in a boost converter, the

output voltage rises as the output capacitor starts charging.

When the output voltage reaches the OVP rising threshold,

the HV9912 detects an over voltage condition and turns off

the converter. The converter is turned back on only when the

output voltage falls below the falling OVP threshold (which

is 10% lower than the rising threshold). This time is mostly

dictated by the R-C time constant of the output capacitor Co

and the resistor network used to sense over voltage (ROVP1

+ ROVP2). In case of a persistent open circuit condition, this

cycle keeps repeating maintaining the output voltage within

a 10% band.

In most designs, the lower threshold voltage of the over volt-

age protection when the converter will be turned on will be

more than the LED string voltage. Thus, when the LED load

is reconnected to the output of the converter, the voltage

differential between the actual output voltage and the LED

string voltage will cause a spike in the output current when

the FLT signal goes high. This causes a short circuit to be

detected and the HV9912 will go into short circuit protec-

tion. This behavior continues till the output voltage becomes

lower than the LED string voltage, at which point no fault

will be detected and normal operation of the circuit will com-

mence (Fig. 4).

The various delay times can be computed as follows:

Output Current

FLT

Short Circuit

Occurs

Normal Operation

Resumes

Hiccup Time

tRC ≈ 0.1 x (ROVP1 + ROVP2) x CO

tHICCUP1 ≈ (CC + CZ) x

9V - VCOMP

5µA

tHICCUP2-n

≈

(CC

+

CZ)

x

9V

5µA

(Eqn. 10)

(Eqn. 11)

(Eqn. 12)

Note that the number of hiccup cycles might be more than

two.

COMP

5.0V

1.0V

tHICCUP

Fig. 3 Short Circuit Protection

Over Voltage Protection

The HV9912 provides hysteretic over voltage protection

allowing the IC to recover in case the LED load is discon-

nected momentarily.

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

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