FC9961

2015. 03. 23

6/7

Revision No : 1

The short circuit protection comparator trips when the volt-

age at CS exceeds 0.44V. When this occurs, the GATE off-

time T

of the inductor current and potentially its saturation due to

insufficient output voltage. The typical short-circuit current is

shown in the waveform of Fig. 2.

Fig.2. Short-circuit inductor current.

A leading-edge blanking delay is provided at CS to prevent

false triggering of the current feedback and the short circuit

protection.

Linear Dimming

When the voltage at LD falls below 1.5V, the internal 275mV

reference to the constant-current feedback becomes over-

ridden by V

remains continuous, the LED current is given by the equa-

tion (3) above. However, when V

GATE output becomes disabled. The GATE signal recovers,

when V

tions to be able to shut the LED lamp off with the same signal

input that controls the brightness. The typical linear dimming

response is shown in Fig.3.

Fig.3. Typical linear dimming response of an FC9961

LED driver

The linear dimming input could also be used for “mixed-

mode” dimming to expand the dimming ratio. In such case a

pulse-width modulated signal of a measured amplitude be-

low 1.5V should be applied at LD.

Input Voltage Regulator

The FC9961 can be powered directly from an 8.0 ~ 450VDC

supply through its VIN input. When this voltage is applied at

the VIN pin, the FC9961 maintains a constant 7.5V level at

VDD. This voltage can be used to power the IC and external

circuitry connected to VDD within the rated maximum cur-

rent or within the thermal ratings of the package, whichever

limit is lower. The VDD pin must be bypassed by a low ESR

capacitor to provide a low impedance path for the high fre-

quency current of the GATE output. The FC9961 can also be

powered through the VDD pin directly with a voltage greater

than the internally regulated 7.5V, but less than 12V.

Despite the instantaneous voltage rating of 450V, continu-

ous voltage at VIN is limited by the power dissipation in the

package. For example, when FC9961 draws I

from the VIN input, and the 8-pin SOIC package is used, the

maximum continuous voltage at VIN is limited to:

V

(T

(5)

R

where the ambient temperature T

O

m

u

m

i

x

a

m

e

h

t

,

C

working junction temperature T

O

C, the junction-

to-ambient thermal resistance R

O

C/W.

In such cases, when it is needed to operate the HV9961

from a higher voltage, a resistor or a Zener diode can be

added in series with the VIN input to divert some of the

power loss from the FC9961. In the above example, using

a 100V Zener diode will allow the circuit to work up to 490V.

The input current drawn from the VIN pin is represented by

the following equation:

I

(6)

In the above equation, f

is the GATE charge of the external FET obtained from the

manufacturer’s datasheet.

GATE Output

The GATE output of the FC9961 is used to drive an external

MOSFET. It is recommended that the gate charge Q

external MOSFET be less than 25nC for switching frequen-

cies ≤100kHz and less than 15nC for switching frequencies

>100kHz.

400µs

0.44V/R

CS

0

0.2

0.4

0 6

0 8

1 0

1.2

1.4

1 6

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

LD (V)

LD Response Characteristics