FC9961C

2019. 03. 14

5/7

Revision No : 0

General Description

Peak-current control (as in FC9910B) of a buck converter is

the most economical and simple way to regulate its output

current. However, it suffers accuracy and regulation prob-

lems that arise from the so-called peak-to-average current

error, contributed by the current ripple in the output inductor

and the propagation delay in the current sense compara-

tor. The full inductor current signal is unavailable for direct

sensing at the ground potential in a buck converter when

the control switch is referenced to the same ground poten-

tial because the control switch is only conducting for small

periods. While it is very simple to detect the peak current in

the switch, controlling the average inductor current is usu-

ally implemented by level translating the sense signal from

+V

V

sively complex and expensive in the offline AC or other high-

voltage DC applications.

The FC9961C provides fast and very accurate control of the

average current in the intermediate inductor by measuring

only the switching current.

No compensation of the current control loop is required. The

LED current response to PWMD input is similar to that of the

FC9910B. The inductor current ripple amplitude does not af-

fect this control scheme significantly, and therefore, the LED

current is independent of the variation in inductance, switch-

ing frequency or output voltage. Constant off-time control of

the buck converter is used for stability and to improve the

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(Note that,unlike FC9910B, the FC9961C does not support

the constant-frequency mode of operation.)

OFF Timer

The timing resistor connected to RT determines the off-time

of the gate driver, and it must be wired to GND. (Wiring this

resistor to GATE as with FC9910B is no longer supported.)

The equation governing the off-time of the GATE output is

given by:

T

R

(1)

25

within the range of 30kΩ ≤ R

Average Current Control Feedback and Output

Short Circuit Protection

The current through the switching MOSFET source is aver-

aged and used to give constant-current feedback. This cur-

rent is detected using a sense resistor at the CS pin. The

feedback operates in a fast open-loop mode. No compensa-

tion is required. Output current is programmed simply as:

I

0.265V

(2)

R

CS

when the voltage at the LD input V

I

V

(3)

R

CS

The above equations are only valid for continuous conduc-

tion of the output inductor. It is a good practice to design the

inductor such that the switching ripple current in it is 30~40%

of its average peak-to-peak, full load, DC current. Hence,

the recommended inductance can be calculated as:

L

V

(4)

0.4 • I

O

The duty-cycle range of the current control feedback is lim-

ited to D ≤ 0.75. A reduction in the LED current may occur

when the LED string voltage V

input voltage V

Reducing the output LED voltage V

D

the loss of regulation of the LED current. This condition,

however, causes an increase in the LED current and can

potentially trip the short-circuit protection comparator.

The typical output characteristic of the FC9961C LED driver is

shown in Fig.1. The corresponding FC9910B characteristic

is given for the comparison.

Fig.1. Typical output characteristic of an FC9961C LED

driver.

Application Information

V

FC9961C

FC9910B

0

10

20

30

40

50

60

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

Output Voltage (V)

Output Characteristics