SM8121A 查看數據表（PDF） - Nippon Precision Circuits

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SM8121A

White LED Driver IC

Nippon Precision Circuits  

SM8121A

LED OPEN-CIRCUIT PROTECTION

When there is no load (LED open-circuit), the FB pin is pulled-down and then switching occurs at maximum

duty. Consequently, the output voltage continues to be boosted and the SW pin voltage may exceed the maxi-

mum rating of 27V. A zener diode can be added so that it acts as the output load when the LED is open-circuit,

preventing the SW voltage from rising. The zener diode must be selected so that the zener does not breakdown

during normal operation. The zener voltage VZD range is given by the following relationship, where N is the

number of LEDs connected in series, VF MAX is the maximum LED forward-bias voltage drop, VOUT MAX is

the SW pin maximum output voltage, VFB is the FB pin voltage, and VSBD is the Schottky-barrier diode for-

ward-bias voltage drop.

(VF MAX × N) ≤ VZD ≤ (VOUT MAX − VFB − VSBD)

When the load is applied using a connector (SM8121A and LEDs on separate boards), the zener diode should

be mounted on the same board as the SM8121A device so that the SW boost prevention function can operate

when the load is disconnected.

Zener Diode (ZD) Only Connection

When the load is removed (LEDs open circuit), the output voltage is determined by the zener voltage, and the

output current is determined by the output current-setting resistance. Consequently, the output current when the

load is removed is not limited, and thus the input current cannot be controlled.

IIN=(VOUT IZD)/VIN=65mA L

10µH

VIN

3.6V

CIN

4.7µF

SBD

COUT

1.0µF

VOUT=VZD+0.5=15.5V

SW

VSS

FB

VDD

SM8121

CE

ZD

(15V)

LED Open

33Ω

IZD=0.5/33=15.15mA

Zener Diode (ZD) and Current-Limiting Resistance Connection

When the load is removed (LEDs open circuit), the output voltage is determined by the zener voltage, and the

output current is determined by the sum of the output current-setting resistance and the current-limiting resis-

tance. Consequently, the output current is limited when the load is removed, and the input current can be con-

trolled.

IIN=(VOUT IZD)/VIN=2mA

L

10µH

VIN

3.6V

CIN

4.7µF

SBD

VOUT=VZD+0.5=15.5V

COUT

1.0µF

SW

VDD

VSS

SM8121

FB

CE

ZD

(15V)

LED Open

1kΩ

33Ω

IZD=0.5/1033=0.48mA

NIPPON PRECISION CIRCUITS INC.—9

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