SM8121A Datasheet, PDF (9/19 Pages) Nippon Precision Circuits Inc – White LED Driver IC

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 V_ZD range is given by the following relationship, where N is the
number of LEDs connected in series, V_{F MAX} is the maximum LED forward-bias voltage drop, V_{OUT MAX} is
the SW pin maximum output voltage, V_FB is the FB pin voltage, and V_SBD is the Schottky-barrier diode for-
ward-bias voltage drop.
(V_{F MAX} × N) ≤ V_ZD ≤ (V_{OUT MAX} − V_FB − V_SBD)
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