ACT111A
Rev 2, 22-Oct-12
efficiency and good thermal condition, the inductor
DC resistance (DCR) should be less than 0.25Ω.
The peak inductor and switch current is:
I
L( PK )
=
I
SW ( PK )
=
I
O
+
Rectifier Diode Selection
Use a Schottky diode as the rectifier to conduct
current when the ACT111A internal top MOSFET
switch is off. In steady state operation, average
forward current in the diode is:
I
D _ AVG
=
I
O
V
IN
−
V
O
V
IN
Δ
I
L
2
(3)
The peak current must be less than the current limit
to maintain output regulation.
(6)
Input Capacitor Selection
A step-down regulator draws pulsing current from
input source. The input capacitor is required to
reduce the voltage ripple at the ACT111A input and
force the pulsing current into a local loop to
minimize EMI. The input capacitor must have low
impedance at the switching frequency to effectively
reduce the voltage ripple and EMI, and it must have
an adequate RMS ripple current rating. The RMS
current for the input capacitor is:
I
CIN _ RMS
=
I
O
×
V
O
(V
IN
−
V
O
)
V
IN
<
I
O
2
The Schottky diode must have current rating
higher than the maximum output current and the
reverse voltage rating higher than the maximum
input voltage.
PWM and Analog Dimming
There are two dimming schemes to control LED
average current during steady state operation. As
those applications requiring a PWM logic signal to
control dimming, the PWM signal could be directly
applied to the DIM pin of the ACT111A as shown in
Figure 1. The LEDs turn on with full load to
completely turn off. The average LED current
increase proportionally to the duty cycle of the
PWM signal. The turn-on threshold voltage is 1.66V
with 100mV hysteresis. The frequency of the PWM
signal is from 100 Hz up to 10 kHz.
If analog dimming scheme is preferred in an
application, a DC voltage to control the FB voltage,
as shown in Figure 2 is used. As the DC voltage
increases from 0 to certain level determined by the
application like 5V, current starts to flow down R
DIM
,
R1 and R
S
. As the control loop maintains the
feedback voltage V
FB
to be 100mV, the current
through the LEDs will linearly decrease to zero.
Figure 1:
PWM Dimming
(4)
For best performance choose a ceramic type
capacitor with X5R or X7R dielectrics due to their
low ESR and small temperature coefficients.
However, low ESR tantalum or electrolytic types
may also be used, provided that the RMS ripple
current rating is higher than 50% of the output
current. For most applications, a 10μF capacitor is
sufficient. The input capacitor should be placed
close to the IN and G pins of the ACT111A, with
shortest possible traces. In the case of tantalum or
electrolytic types, connect a small parallel 0.1μF
ceramic capacitor right next to the ACT111A.
Output Capacitor Selection
A ceramic capacitor with X5R or X7R dielectric
provides the best results over a wide range of
applications. The output capacitor also needs to
have low ESR to keep low output voltage ripple.
The output ripple voltage is:
V
O _ RIPPLE
=
I
O
×
K
RIPPLE
×
ESR
+
I
O
×
K
RIPPLE
8
×
f
×
C
O
ACT111A
PWM
DIM
FB
R1
R
S
(5)
Figure 2:
Analog Dimming
V
DIM
where I
O
is the output current, K
RIPPLE
is the ripple
factor (typically 20% to 30%), ESR is the equivalent
series resistor of the output capacitor, f is 1.4MHz
switching frequency, L is the inductor value, and C
O
is the output capacitance. In the case of ceramic
output capacitors, ESR is very small and does not
contribute to the ripple. In the case of tantalum or
electrolytic type, the ripple is dominated by ESR
multiplied by the ripple current. In this case, the
output capacitor is chosen to have low ESR
capacitor with ESR typically less than 50mΩ.
Innovative Power
TM
-6-
ACT111A
DIM
FB
R
DIM
R1
R
S
www.active-semi.com
Copyright © 2012 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
