mic3201 Micrel Semiconductor, mic3201 Datasheet - Page 9

• Manufacturer Part Number: mic3201
• Description: High Brightness Led Driver With High-side Current Sense
• Manufacturer: Micrel Semiconductor

Application Information
The MIC3201 is a hysteretic step-down constant-current
High-Brightness LED (HB LED) driver. The internal block
diagram is shown in Figure 1. The MIC3201 is
composed of a current sense comparator, voltage and
current reference, 5V regulator, MOSFET driver, and a
MOSFET. Hysteretic mode control, also called bang-
bang control, is the topology that does not employ an
error amplifier, and instead uses an error comparator.
The inductor current is controlled within a hysteretic
window. If the inductor current is too small, the power
MOSFET is turned on; if the inductor current is large
enough, the power MOSFET is turned off. It is a simple
control scheme with no oscillator and no loop
compensation. Since the control scheme does not need
loop compensation, it makes a design easy, and avoids
problems of instability.
Transient response to load and line variation is very fast
and only depends on propagation delay. This makes the
control scheme very popular for certain applications.
LED Current and R
The main feature in MIC3201 is to control the LED
current accurately within ± 5% of set current. Choosing a
high-side R
current irrespective of wide input voltage range. The
following equation gives the R
For V
table.
Micrel, Inc.
January, 2010
R
2.00
1.00
0.63
0.56
0.50
0.40
0.33
0.28
0.24
0.22
0.20
CS
CS(MAX)
(Ω)
R
Table 1. Selecting R
CS
CS
and V
=
resistor helps for setting constant LED
1
2
ILED (A)
(
V
0.35
CS(MIN)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
CS
CS
(
MAX
refer to electrical characteristic
I
)
LED
+
V
CS
CS
CS
for LED Current
value:
0.0200
0.0400
0.0567
0.0691
0.0800
0.1000
0.1188
0.1372
0.1536
0.1782
0.2000
I
(
2
MIN
R (W)
)
)
Size (SMD)
0402
0402
0402
0603
0603
0805
0805
0805
0805
0805
1206
9
Frequency of Operation
To calculate the frequency spread across input supply:
L is the inductance, dI is fixed (the value of the hysteresis)
V
For current rising (MOSFET is ON):
where:
For current falling (MOSFET is OFF):
where:
where
According to the above equation, choose the inductor to make
the operating frequency not beyond 1MHz.
Free Wheeling Diode
The free wheeling diode should have the reverse voltage
rating to accommodate the maximum input voltage. The
forward voltage drop should be small to get the lowest
conduction dissipation for high efficiency. The forward current
rating has to be at least equal to LED current. A Schottky
diode is recommended.
LED Ripple Current
The LED current is the same as inductor current. If LED ripple
current needs to be reduced then place a 10µF capacitor
across LED.
L
voltage across inductor L which varies by supply.
V
V
V
T
dI
t
t
F
r
f
L_RISE
L_FALL
L
SW
=
=
=
=
=
=
t
L
V
L
r
L
V (
V
CS
+
V
= V
dI
dt
V
V
V
I
= V
D
LED
L
t
(
L
D
LED
IN
dI
+
MAX
_
f
dI
_
is Schottky diode forward drop
RISE
I
is input voltage
FALL
,
IN
is average LED current:
D
LED
is total LEDs voltage drop
F
)
R
– I
+ I
−
SW
CS
⋅
R
V
LED
LED
CS
CS
=
+
L
(
T
·R
1
MIN
·R
V
⋅
dI
LED
CS
CS
)
⋅
)
- V
V (
+ V
•
D
V (
LED
+
IN
LED
V
IN
−
I
)
LED
M9999-010710-B
⋅
R
CS
MIC3201
−
V
LED
)