MIC3201 Datasheet, PDF (9/16 Pages) Micrel Semiconductor – High Brightness LED Driver with High-Side Current Sense

Micrel, Inc.
MIC3201
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_CS
The main feature in MIC3201 is to control the LED
current accurately within ± 5% of set current. Choosing a
high-side R_CS resistor helps for setting constant LED
current irrespective of wide input voltage range. The
following equation gives the R_CS value:
R_CS
=
1 ₍^V_{CS (MAX )} ^{+ V}_{CS (MIN ) )}
2
I _LED
R_CS (Ω)
2.00
1.00
0.63
0.56
0.50
0.40
0.33
0.28
0.24
0.22
0.20
ILED (A)
0.1
0.2
0.3
0.35
0.4
0.5
0.6
0.7
0.8
0.9
1.0
I²R (W)
0.0200
0.0400
0.0567
0.0691
0.0800
0.1000
0.1188
0.1372
0.1536
0.1782
0.2000
Size (SMD)
0402
0402
0402
0603
0603
0805
0805
0805
0805
0805
1206
Table 1. Selecting R_CS for LED Current
For V_CS(MAX) and V_CS(MIN) refer to electrical characteristic
table.
Frequency of Operation
To calculate the frequency spread across input supply:
V_L
=
L
dI
dt
L is the inductance, dI is fixed (the value of the hysteresis)
dI = ^{VCS (MAX ) − VCS (MIN )}
R_CS
V_L voltage across inductor L which varies by supply.
For current rising (MOSFET is ON):
t_r
=
dI
L
V_{L _ RISE}
where:
V_{L_RISE} = V_IN – I_LED·R_CS - V_LED
For current falling (MOSFET is OFF):
t_f
= L ^dI
V_{L _ FALL}
where:
V_{L_FALL} = V_D + I_LED·R_CS + V_LED
T
= t_r
+t_f ,
F_SW
=¹
T
F_SW
=
(V_D
+
I_LED ⋅
R_CS
+V_LED) • (V_IN − I_LED ⋅
L⋅ dI ⋅ (V_D +V_IN )
R_CS
− V_LED )
where
V_D is Schottky diode forward drop
V_LED is total LEDs voltage drop
V_IN is input voltage
I_LED is average LED current:
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.
January, 2010
9
M9999-010710-B