MIC3203 Datasheet, PDF (11/21 Pages) Micrel Semiconductor – High-Brightness LED Driver Controller with High-Side Current Sense

Micrel, Inc.
MIC3203
Application Information
The internal block diagram of the MIC3203 is shown in
Figure 1. The MIC3203 is composed of a current-sense
comparator, voltage and current reference, 5V regulator
and MOSFET driver. Hysteretic mode control – also
called bang-bang control – is a topology that does not
employ an error amplifier, using an error comparator
instead.
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 MIC3203 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:
Frequency of Operation
To calculate the frequency spread across input supply:
V_L
=L
ΔI_L
Δt
L is the inductance, ∆I_L is fixed (the value of the hysteresis):
ΔI_L
=
V_CS(MAX) - V_CS(MIN)
R_CS
V_L is the voltage across inductor L which varies by supply.
For current rising (MOSFET is ON):
t_r
=L
ΔI_L
V_{L _ RISE}
where:
V_{L_RISE} = V_IN − I_LED × R_CS − V_LED
For current falling (MOSFET is OFF):
R_CS
=
1 _x( ^V_CS(MAX) ^{+ V}_CS(MIN)
2
I_LED
)
t_f
= L ^ΔIL
V_{L _ FALL}
R_CS (Ω)
1.33
0.56
0.4
0.28
0.2
0.13
0.1
0.08
0.068
Table 1. R_CS for LED Current
I_LED (A)
I²R (W)
Size (SMD)
0.15
0.03
0603
0.35
0.07
0805
0.5
0.1
0805
0.7
0.137
0805
1.0
0.2
1206
1.5
0.3
1206
2.0
0.4
2010
2.5
0.5
2010
3.0
0.6
2010
For V_CS(MAX) and V_CS(MIN), refer to the Electrical
Characteristic table.
where:
V_{L_FALL} = V_D + I_LED × R_CS + V_LED
1
T = t_r + t_f , F_SW = _T
F_SW
=
(V_D
+I_LED×R_CS + V_LED)×(V_IN -I_LED×R_CS
L×ΔI_L ×(V_D + V_IN)
-
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
March 2010
11
M9999-032910-A