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Micrel, Inc.
Application Information
The internal block diagram of the MIC3202 is shown in
Figure 1. The MIC3202 is composed of a current-sense
MIC3202
Frequency of Operation
To calculate the frequency spread across input supply:
comparator, voltage and current reference, 5V regulator
and MOSFET. Hysteretic mode control, also called
bang-bang control, is a topology that does not employ an
error amplifier, using an error comparator instead.
V L = L
Δ I L
Δ t
The inductor current is controlled within a hysteretic
window. If the inductor current is too small, the power
L is the inductance, ? I L is fixed (the value of the hysteresis):
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 simplifies design and avoids
Δ I L =
V CS ( MAX ) - V CS( MIN )
R CS
problems of instability.
Transient response to load and line variation is very fast
and only depends on propagation delay. This makes the
V L is the voltage across inductor L which varies by supply.
For current rising (MOSFET is ON):
control scheme very popular to achieve low dimming
duty cycles.
LED Current and R CS
t r = L
Δ I L
V L _ RISE
The main function of the MIC3202 is to control the LED
current accurately within ± 5% of the set current. A high-
side R CS resistor sets LED current. The following
equation gives the R CS value:
where:
V L_RISE = V IN ? I LED × R CS ? V LED .
For current falling (MOSFET is OFF):
R CS =
1
2
x (
V CS ( MAX ) + V CS( MIN )
I LED
)
t f = L
Δ I L
V L _ FALL
T = t r + t f , f SW =
× R CS + V LED ) × (V IN - I LED × R CS - V LED )
f SW = D LED
R CS ( ? )
1.33
0.56
0.4
0.28
0.2
I LED (A)
0.15
0.35
0.5
0.7
1.0
I 2 R (W)
0.03
0.07
0.1
0.137
0.2
Size (SMD)
0603
0805
0805
0805
1206
where:
V L_FALL = V D + I LED × R CS + V LED
1
T
(V + I
L × Δ I L × (V D + V IN )
Table 1. R CS Values for Various LED Currents
For V CS(MAX) and V CS(MIN), refer to the Electrical
Characteristic table.
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.
September 2010
10
M9999-091710-A
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