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Micrel, Inc.
Application Information
The internal block diagram of the MAQ3203 is shown in
Figure 1. The MAQ3203 is composed of a current-sense
MAQ3203
Frequency of Operation
To calculate the frequency spread across input supply:
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
V L = L
Δ I L
Δ t
instead.
The inductor current is controlled within a hysteretic
L is the inductance, ? I L is fixed (the value of the hysteresis):
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
Δ I L =
V CS ( MAX ) - V CS( MIN )
R CS
loop compensation, it makes a design easy, and avoids
problems of instability.
Transient response to load and line variation is very fast
V L is the voltage across inductor L which varies by supply.
For current rising (MOSFET is ON):
and only depends on propagation delay. This makes the
control scheme very popular for certain applications.
LED Current and R CS
t r = L
Δ I L
V L _ RISE
The main feature in MAQ3203 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:
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
R CS ( ? )
Table 1. R CS for LED Current
I LED (A) I 2 R (W)
Size (SMD)
where:
V L_FALL = V D + I LED × R CS + V LED
1.33
0.56
0.15
0.35
0.03
0.07
0603
0805
T = t r + t f , F SW =
1
T
0.4
0.28
0.2
0.13
0.5
0.7
1.0
1.5
0.1
0.137
0.2
0.3
0805
0805
1206
1206
where :
F SW =
(V D + I LED × R CS + V LED ) × (V IN - I LED × R CS - V LED )
L × Δ I L × ( V D + V IN )
0.1
0.08
0.068
2.0
2.5
3.0
0.4
0.5
0.6
2010
2010
2010
?
?
?
V D is Schottky diode forward drop
V LED is total LEDs voltage drop
V IN is input voltage
For V CS(MAX) and V CS(MIN), refer to the Electrical
Characteristic table.
?
I LED is average LED current
March 2011
11
M9999-032411-A
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