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MAQ3203 View Datasheet(PDF) - Micrel

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MAQ3203 Datasheet PDF : 23 Pages
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Micrel, Inc.
MAQ3203
Application Information
The internal block diagram of the MAQ3203 is shown in
Figure 1. The MAQ3203 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 RCS
The main feature in MAQ3203 is to control the LED
current accurately within ±5% of set current. Choosing a
high-side RCS resistor helps for setting constant LED
current irrespective of wide input voltage range. The
following equation gives the RCS value:
Frequency of Operation
To calculate the frequency spread across input supply:
VL
=L
ΔIL
Δt
L is the inductance, IL is fixed (the value of the hysteresis):
ΔIL
=
VCS(MAX) - VCS(MIN)
RCS
VL is the voltage across inductor L which varies by supply.
For current rising (MOSFET is ON):
tr
=L
ΔIL
VL _ RISE
where:
VL_RISE = VIN ILED × RCS VLED
For current falling (MOSFET is OFF):
RCS
=
1 x( VCS(MAX) + VCS(MIN)
2
ILED
)
tf
= L ΔIL
VL _ FALL
RCS ()
1.33
0.56
0.4
0.28
0.2
0.13
0.1
0.08
0.068
Table 1. RCS for LED Current
ILED (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 VCS(MAX) and VCS(MIN), refer to the Electrical
Characteristic table.
where:
VL_FALL = VD + ILED × RCS + VLED
1
T = tr + tf , FSW = T
FSW
=
(VD
+ILED×RCS + VLED)×(VIN -ILED×RCS
ΔIL ×(VD + VIN)
-
VLED)
where :
VD is Schottky diode forward drop
VLED is total LEDs voltage drop
VIN is input voltage
ILED is average LED current
March 2011
11
M9999-032411-A
 

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