Rainbow Electronics MAX1543 User Manual
Page 16
MAX1542/MAX1543
TFT LCD DC-to-DC Converter with
Operational Amplifiers
16
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operating point (
η
MIN
) taken from an appropriate curve
in the Typical Operating Characteristics:
I
IN(DC,MAX)
= I
MAIN(MAX)
✕
V
MAIN
/ (V
IN(MIN)
✕
η
MIN
)
Calculate the ripple current at that operating point and
the peak current required for the inductor:
I
RIPPLE
= V
IN(MIN)
✕
(V
MAIN
-V
IN(MIN)
) / (L
✕
f
OSC
✕
V
MAIN
)
I
PEAK
= I
IN(DC,MAX)
+ (I
RIPPLE
) / 2
The inductor’s saturation current rating and the
MAX1542/MAX1543s’ LX current limit (I
LIM
) should
exceed I
PEAK
and the inductor’s DC current rating
should exceed I
IN(DC,MAX)
. For reasonable efficiency,
choose an inductor with less than 0.5
Ω series resis-
tance.
Considering the Typical Application Circuits, the maxi-
mum load current (I
MAIN(MAX)
) is 200mA with an 8V
output and a typical input voltage of 3.3V.
Choosing an LIR of 0.6 and estimating efficiency of
85% at this operating point:
L = (3.3V)
2 ✕
0.85
✕
(8V - 3.3V) / ((8V)
2 ✕
0.6
✕
0.2A
✕
1.2MHz) = 4.7µH
Using the circuit’s minimum input voltage (2.7V) and
estimating efficiency of 80% at that operating point,
I
IN(DC,MAX)
= (0.2A
✕
8V / (2.7V
✕
0.8)) = 741mA
The ripple current and the peak current are:
I
RIPPLE
= 2.7V
✕
(8V - 2.7V) / (4.7µH
✕
1.2MHz
✕
8V)
= 317mA
I
PEAK
= 741mA + (317mA / 2) = 900mA
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and dis-
charging of the output capacitance, and the ohmic rip-
ple due to the capacitor’s equivalent series resistance
(ESR):
where I
PEAK
is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors, the
output voltage ripple is typically dominated by V
RIP-
PLE(C)
. The voltage rating and temperature characteris-
tics of the output capacitor must also be considered.
Input Capacitor Selection
The input capacitor (C
IN
) reduces the current peaks
drawn from the input supply and reduces noise injec-
tion into the device. A 10µF ceramic capacitor is used
in the Typical Application Circuits (Figures 1 and 2)
because of the high source impedance seen in typical
lab setups. Actual applications usually have much
lower source impedance since the step-up regulator
often runs directly from the output of another regulated
supply. Typically, C
IN
can be reduced below the values
used in the Typical Application Circuits. Ensure a low-
noise supply at IN by using adequate C
IN
.
Output Voltage
The MAX1542/MAX1543 operate with an adjustable out-
put from V
IN
to 13V. Connect a resistive voltage-divider
to FB (Typical Application Circuits) from the output
(V
MAIN
) to AGND. Select the resistor values as follows:
where V
FB
, the step-up converter feedback set point, is
1.24V. Since the input bias current into FB is typically
zero, R
2
can have a value up to 100k
Ω without sacrific-
ing accuracy, although lower values provide better
noise immunity. Connect the resistor-divider as close to
the IC as possible.
Loop Compensation
Choose R
COMP
to set the high-frequency integrator
gain for fast transient response. Choose C
COMP
to set
the integrator zero to maintain loop stability.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
To further optimize transient response, vary R
COMP
in
20% steps and C
COMP
in 50% steps while observing
transient response waveforms.
Charge Pumps
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meet the output
requirements. Figures 5 and 6 show the positive and
R
x V
x V
x C
L x I
C
V
x C
x I
x R
COMP
IN
OUT
OUT
MAIN MAX
COMP
OUT
OUT
MAIN MAX
COMP
≅
≅
500
10
(
)
(
)
R
R
V
V
MAIN
FB
1
2
1
=
−
V
V
V
V
I
x R
and
V
I
C
V
V
V
RIPPLE
RIPPLE ESR
RIPPLE C
RIPPLE ESR
PEAK
ESR COUT
RIPPLE C
MAIN
OUT
MAIN
IN
MAIN
OSC
=
+
≅
≅
−
× ƒ
(
)
( )
(
)
(
)
( )
,