MAX1542 Datasheet, PDF (16/20 Pages) Maxim Integrated Products – TFT LCD DC-to-DC Converter with Operational Amplifiers

TFT LCD DC-to-DC Converter with
Operational Amplifiers
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 0.5Ω 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):
V_RIPPLE = V_RIPPLE(ESR) + V_RIPPLE(C)
V_RIPPLE(ESR) ≅ I_PEAK x R_ESR(COUT), and
V_RIPPLE(C)
≅
I_MAIN
C_OUT


V_MAIN − V_IN
V_MAIN × ƒ_OSC


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:
R₁
=

R_{2 }
V_MAIN
V_FB
−

1_
where V_FB, the step-up converter feedback set point, is
1.24V. Since the input bias current into FB is typically
zero, R₂ can have a value up to 100kΩ 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:
R_COMP ≅ ⁵⁰⁰
x V_IN
Lx
x V_OUT x
I_MAIN(MAX)
C_OUT
C_COMP ≅
10
V_OUT x C_OUT
x I_MAIN(MAX) x R_COMP
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
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