MAX1516 Datasheet, PDF (20/26 Pages) Maxim Integrated Products – TFT-LCD DC-DC Converters with Operational Amplifiers

TFT-LCD DC-DC Converters with
Operational Amplifiers
Output-Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing of the output capacitance, and the ohmic ripple due
to the capacitor’s equivalent series resistance (ESR).
V_RIPPLE = V_RIPPLE(C) + V_RIPPLE(ESR)
V_RIPPLE(C)
≈
I_MAIN
C_OUT


V_MAIN − V_IN
V_MAINf_OSC

^,
and
V_RIPPLE(ESR) ≈ I_PEAKR_ESR(COUT)
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_RIPPLE(C). The voltage rating and temperature charac-
teristics 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 IC. A 22µF ceramic capacitor is used in the
Typical Applications Circuit (Figure 1) 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 Applications Circuit. Ensure a low-noise
supply at IN by using adequate C_IN. Alternately,
greater voltage variation can be tolerated on C_IN if IN is
decoupled from C_IN using an RC lowpass filter (see
R10 and C18 in Figure 1).
Rectifier Diode
The MAX1516/MAX1517/MAX1518s’ high switching fre-
quency demands a high-speed rectifier. Schottky
diodes are recommended for most applications
because of their fast recovery time and low forward
voltage. In general, a 2A Schottky diode complements
the internal MOSFET well.
Output-Voltage Selection
The output voltage of the main step-up regulator can be
adjusted by connecting a resistive voltage-divider from
the output (V_MAIN) to AGND with the center tap connect-
ed to FB (see Figure 1). Select R2 in the 10kΩ to 50kΩ
range. Calculate R1 with the following equation:
R1= R2
×


V_MAIN
V_FB
− 1^{}
where V_FB, the step-up regulator’s feedback set point,
is 1.236V. Place R1 and R2 close to the IC.
Generating Output Voltages >13V
The maximum output voltage of the step-up regulator is
13V, which is limited by the absolute maximum rating of
the internal power MOSFET. To achieve higher output
voltages, an external n-channel MOSFET can be cascod-
ed with the internal FET (Figure 8). Since the gate of the
external FET is biased from the input supply, use a logic-
level FET to ensure that the FET is fully enhanced at the
minimum input voltage. The current rating of the FET
needs to be higher than the IC’s internal current limit.
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
≈
315
× V_IN × V_OUT × C_OUT
L × I_MAIN(MAX)
C_COMP
≈
10
V_OUT × C_OUT
× I_MAIN(MAX) × 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
requirement. Figures 9 and 10 show the positive and
negative charge-pump output voltages for a given
V_MAIN for one-, two-, and three-stage charge pumps.
The number of positive charge-pump stages is given by:
n_POS
=
V_GON
+ V_DROPOUT −
V_MAIN − 2 × V_D
V_MAIN
where n_POS is the number of positive charge-pump
stages, V_GON is the gate-on linear-regulator REG P out-
put, V_MAIN is the main step-up regulator output, V_D is
the forward-voltage drop of the charge-pump diode,
and V_DROPOUT is the dropout margin for the linear reg-
ulator. Use V_DROPOUT = 0.3V.
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