Rainbow Electronics MAX8758 - Manual (Page 18)

MAX8758
Charge-Pump Flying Capacitors
Increasing the flying capacitor (C6, C17, C18) value
lowers the effective source impedance and increases
the output-current capability. Increasing the capaci-
tance indefinitely has a negligible effect on output-cur-
rent capability because the diode impedance places a
lower limit on the source impedance. Ceramic capaci-
tors of 0.1µF or greater work well in most applications
that require output currents in the order of 10mA to
20mA.
The flying capacitor’s voltage rating must exceed the
following:
V C > n x V MAIN
where n is the stage number in which the flying capaci-
tor appears, and V
MAIN is the output voltage of the
main step-up regulator.
Charge-Pump Output Capacitor
Increasing the output capacitance or decreasing the
ESR reduces the output voltage ripple and the peak-to-
peak voltage during load transients. With ceramic
capacitors, the output voltage ripple is dominated by
the capacitance value. Use the following equation to
approximate the required capacitor value:
where C MAIN_CP is the output capacitor of the charge
pump, I LOAD_CP is the load current of the charge
pump, and V
RIPPLE_CP
is the peak-to-peak value of the
output ripple.
The charge-pump output capacitor is typically also the
input capacitor for a linear regulator. Often, its value must
be increased to maintain the linear regulator’s stability.
Charge-Pump Rectifier Diodes
Use low-cost, silicon-switching diodes with a current
rating equal to or greater than two times the average
charge-pump input current. If it helps avoid an extra
stage, some or all of the diodes can be replaced with
Schottky diodes with equivalent current ratings.
PC Board Layout and Grounding
Careful PC board layout is important for proper operation.
Use the following guidelines for good PC board layout:
1) Minimize the area of high-current loops by placing
the step-up regulator’s inductor, diode, and output
capacitors near its input capacitors, its LX, and
PGND pin. The high-current input loop goes from
the positive terminal of the input capacitor to the
inductor, to the IC’s LX pin, out of PGND, and to
the input capacitor’s negative terminal. The high-
current output loop is from the positive terminal of
the input capacitor to the inductor, to the output
diode (D1), to the positive terminal of the output
capacitors, reconnecting between the output
capacitor and input capacitor ground terminals.
Connect these loop components with short, wide
connections. Avoid using vias in the high-current
paths. If vias are unavoidable, use many vias in
parallel to reduce resistance and inductance.
2) Create a power ground island (PGND) for the
step-up regulator, consisting of the input and out-
put capacitor grounds and the PGND pin.
Maximizing the width of the power ground traces
improves efficiency and reduces output voltage
ripple and noise spikes. Create an analog ground
plane (GND) consisting of the GND pin, the feed-
back-divider ground connection, the COMP and
DLP capacitor ground connections, and the
device’s exposed backside pad. Connect the
PGND and GND islands by connecting the two
ground pins directly to the exposed backside pad.
Make no other connections between these sepa-
rate ground planes.
3) Place the feedback voltage-divider resistors as
close to the feedback pin as possible. The
divider’s center trace should be kept short.
Placing the resistors far away causes the FB trace
to become antennas that can pick up switching
noise. Care should be taken to avoid running the
feedback trace near LX.
4) Place the IN pin bypass capacitor as close to the
device as possible. The ground connection of the
IN bypass capacitor should be connected directly
to the GND pin with a wide trace.
5) Minimize the length and maximize the width of the
traces between the output capacitors and the load
for best transient responses.
6) Minimize the size of the LX node while keeping it
wide and short. Keep the LX node away from
feedback node (FB) and analog ground. Use DC
traces as shield if necessary.
Refer to the MAX8758 evaluation kit for an example of
proper board layout.
C
I
f V
MAIN CP
LOAD CP
OSC RIPPLE CP
_
_
_
≥
Ч Ч 2
Step-Up Regulator with Switch Control and
Operational Amplifier for TFT LCD
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