Microchip Technology DM183037 Datenbogen

 2012 Microchip Technology Inc.

DS30575A-page 263

When designing applications that use the LCD regula-
tor with the charge pump enabled, users must always
consider both the dynamic current and RMS (static)
current requirements of the display, and what the
charge pump can deliver. Both dynamic and static
current can be determined by 

For dynamic current, C, is the value of the capacitors
attached to LCDBIAS3 and LCDBIAS2. The variable,
dV, is the voltage drop allowed on C2 and C3 during a
voltage switch on the LCD display, and dt is the duration
of the transient current after a clock pulse occurs.

For practical design purposes, it will be assumed to be
0.047 µF for C, 0.1V for dV and 1 µs for dt. This yields
a dynamic current of 4.7 mA for 1 µs.
RMS current is determined by the value of C

FLY

 for C,

the voltage across V

LCAP

1 and V

LCAP

2 for dV and the

regulator clock period (TPER) for dt. Assuming a C

FLY

value of 0.047 µF, a value of 1.02V across C

FLY

 and

TPER of 30 µs, the maximum theoretical static current
will be 1.8 mA. Since the charge pump must charge five
capacitors, the maximum current becomes 360 µA.
For a real-world assumption of 50% efficiency, this
yields a practical current of 180 µA. Users should com-
pare the calculated current capacity against the
requirements of the LCD. While dV and dt are relatively
fixed by device design, the values of C

FLY

 and the

capacitors on the LCDBIAS pins can be changed to
increase or decrease current. As always, any changes
should be evaluated in the actual circuit for their impact
on the application.

I = C x dV

dt