MAX1559 Datasheet, PDF (11/14 Pages) Maxim Integrated Products – 5-Output Power-Management IC For Low-Cost PDAs

5-Output Power-Management IC For
Low-Cost PDAs
IN
SWIN
MAX1559
LDO
CONTROL
REF
MAIN
RS
3.3V, 500mA
RESET
OUTPUT
ON
OFF
ENSD
ENC2
ENLCD
LDO
CONTROL
LDO
CONTROL
LDO
CONTROL
LCD OFF
SWITCH
COR1
SDIG
COR2
SW
1V, 250mA
3.3V, 400mA
1.8V, 30mA
LCD
BOOST
LX
LCD
20V
1mA
LFB
BIAS
REF
CURRENT
REF
GND
THSD
Figure 2. Functional Diagram
The circuit consists of the PWM source, capacitor C10,
and resistors R_D and R_W. To analyze the transfer func-
tion of the PWM circuit, it is easiest to first simplify it to
its Thevenin equivalent. The Thevenin voltage can be
calculated using the following formula:
V_THEV = (D ✕ V_OH) + (1 - D) ✕ V_OL
where D is the duty cycle of the PWM signal, V_OH is the
PWM output high level (often 3.3V), and V_OL is the
PWM output low level (usually 0V). For CMOS logic, this
equation simplifies to:
V_THEV = D ✕ V_DD
______________________________________________________________________________________ 11