Rainbow Electronics MAX1801 - Manual (Page 8)

MAX1801
Digital Camera Step-Up Slave
DC-DC Controller
8 _______________________________________________________________________________________
Oscillator
The MAX1801 requires a 0 to 1.25V sawtooth oscillator
signal that is obtained from the MAX1800 or the
MAX1802 (at OSC). The 100kHz to 1MHz oscillator sig-
nal sets the converter switching frequency, and it is
used to control pulse-width modulation and maximum
duty cycle.
Maximum Duty Cycle
The MAX1801 uses the master-generated oscillator sig-
nal at OSC, the voltage at DCON, and an internal com-
parator to limit its maximum switching duty cycle (see
Setting the Maximum Duty Cycle). Limiting the duty
cycle can prevent saturation in some magnetic compo-
nents. A low maximum duty cycle can also force the
converter to operate in discontinuous current mode,
simplifying design stability at the cost of a slight reduc-
tion in efficiency.
Soft-Start
The MAX1801 features a soft-start function that limits
inrush current and prevents excessive battery loading
at startup by ramping the output voltage to the regula-
tion voltage. This is achieved by increasing the internal
reference to the transconductance amplifier from 0 to
the 1.25V reference voltage over 1024 oscillator cycles
when initial power is applied or when the part is taken
out of shutdown or sleep mode.
R Q
S
CLK
REFI
DL
FAULT
PROTECTION ENABLE
LEVEL
SHIFT
REF
COMP
FB
DCON
OSC
0.4V
1.1V
2.35V
IN
SOFT-
START*
*SOFT-START RAMPS REFI FROM 0 TO V REF IN 1024 CLK CYCLES.
IC
POWER
POWER-ON
Figure 2. PWM Controller Block Diagram