Rainbow Electronics MAX1800 - Manual (Page 13)

MAX1800
Digital Camera Step-Up
Power Supply
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heavy loads (see the Startup Current vs. Input Voltage
graph in the Typical Operating Characteristics).
Maximum Duty Cycle
The MAX1800 auxiliary controllers use the sawtooth
oscillator signal generated at OSC, the voltage at
DCON_, and an internal comparator to limit their maxi-
mum duty cycles (see Setting the Maximum Duty
Cycle). Limiting the duty cycle can prevent saturation in
some magnetic components. A low maximum duty
cycle can also force the converter to operate in discon-
tinuous current mode, simplifying design stability at the
cost of a slight reduction in efficiency.
Soft-Start
The MAX1800 gain block and auxiliary controllers fea-
ture a soft-start function that limits inrush current and
prevents excessive battery loading at startup by ramp-
ing the output voltage to the regulation voltage. This is
achieved by increasing the internal reference inputs to
the controller transconductance amplifiers from 0 to the
1.25V reference voltage over 1024 oscillator cycles
when initial power is applied or when the controller is
enabled.
Overload Protection
The MAX1800 auxiliary controllers have a fault protec-
tion that prevents damage to transformer-coupled or
single-ended primary inductance converter (SEPIC) cir-
cuits due to an output overload. When the output volt-
age drops out of regulation for 1024 oscillator clock
periods, the auxiliary controller is disabled to prevent
excessive output current. Restart the controller by
cycling the voltage at ON_ or DCON_ to GND and back
to the on state. For a step-up application, short-circuit
current is not limited, due to the DC current path
through the inductor and output rectifier to the short-cir-
cuit. If short-circuit protection is required in a step-up
configuration, a protection device such as a fuse must
be used to limit short-circuit current.
Ready-Main (RDYM) Output
The MAX1800 power-ready RDYM comparator open-
drain output sinks up to 1mA if the main output drops
10% below its regulation voltage. When FBM exceeds
the RDYM trip level, the RDYM output becomes high
impedance to indicate that the main output is within the
limits of regulation. The RDYM comparator has 1% hys-
teresis to prevent oscillations near the trip threshold.
Connect RDYM to POUT with a 1M
Ω pullup resistor.
Shutdown
The main DC-DC converter shuts down with a low input
at ONM. Auxiliary DC-DC converters 1, 2, and 3, and the
uncommitted gain block shut down with low inputs at
ON1, ON2, ON3, and ONA, respectively. The auxiliary
converters and the gain block cannot be activated until
the MAIN output reaches the RDYM trip threshold.
Typical shutdown supply current is 2nA. For automatic
startup, connect ON_ to POUT. When ONA is low to dis-
able the gain block, AO is driven to POUT.
Design Procedure
Setting the Switching Frequency
Choose a switching frequency to optimize external com-
ponent size or circuit efficiency for the particular
MAX1800 application. Typically, switching frequencies
between 400kHz and 500kHz offer a good balance
between component size and circuit efficiency—higher
frequencies generally allow smaller components, and
lower frequencies give better conversion efficiency.
The switching frequency is set with an external timing
resistor (R
OSC ) and capacitor (C OSC ). At the beginning
of a cycle, the timing capacitor charges through the
resistor until it reaches V
REF . The charge time t 1 is:
and it decays to zero over time t 2 = 100ns. The oscilla-
tor frequency is f OSC = 1 / (t 1 + t 2 ). Choose f OSC in the
range 100kHz < f OSC < 1MHz. Choose C OSC between
22pF and 470pF. Determine R
OSC
from the relation:
See the Typical Operating Characteristics for f OSC ver-
sus R OSC using different values of C OSC .
R
ns
f
C
V
OSC
OSC
OSC
POUT
=
−
−






100
1
1
1 25
ln
.
t R C
V
V
OSC OSC
REF
POUT
1
1 = − −





 ln
C OSC
V
REF
(1.25V)
V POUT
R OSC
OSC
100ns
ONE-SHOT
MAX1800
Figure 4. Master Oscillator