MAX1565 Datasheet, PDF (17/26 Pages) Maxim Integrated Products – Small, High-Efficiency, Five-Channel Digital Still Camera Power Supply

Small, High-Efficiency, Five-Channel
Digital Still Camera Power Supply
Low-Voltage Startup Oscillator
The MAX1565 internal control and reference-voltage
circuitry receive power from OUTSU and do not function
when OUTSU is less than 2.5V. To ensure low-voltage
startup, the step-up employs a low-voltage startup
oscillator that activates at 0.9V. The startup oscillator
drives the internal N-channel MOSFET at LXSU until
OUTSU reaches 2.5V, at which point voltage control is
passed to the current-mode PWM circuitry.
Once in regulation, the MAX1565 operates with inputs
as low as 0.7V since internal power for the IC is sup-
plied by OUTSU. At low input voltages, the MAX1565
can have difficulty starting into heavy loads.
Soft-Start
The MAX1565 step-down and AUX_ channels feature a
soft-start function that limits inrush current and prevents
excessive battery loading at startup by ramping the
output voltage to the regulation voltage. This is
achieved by increasing the internal reference inputs to
the controller transconductance amplifiers from 0V to
the 1.25V reference voltage over 4096 oscillator cycles
(8ms at 500kHz) when initial power is applied or when
a channel is enabled. Soft-start is not included in the
step-up converter in order to avoid limiting startup
capability with loading.
Shutdown
The step-up converter is activated with a high input at
ONSU. The step-down and auxiliary DC-to-DC convert-
ers 1, 2, and 3 activate with a high input at ONSD,
ON1, ON2, and ON3, respectively. The auxiliary con-
trollers and step-down cannot be activated until
OUTSU is in regulation. For automatic startup, connect
ON_ to OUTSU or a logic level greater than 1.6V.
Design Procedure
Setting the Switching Frequency
Choose a switching frequency to optimize external
component size or circuit efficiency for any particular
MAX1565 application. Typically, switching frequencies
between 300kHz and 600kHz 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, t₁, is:
t₁ = -R_OSCC_OSC ln [1 - 1.25/V_OUTSU]
Table 1. Voltage Setting Summary
CHANNEL FB_ ^{FB THRESHOLD}
(FBSEL_ LOW)
FB THRESHOLD
(FBSEL_ HIGH)
FBSU
3.35V
FBSD
1.5V
1.25V
FB1
5V
FB2
Always 1.25V
FB2
(FBSEL is not provided for these channels)
The capacitor voltage is then given time (t₂ = 300ns) to
discharge. The oscillator frequency is
f_OSC = 1/(t₁ + t₂)
f_OSC can operate from 100kHz to 1MHz. Choose C_OSC
between 47pF and 470pF. Determine R_OSC from the
equation:
R_OSC = (300ns - 1/f_OSC)/(C_OSC ln [1 - 1.25/V_OUTSU])
See the Typical Operating Characteristics for f_OSC versus
ROSC using different values of COSC.
Setting Output Voltages
The MAX1565 step-up/step-down converters and the
AUX1 controllers have both factory-set and adjustable
output voltages. These are selected by FBSEL_ for the
appropriate channel. When FBSEL_ is low, the channel
output regulates at its preset voltage. When FBSEL_ is
high, the channel regulates FB_ at 1.25V for use with
external feedback resistors.
When setting the voltage for auxiliary channels 2 and 3,
or when using external feedback at FBSU, FBSD, or FB1,
connect a resistive voltage-divider from the output volt-
age to the corresponding FB_ input. The FB_ input bias
current is less than 100nA, so choose the low-side (FB_-
to-GND) resistor (R_L), to be 100kΩ or less. Then calcu-
late the high-side (output-to-FB_) resistor (R_H) using:
R_H = R_L [(V_OUT/1.25) - 1]
General Filter Capacitor Selection
The input capacitor in a DC-to-DC converter reduces
current peaks drawn from the battery, or other input
power source, and reduces switching noise in the con-
troller. The impedance of the input capacitor at the
switching frequency should be less than that of the
input source so that high-frequency switching currents
do not pass through the input source.
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