MAX1565ETJ Maxim Integrated Products, MAX1565ETJ Datasheet - Page 17

• Manufacturer Part Number: MAX1565ETJ
• Description: DC/DC Switching Converters 5Ch Digital Camera Power Supply
• Manufacturer: Maxim Integrated Products

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.
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.
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.
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
of a cycle, the timing capacitor charges through the
resistor until it reaches V
t
1
OSC
= -R
Setting the Switching Frequency
) and capacitor (C
Low-Voltage Startup Oscillator
OSC
______________________________________________________________________________________
C
OSC
REF
ln [1 - 1.25/V
Design Procedure
. The charge time, t
Small, High-Efficiency, Five-Channel
OSC
Digital Still Camera Power Supply
). At the beginning
OUTSU
Shutdown
Soft-Start
]
1
, is:
The capacitor voltage is then given time (t
discharge. The oscillator frequency is
f
between 47pF and 470pF. Determine R
equation:
See the Typical Operating Characteristics for f
ROSC using different values of COSC.
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
late the high-side (output-to-FB_) resistor (R
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.
Table 1. Voltage Setting Summary
OSC
FBSU
FBSD
FB1
FB2
FB2
CHANNEL FB_
R
OSC
can operate from 100kHz to 1MHz. Choose C
= (300ns - 1/f
General Filter Capacitor Selection
R
H
FB THRESHOLD
(FBSEL_ LOW)
(FBSEL is not provided for these channels)
= R
L
f
OSC
), to be 100kΩ or less. Then calcu-
OSC
3.35V
L
1.5V
5V
[(V
Setting Output Voltages
= 1/(t
)/(C
OUT
OSC
1
Always 1.25V
/1.25) - 1]
+ t
ln [1 - 1.25/V
2
)
FB THRESHOLD
(FBSEL_ HIGH)
OSC
1.25V
2
H
) using:
from the
= 300ns) to
OSC
OUTSU
versus
OSC
])
17