Rainbow Electronics MAX747 - Manual (Page 11)

MAX747
High-Efficiency PWM, Step-Down
P-Channel DC-DC Controller
______________________________________________________________________________________ 11
Setting the Low-Battery Detector Voltage
Select R1 between 10k
Ω
and 1M
Ω
.
Connect a pull-up resistor (e.g., 100k
Ω
) between LBO
and V
OUT
(Figure 4).
__________Applications Information
Layout Considerations
Due to high current levels and fast switching
waveforms, which radiate noise, proper MAX747 PC
board layout is essential. Protect sensitive analog
grounds by using a star ground configuration. Use an
adequate ground plane and minimize ground noise by
connecting GND, the anode of the steering Schottky
diode, the input bypass capacitor ground lead, and the
output filter capacitor ground lead to a single point
(star ground configuration). Also, minimize lead lengths
to minimize stray capacitance, trace resistance, and
radiated noise. Place bypass capacitor C3 as close as
possible to V+ and GND.
AV+ and CS are the inputs to the differential-input
current-sense amplifier. Use a Kelvin connection
across the sense resistor as shown in Figure 6. Note
that even though AV+ also functions as the supply
voltage for sensitive analog circuitry, a separate AV+
bypass capacitor should not be used. By not using a
capacitor, any noise appearing at the CS input will also
appear at the AV+ input and will appear as a common-
mode signal to the current-sense amplifier. A separate
AV+ capacitor causes the noise to appear only on one
input, and this differential noise will be amplified,
adversely affecting circuit operation.
Similarly, CC (or FB in adjustable-output operation) is a
sensitive input that should not be shorted to any node.
Avoid shorting CC when probing the circuit, as this
may damage the device.
Switching Waveforms
A region exists between CCM and DCM where the
inductor current operates in both modes, as shown in
the Idle-Mode Moderate current EXT waveform in the
Typical Operating Characteristics. As the output
voltage varies, it is fed back into CC and the duty cycle
is adjusted to compensate for this change. The switch
is considered off when V
EXT
≤
the P-FET’s V
GS
threshold voltage. Once the switch is off, the voltage at
EXT is pulled to V+ and the P-FET drain voltage is a
Schottky diode drop below GND. However, in this “in-
between” mode (due to the changing duty cycle
inherent with DCM), when the device is at maximum
duty cycle, EXT turns off at V+ - V
GS
. But it is not
always pulled to V+ because the switch sometimes
turns on again after a minimum off-time before EXT can
be pulled to V+. The result is short spikes that appear
on the EXT waveform in the
Typical Operating
Characteristics.
AC Stability with Low
Input/Output Differentials
At low input/output differentials, the inductor current
cannot slew quickly to respond to load changes, so the
output filter capacitor must hold up the voltage as the
load transient is applied. In Figure 1a’s circuit, for
V+ = 6.5V, increase the output filter capacitor to 700µF
(Sprague 595D low-ESR capacitors) to obtain a
transient response less than 250mV with a load step
from 200mA to 2.5A. For V+ = 6V and V
OUT
= 5V,
increase the output filter capacitor to approximately
1000µF. As V+ increases, the device will no longer be
operating near full duty cycle with light loads, allowing
it to adjust to full duty cycle when the load transient is
applied and, in turn, allowing smaller output filter
capacitors to be used.
Dual-Mode Operation
The MAX747 is designed in either fixed-output mode
(5V-output, FB = GND) or in adjustable mode (FB = 2V)
using a resistor divider. It is not designed to be
switched from one mode to another when powered up;
however, in adjustment mode, switching between two
different resistor dividers is acceptable.
R2 R1
(V V
V
TRIP REF
REF
=
− 





)
AV+
MAX747
V IN
CS
RSENSE
P EXT
L1 V OUT
KELVIN SENSE
CONNECTION
V+
Figure 6. Kelvin Connection for Current-Sense Amplifier