QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1242A
600MA SYNCHRONOUS BOOST DC/DC CONVERTER AND 100MA LDO REGULATOR
3
regulated output voltage. A data sheet curve
shows the minimum resistive load vs. Vin that
will allow the boost converter to start.
Increase the input voltage to approximately
1.5V or more and increase the load current.
The maximum load current depends on the
input and output voltage settings. A data sheet
curve shows the maximum output current for
different input and output voltages.
The PWRGOOD output terminals are high
when the output voltages are in regulation and
pull low if the output voltage drops by ap-
proximately 10%, for any reason.
The LOW BAT output terminal is high when
the voltage present on the VINBOOST termi-
nal is greater than the voltage threshold se-
lected by jumper JP2.
The input voltage can exceed the output volt-
age and still maintain regulation, although the
load current is less and efficiency is lower.
When Vin is near VOUT and in the range of
approximately 300mV to 100mV below Vout,
the converter is approaching the switch mini-
mum ON time resulting in an increased output
voltage ripple similar to BURST mode opera-
tion. This ripple can be reduced by adding ad-
ditional output capacitance.
Either regulator can be individually shut down
by moving jumpers JP7 or JP8 to the OFF po-
sition. Shutting down the Boost Converter
drops the converter’s output voltage to near
0V.
Jumpers JP7 and JP8 also can also be set for
regulator output voltage sequencing. Depend-
ing on the jumper setting, one of the regula-
tors will remain shut down until the other regu-
lator has reached regulation.
The linear regulator operation can be verified
by increasing the input supply voltage to 5V
and applying a suitable load on the VOUT
LDO terminal.
If LED indicators for the two Power Good out-
puts and the Low Battery output are desired,
the surface mount 1206 resistors (R5, R7 &
R10) can be removed and replaced by sur-
face mount LEDs.
When evaluating the circuit at low input volt-
ages, it is important to monitor the input volt-
age directly at the input terminals of the circuit
board. At very low input voltages, voltage
drops in the power supply wire and Ammeter
will result in the input voltage at the input ter-
minals dropping below the minimum voltage
required for operation.
Additional pc board pads are provided for an
optional input bypass capacitor (C1). It maybe
necessary when using long wires between the
power supply and circuit board, or for adding
a tantalum capacitor to minimize input voltage
transients that may occur when the input is
hot-switched. Also, pads on the board back-
side are provided for adding a small Schottky
diode (D1), which can increase the efficiency
slightly under some conditions. But adding a
diode defeats the output disconnect and short
circuit protection features.
When verifying output ripple, it is important to
use the scope probe connection as shown in
figure 2.
With JP1 in the Burst position, the converter
will operate in a low quiescent current burst
mode for light load current conditions and
switch to fixed frequency (PWM) operation
under heavier loads. In Burst mode, with light
loads, the output ripple voltage is higher than
in non-burst mode.
Applications requiring BURST mode operation
with input voltages below 1.2V, boosting to a
voltage between 4 and 5V out, may experi-
ence non-BURST mode operation at light load
conditions. This will result in higher light load
quiescent current. See data sheet curves
“Burst Mode Threshold Current vs VIN” for
additional information.
