10
Date: 11/30/04 SP6644/6645 High Efficiency Boost Regulator © Copyright 2004 Sipex Corporation
Internal Bootstrap Circuitry
The internal bootstrap circuitry contains a
low-voltage start-up oscillator that pumps up the
output voltage to approximately 1.9V so the
main DC-DC converter can function. At lower
battery supply voltages, the circuitry can start up
with low-load conditions. Designers can reduce
the load as needed to allow start-up with input
voltages below 1V. Refer to Figures 10 to
13. Once started, the output voltage can maintain
the load as the battery voltage decreases below
the initial start-up voltage. The start-up oscillator
is powered by V
BATT
driving a charge pump and
NMOS switch. During start-up, the P-channel
synchronous rectifier remains off and either its
body diode or an external diode is used as an
output rectifier.
BATTLO Circuitry
The SP6644 device has an internal comparator
for low-battery detection. If V
BATT
drops below
1V, BATTLO will sink current. BATTLO is an
open-drain output. The SP6645 operates in the
same manner with a threshold voltage of 2V.
Shutdown for the SP6644
A logic LOW at SHDN will drive the SP6644
into a shutdown mode where BATTLO goes
into a high-impedance state, the internal
switching MOSFET turns off, and the
synchronous rectifier turns off to prevent
reverse current from flowing from the output
back to the input. Designers should note that
in shutdown, the output can drift to one diode
drop below V
BATT
because there is still a forward
current path through the synchronous-rectifier
body diode from the input to the output.
To disable the shutdown feature, designers can
connect SHDN to V
BATT
.
Adjustable Output Voltage
Driving FB to ground (logic LOW) will drive the
output voltage to the fixed-voltage operation of
+3.3V + 4%. Connecting FB to a voltage divider
between V
OUT
and ground will select an adjustable
output voltage between +2V and +5.5V. Refer to
Figure 28. FB regulates to +1.25V.
Since the FB leakage current is 10nA maximum,
designers should select the feedback resistor
R2 in the 100kΩ to 1MΩ range. R1 can be
determined with the following equation:
R1 = R2 x -1
V
OUT
V
REF
where R1 [Ω] and R2 [Ω] are the feedback
resistors in Figure 29, V
OUT
[V] is the output
voltage, and V
REF
[V] is 1.25V.
Battery Reversal Protection
The SP6644/6645 devices will tolerate single-
cell battery reversal up to the package power-
dissipation limits noted in the ABSOLUTE
MAXIMUM RATINGS section. An internal
diode in series with an internal 5Ω resistor limits
any reverse current to less than 220mA
preventing damage to the devices. Prolonged
operation above 220mA reverse-battery
current can degrade performance of the devices.
The Inductor
The programmable peak inductor current feature
of the SP6644/6645 devices affords a great deal
of flexibility in choosing an inductor. The most
important point to consider when choosing an
inductor is to insure that the peak inductor current
is programmed below the saturation rating of the
inductor. If the inductor goes into saturation, the
internal switches and the inductor will be stressed
due to current peaking, potentially leading to
reliability problems with the application circuit.
The peak inductor current is programmed by
putting a resistor between the R
LIM
pin and ground.
The usable current range is between 150mA and
560mA. This is defined by:
where I
PEAK
[A] is the peak inductor current, and
R
LIM
[Ω] is the value of the resistor connected
from pin R
LIM
to ground.
I
PEAK
=
1400
R
LIM
