MAX1705/MAX1706
1- to 3-Cell, High-Current, Low-Noise,
Step-Up DC-DC Converters with Linear Regulator
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regulation for the first time. However when power is first
applied, LDO may be on before POUT reaches regula-
tion. If this is not acceptable, the chip should be held in
shutdown when input voltage is first appled to ensure
that the linear regulator is off until POUT is ready.
The linear regulator in the MAX1705/MAX1706 features
a 0.5Ω, p-channel MOSFET pass transistor. This pro-
vides several advantages, including longer battery life,
over similar designs using a pnp pass transistor. The p-
channel MOSFET requires no base-drive current, which
reduces quiescent current considerably. PNP-based
regulators tend to waste base-drive current in dropout
when the pass transistor saturates. The
MAX1705/MAX1706 eliminate this problem.
The linear-regulator error amplifier compares the output
feedback sensed at the FBLDO input against the inter-
nal 1.250V reference, and amplifies the difference
(Figure 1). The MOSFET driver reads the error signal
and applies the appropriate drive to the p-channel
pass transistor. If the feedback signal is lower than the
reference, the pass-transistor gate is pulled lower,
allowing more current to pass to the output, thereby
increasing the output voltage. If the feedback voltage is
too high, the pass-transistor gate is pulled up, allowing
less current to pass to the output. Additional blocks
include a current-limiting block and a thermal-overload
protection block.
Low-Voltage Startup Oscillator
The MAX1705/MAX1706 use a CMOS, low-voltage start-
up oscillator for a 1.1V guaranteed minimum startup
input voltage at +25°C. On startup, the low-voltage oscil-
lator switches the n-channel MOSFET until the output
voltage reaches 2.15V. Above this level, the normal step-
up converter feedback and control circuitry take over.
Once the device is in regulation, it can operate down to a
0.7V input, since internal power for the IC is boot-
strapped from the output using the OUT pin.
To reduce current loading during step-up, the linear
regulator is kept off until the startup converter goes into
regulation. Minimum startup voltage is influenced by
load and temperature (see the Typical Operating
Characteristics). To allow proper startup, do not apply
a full load at POUT until after the device has exited
startup mode and entered normal operation.
Shutdown
The MAX1705/MAX1706 feature a shutdown mode that
reduces quiescent current to less than 1µA, preserving
battery life when the system is not in use. During shut-
down, the reference, the low-battery comparator, and
all feedback and control circuitry are off. The step-up
converter’s output drops to one Schottky diode drop
below the input, but the linear regulator output is
turned off.
Entry into shutdown mode is controlled by logic input
pins ONA and ONB (Table 2). Both inputs have trip
points near 0.5V
OUT
with 0.15V
OUT
hysteresis.
Tracking
Connecting TRACK to the step-up converter output
implements a tracking mode that sets the step-up
converter output to 300mV above the linear-regulator
output, improving efficiency. In track mode, feedback
for the step-up converter is derived from the OUT pin.
When TRACK is low, the step-up converter and linear
regulator are separately controlled by their respective
feedback inputs, FB and FBLDO. TRACK is a logic
input with a 0.5V
OUT
threshold, and should be hard-
wired or switched with a slew rate exceeding 1V/µs.
V
LDO
must be set above 2.3V for track mode to operate
properly.
On power-up with TRACK = OUT, the step-up convert-
er initially uses the FB input to regulate its output. After
the step-up converter goes into regulation for the first
time, the linear regulator turns on. When the linear regu-
lator reaches 2.3V, track mode is enabled and the step-
up converter is regulated to 300mV above the linear-
regulator output.
Low-Battery Comparator
The internal low-battery comparator has uncommitted
inputs and an open-drain output capable of sinking
1mA. To use it as a low-battery-detection comparator,
connect the LBN input to the reference, and connect
the LBP input to an external resistor-divider between
the positive battery terminal and GND (Figure 2). The
resistor values are then as follows:
where V
IN,TH
is the desired input voltage trip point and
V
LBN
= V
REF
= 1.25V. Since the input bias current into
