MAX1749
SOT23 Vibrator Motor Driver
6 _______________________________________________________________________________________
The P-channel MOSFET requires no base drive current,
which reduces quiescent current considerably. PNP-
based regulators waste considerable amounts of base
current under large loads. The MAX1749 does not suf-
fer from these problems and consumes only 80µA of
quiescent current independent of the load (see
Typical
Operating Characteristics
).
Output Voltage Selection
To select the preset 1.25V output, connect OUT directly
to SET. To adjust the output (1.25V to 6.5V), use two
external resistors connected as a voltage divider to SET
(Figure 2). The output voltage is set by the following
equation:
V
OUT
= V
SET
(1 + R1 / R2)
where V
SET
= 1.25V. To simplify resistor selection:
Choose R2 = 100kΩ to optimize power consumption,
accuracy, and high-frequency power-supply rejection.
The total current through the external resistive feedback
and load should not be less than 10µA.
ON/
OOFFFF
Drive ON high to provide power to the load. Drive ON
low to disable power to the load and reduce the supply
current to typically 0.1nA (1µA max). Refer to the
ON/OFF waveforms in the
Typical Operating Char-
acteristics
. When ON goes high, output current rises to
the current limit until V
OUT
reaches regulation. While in
regulation, the output current drops to a lower value
sufficient to maintain motor speed. When ON goes low,
the regulator turns off; however, inertial energy in the
motor exhibits a slow output voltage decline. The
MAX1749 is designed to withstand this condition with
no negative effects.
Current Limit
The MAX1749 includes a current limiter that monitors
and controls the pass transistor’s gate voltage, estimat-
ing the output current and limiting it to about 280mA.
For design purposes, the current limit should be con-
sidered 120mA (min) to 420mA (max). The output can
be shorted to ground for an infinite time period without
damaging the part.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX1749. When the junction temperature
exceeds T
J
= +170°C, the thermal sensor sends a sig-
nal to the ON/ OFF logic, turning off the pass transistor
and allowing the IC to cool. The thermal sensor will turn
the pass transistor on again after the IC’s junction tem-
perature cools by typically 20°C, resulting in a pulsed
output during continuous thermal-overload conditions.
Operating Region and Power Dissipation
Maximum power dissipation of the MAX1749 depends
on the thermal resistance of the case and circuit board,
the temperature difference between the die junction
and ambient air, and the rate of air flow. The power dis-
sipation across the device is P = I
OUT
(V
IN
- V
OUT
). The
resulting maximum power dissipation is:
P
MAX
= (T
J
- T
A
) / θ
JA
where (T
J
- T
A
) is the temperature difference between
the MAX1749 die junction and the surrounding air, and
θ
JA
is +140°C/W.
GND performs the dual function of providing an electri-
cal connection to ground and channeling heat away.
Connect GND to a large pad or ground plane.
Reverse Battery Protection
The MAX1749 has a unique protection scheme that lim-
its the reverse supply current to less than 1mA when
either V
IN
or V
ON
falls below ground. The circuitry moni-
tors the polarity of these two pins, disconnecting the
internal circuitry and parasitic diodes when the battery
is reversed. This feature prevents the device from over-
heating and damaging the battery.
V
IN
> 5.5V Minimum Load Current
When operating the MAX1749 with an input voltage
above 5.5V, the minimum current through the external
feedback resistors and load must be 30µA.
