MAX1672 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – Step-Up/Down DC-DC Converter in QSOP Package

Step-Up/Down DC-DC Converter
in QSOP Package
__________Applications Information
Using a Single, Pushbutton On/Off Switch
A single pushbutton switch can be used to turn the
MAX1672 on and off. As shown in Figure 3, ONA is
pulled low and ONB is pulled high when the part is off.
When the momentary switch is pressed, ONB is pulled
low and the regulator turns on. The switch should be on
long enough for the µC to exit reset. The controller
issues a logic high to ONA, which guarantees the part
will stay on regardless of the switch state.
To turn off the regulator, press the switch again. The
controller reads the switch status and pulls ONA low.
When the switch is released, ONB goes high, turning
off the MAX1672.
Thermal Overload Protection
Thermal overload protection limits total power dissipa-
tion in the MAX1672. When the junction temperature
exceeds T_J = +150°C, the pass transistor turns off,
allowing the MAX1672 to cool. The pass transistor turns
on again after the IC’s junction temperature cools by
20°C, resulting in a pulsed output during thermal over-
load conditions.
Thermal overload protection is designed to protect the
MAX1672 if fault conditions occur. It is not intended to
be used as an operating mode. Prolonged operation in
thermal shutdown mode may reduce the IC’s reliability.
For continual operation, do not exceed the absolute
maximum junction temperature rating T_J = +150°C.
Power Dissipation and Operating Region
The MAX1672’s maximum power dissipation in step-
down mode depends on the thermal resistance of the
case and circuit board, the temperature difference
between the die junction and ambient air, and the air
flow rate. The power dissipated in the device is
P = I_OUT (V_IN - V_OUT) during step-down operation. The
maximum power dissipation is as follows:
P_MAX = (T_J - T_A)/(θ_JB + θ_BA)
where (T_J - T_A) is the temperature difference between
the MAX1672 die junction and the surrounding air, θ_JB
(or θ_JC) is the thermal resistance of the package, and
θ_BA is the thermal resistance throughout the printed cir-
cuit board, copper traces, and other materials to the
surrounding air. The MAX1672’s thermal resistance is
120°C/W. See the Typical Operating Characteristics for
Maximum Output Current vs. Input Voltage.
1M
ONB
OUT
MAX1672
ONA
V_DD
I/O µC
I/O
1M
Figure 3. Momentary Pushbutton On/Off Control
Layout Considerations
Proper PC board layout is essential to minimize noise
due to high inductor current levels and fast switching
waveforms. To maximize output power and efficiency
and minimize output ripple voltage and ground noise,
use the following guidelines when designing your
board:
• Use a ground plane.
• Keep the IC’s GND pin and the ground leads of C1
and C2 (Figure 2) less than 0.2in. (5mm) apart.
• Make all connections to the FB and LX pins as short
as possible.
• Solder the IC’s GND pin directly to the ground
plane.
Refer to the MAX1672 EV kit for a suggested PC board
layout.
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