MAX15027ATBT データシートの表示（PDF） - Maxim Integrated
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MAX15027ATBT Datasheet PDF : 12 Pages
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MAX15027/MAX15028
1.425V to 3.6V Input, 1A Low-Dropout
Regulators with BIAS Input
Applications Information
Programming the Output Voltage
The MAX15027/MAX15028 feature an adjustable output
voltage from 0.5V to VIN using two external resistors con-
nected as a voltage-divider to FB as shown in Figure 1.
The output voltage is set by the following equation:
V= OUT
VFB
1
+
R1
R2
where typically VFB = 0.5V. Choose R2 to be 10kΩ. Or,
to optimize load-transient response for no load to full load
transients, use the resistor-divider as a minimum load and
choose R2 to be 500Ω. To simplify resistor selection:
R1
=
R2
VOUT
VFB
− 1
Capacitor Selection and Regulator Stability
Capacitors are required at the device’s inputs and outputs
for stable operation over the full temperature range and
with load currents up to 1A. Connect a 1μF capacitor
between IN and ground and a 4.7μF capacitor with low
equivalent series resistance (ESR) between OUT and
ground for 1A output current.
The input capacitor (CIN) lowers the source impedance of
the input supply. If input supply source impedance is high,
place a larger input capacitor close to IN to prevent VIN
sagging due to load transients. Smaller output capacitors
can be used for output currents less than 1A. Calculate
the minimum COUT as follows:
= C OUT
IOUT(MAX)
×
1µF
0.25A
Operating Region and Power Dissipation
The maximum power dissipation depends on the thermal
resistance of the IC package and circuit board, the tem-
perature difference between the die junction and ambi-
ent air, and the rate of airflow. The power dissipated in
the device is PDISS = IOUT (VIN - VOUT). The package
features an exposed thermal pad on its underside. This
pad lowers the thermal resistance of the package by
providing a direct heat conduction path from the die to
the PCB. Connect the exposed backside pad and GND to
the system ground using a large pad or ground plane and
multiple vias to the ground plane layer.
Noise, PSRR, and Transient Response
The devices are designed to operate with low-dropout
voltages and low quiescent currents while still maintaining
good noise performance, transient response, and AC rejec-
tion (see the Typical Operating Characteristics for a plot
of Power-Supply Rejection Ratio (PSRR) vs. Frequency).
When operating from noisy sources, improved supply-
noise rejection and transient response can be achieved
by increasing the values of the input and output bypass
capacitors and through passive filtering techniques. The
MAX15027/MAX15028 load-transient response graphs
(see the Typical Operating Characteristics) show two
components of the output response: a DC shift from the
output impedance due to the load current change, and
the transient response. A typical transient overshoot for a
step change in the load current from 300mA to 800mA is
15mV. Use ceramic output capacitors greater than 4.7μF
(up to 100μF) to attenuate the overshoot.
Layout Guidelines
The TDFN package has an exposed thermal pad on its
underside. This pad provides a low thermal resistance
path for heat transfer into the PCB. This low thermally
resistive path carries a majority of the heat away from
the IC. The PCB is effectively a heatsink for the IC. The
exposed pad should be connected to a large ground
plane for proper thermal and electrical performance. The
minimum size of the ground plane is dependent upon
many system variables. To create an efficient path, the
exposed pad should be soldered to a thermal landing,
which is connected to the ground plane by thermal vias.
The thermal landing should be at least as large as the
exposed pad.
ALWAYS-ON
INPUT 3V
TO 5.5V
INPUT
1.2V TO 3.6V
1µF
ENABLE
SHUTDOWN
BIAS
OUT
IN
MAX15028 FB
I.C.
EN
SS
GND
0.1uF
OUTPUT
R1
4.7µF
R2
Figure 1. MAX15028 Typical Application Circuit
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