MAX1534
input capacitor that exhibits less than +10°C tempera-
ture rise at the RMS input current for optimal circuit
longevity.
Diode Selection
The current in the external diode (D1 in Figure 1)
changes abruptly from zero to its peak value each time
the LX switch turns off. To avoid excessive losses, the
diode must have a fast turn-on time and a low forward
voltage. Make sure that the diode’s peak current rating
exceeds the peak current set by the current limit, and
that its breakdown voltage exceeds V
IN
. Use Schottky
diodes when possible.
Linear Regulators
Capacitor Selection and LDO Stability
Use a 2.2µF capacitor on the MAX1534 LDOIN pin and
a 2.2µF capacitor on the outputs. Larger input capaci-
tor values and lower ESRs provide better supply-noise
rejection and line-transient response. To reduce noise,
improve load transients, and for loads up to 160mA,
use larger output capacitors (up to 10µF). For stable
operation over the full temperature range and with load
currents up to 80mA, use 2.2µF. Note that some ceram-
ic dielectrics exhibit large capacitance and ESR varia-
tion with temperature. With dielectrics such as Z5U and
Y5V, it may be necessary to use 4.7µF or more to
ensure stability at temperatures below -10°C. With X7R
or X5R dielectrics, 2.2µF is sufficient at all operating
temperatures. These regulators are optimized for
ceramic capacitors, and tantalum capacitors are not
recommended.
Use a 0.01µF bypass capacitor at BP for low output volt-
age noise. Increasing the capacitance slightly decreas-
es the output noise, but increases the startup time.
Applications Information
Buck Dropout Performance
A step-down converter’s minimum input-to-output volt-
age differential (dropout voltage) determines the lowest
usable supply voltage. In battery-powered systems,
this limits the useful end-of-life battery voltage. To maxi-
mize battery life, the MAX1534 operates with duty
cycles up to 100%, which minimizes the dropout volt-
age and eliminates switching losses while in dropout.
When the supply voltage approaches the output volt-
age, the P-channel MOSFET remains on continuously to
supply the load.
For a step-down converter with 100% duty cycle,
dropout depends on the MOSFET drain-to-source on-
resistance and inductor series resistance; therefore, it
is proportional to the load current:
V
DROPOUT(BUCK)
= I
OUT3
✕ (R
LX
+ R
INDUCTOR
)
LDO PSRR
The MAX1534’s linear regulators are designed to deliv-
er low dropout voltages and low quiescent currents in
battery-powered systems. Power-supply rejection is
55dB at low frequencies and rolls off above 20kHz.
(See the LDO PSRR vs. Frequency graph in the Typical
Operating Characteristics.)
To improve supply-noise rejection and transient
response, increase the values of the input and output
bypass capacitors or use passive filtering techniques.
LDO Dropout Voltage
A linear regulator’s minimum input-output voltage differ-
ential (or dropout voltage) determines the lowest usable
supply voltage. Because the MAX1534 uses a P-chan-
nel MOSFET pass transistor, its dropout voltage is a
function of drain-to-source on-resistance (R
DS(ON)
)
multiplied by the load current (see LDO Dropout
Voltage vs. Load Current in the Typical Operating
Characteristics).
PC Board Layout Guidelines
High switching frequencies and large peak currents
make PC board layout an important part of the design.
Poor layout introduces switching noise into the feedback
path, resulting in jitter, instability, or degraded perfor-
mance. High current traces, highlighted in the Typical
Application Circuit (Figure 1), should be as short and
wide as possible. Additionally, the current loops formed
by the power components (C
IN
, C
OUT3
, L1, and D1)
should be as short as possible to avoid radiated noise.
Connect the ground pins of these power components at
a common node in a star-ground configuration.
Separate the noisy traces, such as the LX node, from
the feedback network with grounded copper.
Furthermore, keep the extra copper on the board and
integrate it into a pseudoground plane. When using
external feedback, place the resistors as close to the
feedback pin as possible to minimize noise coupling.
High-Efficiency, Triple-Output, Keep-Alive
Power Supply for Notebook Computers
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