MAX1534
ous operation, do not exceed the absolute maximum
junction temperature rating of T
J
= +150°C.
Operating Region and Power Dissipation
The MAX1534’s maximum power dissipation 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-
sipated in the device is the sum of the buck MOSFET
switching and conduction losses and the linear regula-
tors’ conduction losses. The maximum power dissipa-
tion is:
P
MAX
= (T
J
- T
A
) / (θ
JB
+ θ
BA
)
where T
J
- T
A
is the temperature difference between the
MAX1534 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 through the printed circuit board,
copper traces, and other materials to the surrounding
air. The exposed backside pad of the MAX1534 pro-
vides a low thermal impedance to channel heat out of
the package. Connect the exposed backside pad to
ground using a large pad or ground plane.
Preset and Adjustable Output Voltages
(
PRESET
)
The MAX1534 features dual mode operation; it oper-
ates in either a preset voltage mode (see Table 4) or an
adjustable mode. In preset voltage mode, internal
trimmed feedback resistors set the MAX1534 outputs to
3.3V for V
OUT1
, 1.8V for V
OUT2
, and 5.0V for FB3 (buck
regulator). Select this mode by connecting PRESET to
ground. Connect PRESET to IN to operate the
MAX1534 in the adjustable mode. Select an output volt-
age using two external resistors connected as a volt-
age-divider to FB_ (Figure 4). The output voltage is set
by the following equation:
where V
FB_
= 1.0V, V
OUT1
and V
OUT2
can range from
1.0V to V
LDOIN
, and V
OUT3
can range from 1.0V to V
IN
.
To simplify resistor selection:
Choose R
BOT_
= 100kΩ to optimize power consump-
tion, accuracy, and high-frequency power-supply rejec-
tion. The total current through the external resistive
feedback and load resistors should not be less than
10µA. Since the V
FB_
tolerance is typically less than
±15mV, the output can be set using fixed resistors
instead of trim pots.
Design Procedure
Buck Converter
Inductor Selection
When selecting the inductor, consider these four para-
meters: inductance value, saturation rating, series
resistance, and size. The MAX1534 operates with a
wide range of inductance values. For most applica-
tions, values between 10µH and 50µH work best with
the controller’s high switching frequency. Larger induc-
tor values reduce the switching frequency and thereby
improve efficiency and EMI. The trade-off for improved
efficiency is a higher output ripple and slower transient
response. On the other hand, low-value inductors
respond faster to transients, improve output ripple, offer
smaller physical size, and minimize cost. If the inductor
value is too small, the peak inductor current exceeds
the current limit due to current-sense comparator prop-
agation delay, potentially exceeding the inductor’s cur-
rent rating. Calculate the minimum inductance value as
follows:
where t
ON(MIN)
= 0.5µs.
The inductor’s saturation current rating must be greater
than the peak switch current limit, plus the overshoot
due to the 150ns current-sense comparator propaga-
tion delay. Saturation occurs when the inductor’s mag-
netic flux density reaches the maximum level the core
can support and the inductance starts to fall. Choose
an inductor with a saturation rating greater than I
PEAK
in the following equation:
I
PEAK
= I
LX(PEAK)
+ (V
IN
- V
OUT3
) ✕ 150ns / L
Inductor series resistance affects both efficiency and
dropout voltage (see the Buck Dropout Performance
section).
High series resistance limits the maximum current avail-
able at lower input voltages, and increases the dropout
