MAX1534 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1534

High-Efficiency, Triple-Output, Keep-Alive Power Supply for Notebook Computers

Maxim Integrated 

High-Efficiency, Triple-Output, Keep-Alive

Power Supply for Notebook Computers

ous operation, do not exceed the absolute maximum

junction temperature rating of TJ = +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:

PMAX = (TJ - TA) / (θJB + θBA)

where TJ - TA is the temperature difference between the

MAX1534 die junction and the surrounding air, θJB (or

θJC) is the thermal resistance of the package, and θBA 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 VOUT1, 1.8V for VOUT2, 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:

VOUT _

=



VFB_ 1+

RTOP _

RBOT _





where VFB_ = 1.0V, VOUT1 and VOUT2 can range from

1.0V to VLDOIN, and VOUT3 can range from 1.0V to VIN.

To simplify resistor selection:

RTOP _



= RBOT _ 

VOUT _

VFB _



− 1

Choose RBOT_ = 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 VFB_ tolerance is typically less than

Table 4. PRESET Setting

PRESET

IN

GND

MODE

Adjustable

Preset

OUT_ AND FB_

FB_ regulates to 1.0V

OUT1 = 3.3V, FB1 = GND,

OUT2 = 1.8V, FB2 = GND,

OUT3 = FB3 = 5.0V

±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:

( ) L(MIN) =

VIN(MAX) - VOUT3 × tON(MIN)

ILX(PEAK)

where tON(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 IPEAK

in the following equation:

IPEAK = ILX(PEAK) + (VIN - VOUT3) ✕ 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

12 ______________________________________________________________________________________

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