MAX8745 datasheet(32/36 Pages) MAXIM

32 / 36 page

High-Efficiency, Quad Output, Main Power-

Supply Controllers for Notebook Computers

______________________________________________________________________________________

the FDS6612A n-channel MOSFET is used on the high

side. According to the manufacturer’s data sheet, a sin-

gle FDS6612A has a maximum gate charge of 13nC

(VGS = 5V). Using the above equation, the required

boost capacitance would be:

Selecting the closest standard value, this example

requires a 0.1µF ceramic capacitor.

LDOA Design Procedure

Output Voltage Selection

Adjust the auxiliary linear regulator’s output voltage by

connecting a resistive divider between OUTA and ana-

log ground with the center tap connected to FBA

(Figure 1). Select R6 in the 10k

Ω to 30kΩ range, and

calculate R5 with the following equation:

where VFBA = 1.0V.

Transistor Selection

The pass transistor must meet specifications for current

gain (

β), input capacitance, collector-emitter saturation

voltage, and power dissipation. The transistor’s current

gain limits the guaranteed maximum output current to:

where IDRV is the minimum guaranteed base drive cur-

rent, VBE is the base-to-emitter voltage of the transistor,

and RBE is the pullup resistor connected between the

transistor’s base and emitter. Furthermore, the transis-

tor’s current gain increases the linear regulator’s DC

loop gain (see the LDOA Stability Requirements sec-

tion), so excessive gain destabilizes the output.

Therefore, transistors with current gain over 100 at the

maximum output current can be difficult to stabilize and

are not recommended. The transistor’s input capaci-

tance and input resistance also create a second pole,

which could be low enough to make the output unsta-

ble when heavily loaded.

The transistor’s saturation voltage at the maximum out-

put current determines the minimum input-to-output

voltage differential that the linear regulator supports.

Alternatively, the package’s power dissipation could

limit the useable maximum input-to-output voltage dif-

ferential. The maximum power dissipation capability of

the transistor’s package and mounting must exceed the

actual power dissipation in the device. The power dissi-

pation equals the maximum load current times the max-

imum input-to-output differential:

PWR = ILOAD(MAX) (VINA -VOUTA)

PWR = ILOAD(MAX) VCE

LDOA Stability Requirements

The MAX8744/MAX8745 linear-regulator controller uses

an internal transconductance amplifier to drive an exter-

nal pnp pass transistor. The transconductance amplifier,

the pass transistor, the base-to-emitter resistor, and the

output capacitor determine the loop stability.

The transconductance amplifier regulates the output

voltage by controlling the pass transistor’s base cur-

rent. The total DC loop gain is approximately:

where VT is 26mV at room temperature, hFE is the pass

transistor’s DC gain, and IBIAS is the current through

the base-to-emitter resistor (RBE). The 680

Ω base-to-

emitter resistor used in Figure 1 was chosen to provide

a 1mA bias current (IBIAS).

V LDO

BIAS FE

LOAD

()

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

LOAD MAX

DRV

MIN

() =

⎛

⎝

⎜

⎞

⎠

⎟

−

OUTA

FBA

=−

⎛

⎝⎜

⎞

⎠⎟

BST ==

200

0 065

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