ML1565 datasheet(14/21 Pages) MINILOGIC

14 / 21 page

P14/21

Rev. C, Sep 2005

ML1565

Design Procedure (2)

Setting Output voltages

The ML1565 step-up / step-down converters and the

AUX1 controllers have both factory-set and adjustable

output voltages. These are selected by FBSEL_ for the

appropriate channel. When FBSEL_ is low, the channel

output regulates at its preset voltage. When FBSEL_ is

high, the channel regulates FB_ at 1.25V for use with

external feedback resistors.

When setting the voltage for auxiliary channels 2 and 3, or

when using external feedback at FBSU, FBSD, or FB1,

connect a resistive voltage-divider from the output voltage

to the corresponding FB_ input. The FB_ input bias

current is less than 100nA, so choose the low-side

(FB_-to-GND) resistor (RL), to be 100kΩ or less. Then

calculate the high-side (output-to-FB_) resistor (RH) using:

RH = RL [ (VOUT / 1.25) -1 ]

General Filter Capacitor Selection

The input capacitor in DC-to-DC converter reduces current

peaks drawn from the battery, or other input power source,

and reduces switching noise in the controller. The

impedance of the input capacitor at the switching

frequency should be less than that of the input source so

that high-frequency switching currents do not pass through

the input source.

The output capacitor keeps output ripple small and ensure

control-loop stability. The output capacitor must also have

low impedance at the switching frequency. Ceramic,

polymer, and tantalum capacitors are suitable, with

ceramic exhibiting the lowest ESR and high-frequency

impedance.

Output

ripple

with

ceramic

output

capacitor

approximately:

VRIPPLE =

IL( PEAK ) [ 1 / (2πfOSCCOUT) ]

If the capacitor has significant ESR, the output ripple

component due to capacitor ESR is:

VRIPPLE =

IL( PEAK ) ESR

Output capacitor specifics are also discussed in the

Step-Up Compensation section and the Step-Down

Compensation section.

Step-Up Component Selection

The external components required for the step-up are

an inductor,

input

and output

filter capacitor, and

compensation RC. Typically, the inductor is selected to

operate with continuous current for best efficiency. An

exception might be if the step-up ratio, (VOUT / VIN ), is

greater than 1/(1 – DMAX), where DMAX is the maximum

PWM duty factor of 80%.

When using the step-up channel to boost from a low input

voltage, loaded startup is aided by connecting a Schottky

diode from the battery to OUTSU. See the Minimum

Startup Voltage vs. Load Current graph in the Typical

Operating characteristics.

Step-Up Inductor

In most step-up designs, a reasonable inductor value

(LIDEAL) can be derived from the following equation. Which

sets continuous peak-to-peak inductor current at one-half

the DC inductor current:

LIDEAL = [ 2 VIN(MAX) D (1-D) ] / (IOUT fOSC)

where D is the duty factor given by:

D = 1 – ( VIN / VOUT )

Given LIDEAL , the consistent peak-to peak inductor

current is 0.5IOUT / (1-D).The peak inductor current,

IIND(PK)=1.25IOUT / (1-D). Inductance values smaller than

LIDEAL can be used to reduce inductor size. However, if

much smaller values are used, the inductor current rises

and a larger output capacitance may be required to

suppress output ripple.

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