Rainbow-electronics MAX1715 Manual do Utilizador descarregar pdf (Página 21)

MAX1715

Ultra-High Efficiency, Dual Step-Down

Controller for Notebook Computers

______________________________________________________________________________________ 21

when going abruptly from full-load to no-load condi-

tions, unless there are some bulk tantalum or electrolyt-

ic capacitors in parallel to absorb the stored energy in

the inductor. In some cases, there may be no room for

electrolytics, creating a need for a DC-DC design that

uses nothing but ceramics.

The all-ceramic-capacitor application of Figure 8

replaces the standard tantalum output capacitors with

ceramics. This design relies on having a minimum of

5mΩ parasitic PC board trace resistance in series with

the capacitor to reduce the ESR zero frequency. This

small amount of resistance is easily obtained by locat-

ing the MAX1714A circuit 2 or 3 inches away from the

CPU, and placing all the ceramic capacitors close to

the CPU. Resistance values higher than 5mΩ just

improve the stability (which can be observed by exam-

ining the load-transient response characteristic as

shown in the Typical Operating Characteristics). Avoid

adding excess PC board trace resistance, as there’s an

efficiency penalty; 5mΩ is sufficient for a 7A circuit:

Output overshoot (∆V) determines the minimum output

capacitance requirement. In this example, the switch-

ing frequency has been increased to 600kHz and the

inductor value has been reduced to 0.5µH (compared

to 300kHz and 2µH for the standard 8A circuit) to mini-

mize the energy transferred from inductor to capacitor

during load-step recovery. The overshoot must be cal-

culated to avoid tripping the OVP latch. The efficiency

penalty for operating at 540kHz is about 2% to 3%,

depending on the input voltage.

An optional 1Ω resistor is placed in series with OUT.

This resistor attenuates high-frequency noise in some

bands, which causes double pulsing.

Fixed Output Voltages

The MAX1715’s Dual Mode™ operation allows the

selection of common voltages without requiring external

components (Figure 9). Connect FB to AGND for a

fixed +2.5V output or to V

for a +3.3V output, or con-

nect FB directly to OUT for a fixed +1.0V output.

Setting V

OUT

with a Resistor-Divider

The output voltage can be adjusted with a resistor-

divider if desired (Figure 8). The equation for adjusting

the output voltage is:

where V

is 1.0V and R2 is about 10kΩ.

Two-Stage (5V-Powered) Notebook

CPU Buck Regulator

The most efficient and overall cost-effective solution for

stepping down a high-voltage battery to a very low out-

put voltage is to use a single-stage buck regulator

that’s powered directly from the battery. However, there

may be situations where the battery bus can’t be routed

near the CPU, or where space constraints dictate the

smallest possible local DC-DC converter. In such

cases, the 5V-powered circuit of Figure 10 may be

appropriate. The reduced input voltage allows a higher

V V 1

OUT FB













2FC

ESR

OUT

≥

MAX1715

TO ERROR

AMP1

TO ERROR

AMP2

OUT2

FB2

0.2V

FB1

FIXED

2.5V

FIXED

1.8V

FIXED

3.3V

OUT1

Figure 9. Feedback Mux

AGND

OUT

PGND

1/2

BATT

OUT

MAX1715

Figure 8. Setting V

OUT

with a Resistor-Divider

Dual Mode is a trademark of Maxim Integrated Products.

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