Rainbow-electronics MAX1717 Manual de usuario descargar pdf (Pagina 17)

MAX1717

Dynamically Adjustable, Synchronous

Step-Down Controller for Notebook CPUs

______________________________________________________________________________________ 17

For a battery range of 7V to 24V, this threshold is rela-

tively constant, with only a minor dependence on bat-

tery voltage:

where K is the on-time scale factor (Table 3). The load-

current level at which PFM/PWM crossover occurs,

LOAD(SKIP)

, is equal to 1/2 the peak-to-peak ripple cur-

rent, which is a function of the inductor value (Figure 4).

For example, in the standard application circuit this

becomes:

The crossover point occurs at an even lower value if a

swinging (soft-saturation) inductor is used.

The switching waveforms may appear noisy and asyn-

chronous when light loading causes pulse-skipping

operation, but this is a normal operating condition that

results in high light-load efficiency. Trade-offs in PFM

noise vs. light-load efficiency are made by varying the

inductor value. Generally, low inductor values produce

a broader efficiency vs. load curve, while higher values

result in higher full-load efficiency (assuming that the

coil resistance remains fixed) and less output voltage

ripple. Penalties for using higher inductor values

include larger physical size and degraded load-tran-

sient response (especially at low input voltage levels).

33 16

12 1 6

. .

×=

−

LOAD SKIP

OUT BATT OUT

BATT

()

≈

−

BATT

+5V

BIAS SUPPLY

POWER-GOOD

INDICATOR

1µH

0.005Ω

SKP/SDN

V+

221

CMPSH-3

1µF

0.1µF

1µF

470pF

TO V

100kΩ

1µF

20Ω

TIME

ON/OFF

CONTROL

BST

GND

FBS

GNDS

D1 = INTL RECT 10MQ040N.

FOR OTHER COMPONENTS,

SEE TABLE 1 VALUES.

VGATE

MAX1717

+5V

TON

REF

A/B

ILIM

120kΩ

100kΩ

TO V

HIGH/LOW

2kΩ

150kΩ

TO V

REF

A/B = LOW = 1.60V

A/B = HIGH = 1.35V

OUT

Figure 3. Voltage-Positioned Circuit

1 2 ... 12 13 14 15 16 17 18 19 20 21 22 ... 32 33

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