Rainbow-electronics MAX1718 Manual do Utilizador descarregar pdf (Página 15)

MAX1718

Notebook CPU Step-Down Controller for Intel

Mobile Voltage Positioning (IMVP

II)

______________________________________________________________________________________ 15

Thus, it is necessary to use forced PWM mode during all

negative transitions. Most applications should use PWM

mode exclusively, although there is some benefit to

using skip mode while in the low-power suspend state

(see the Using Skip Mode During Suspend (SKP/

SDN

) section.)

Automatic Pulse-Skipping Switchover

In skip mode (SKP/SDN high), an inherent automatic

switchover to PFM takes place at light loads (Figure 6).

This switchover is effected by a comparator that trun-

cates the low-side switch on-time at the inductor current’s

zero crossing. This mechanism causes the threshold

between pulse-skipping PFM and nonskipping PWM

operation to coincide with the boundary between con-

tinuous and discontinuous inductor-current operation.

The load-current level at which PFM/PWM crossover

occurs, I

LOAD(SKIP)

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

ripple current, which is a function of the inductor value

(Figure 6). For a battery range of 7V to 24V, this thresh-

old is relatively constant, with only a minor dependence

on battery voltage:

where K is the on-time scale factor (Table 2). For exam-

ple, in the standard application circuit this becomes:

The crossover point occurs at a lower value if a swing-

ing (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.

Current-Limit Circuit

The current-limit circuit employs a unique “valley” current-

sensing algorithm that uses the on-resistance of the

low-side MOSFET as a current-sensing element. If the

current-sense signal is above the current-limit thresh-

old, the PWM is not allowed to initiate a new cycle

33 125

2068

12 1 25

. .

−

LOAD SKIP

OUT BATT OUT

BATT

()

≈

−

REF

MAX1718

POS

NEG

Figure 3. Resistive Divider from REF

MAX1718

POS

NEG

Figure 4. Resistive Divider from OUTPUT

MAX1718

SEL

MUX

POS

NEG

MAX4524

Figure 5. Programmable Offset Voltage

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 34 35

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