Rainbow-electronics MAX1717 Manual de usuario descargar pdf (Pagina 20)

MAX1717

Dynamically Adjustable, Synchronous

Step-Down Controller for Notebook CPUs

20 ______________________________________________________________________________________

Internal Mux

The MAX1717 contains an internal mux that can be used

to select one of two programmed DAC codes and output

voltages. The internal mux is controlled with the A/B pin,

which selects between the A mode and the B mode. In

the A mode, the voltage levels on D0–D4 select the out-

put voltage according to Table 4. Do not leave D0–D4

floating; there are no internal pull-up resistors.

The B mode is programmed by external resistors in

series with D0–D4, using a unique scheme that allows

two sets of data bits using only one set of pins (Figure

7). When A/B goes low (or during power-up with A/B

low), D0–D4 are tested to see if there is a large resis-

tance in series with the pin. If the voltage level on the

pin is a logic low, an internal switch connects the pin to

an internal 40kΩ pull-up for about 4µs to see if the pin

voltage can be forced high (Figure 8). If the pin voltage

cannot be pulled to a logic high, the pin is considered

low impedance and its B-mode logic state is low. If the

pin can be pulled to a logic high, the impedance is

considered high and so is the B-mode logic state.

Similarly, if the voltage level on the pin is a logic high,

an internal switch connects the pin to an internal 8kΩ

pull-down to see if the pin voltage can be forced low. If

so, the pin is high-impedance and its B-mode logic

state is high. Otherwise, its logic state is low.

A high pin impedance (and logic high) is 100kΩ or

greater, and a low impedance (and logic low) is 1kΩ or

less. The Electrical Characteristics guaranteed levels for

these impedances are 95kΩ and 1.05kΩ to allow the use

of standard 100kΩ and 1kΩ resistors with 5% tolerance.

If the output voltage codes are fixed at PC board

design time, program both codes with a simple combi-

nation of pin-strap connections and series resistors

(Figure 7). If the output voltage codes are chosen dur-

ing PC board assembly, both codes can be indepen-

dently programmed with resistors (Figure 9). This

matrix of 10 resistor-footprints can be programmed to all

possible A-mode and B-mode code combinations with

only five resistors.

Often, one or more output-voltage codes are provided

directly by the CPU’s VID pins. If the CPU actively dri-

ves these pins, connect A/B high (A mode) and let the

CPU determine the output voltages. If the B mode is

needed for startup or other reasons, insert resistors in

series with D0–D4 to program the B-mode voltage. Be

sure that the VID pins are actively driven at all times.

If the CPU’s VID pins float, the open-circuit pins can

present a problem for the MAX1717’s internal mux. The

processor’s VID pins can be used for the A-mode set-

ting, together with suitable pull-up resistors. However,

the B-mode VID code is set with resistors in series with

D0–D4, and in order for the B-mode to work, any pins

intended to be B-mode logic low must appear to be low

impedance, at least for the 4µs sampling interval.

D4 D3 D2 D1 D0 V

OUT

(V)

0 0 0 0 0 2.00

0 0 0 0 1 1.95

0 0 0 1 0 1.90

0 0 0 1 1 1.85

0 0 1 0 0 1.80

0 0 1 0 1 1.75

0 0 1 1 0 1.70

0 0 1 1 1 1.65

0 1 0 0 0 1.60

0 1 0 0 1 1.55

0 1 0 1 0 1.50

0 1 0 1 1 1.45

0 1 1 0 0 1.40

0 1 1 0 1 1.35

0 1 1 1 0 1.30

0 1 1 1 1 No CPU

1 0 0 0 0 1.275

1 0 0 0 1 1.250

1 0 0 1 0 1.225

1 0 0 1 1 1.200

1 0 1 0 0 1.175

1 0 1 0 1 1.150

1 0 1 1 0 1.125

1 0 1 1 1 1.100

1 1 0 0 0 1.075

1 1 0 0 1 1.050

1 1 0 1 0 1.025

1 1 0 1 1 1.000

1 1 1 0 0 0.975

1 1 1 0 1 0.950

1 1 1 1 0 0.925

1 1 1 1 1 No CPU

Table 4. Output Voltage vs. DAC Codes

Note: In the no-CPU state, DH and DL are held low and the

slew-rate controller is set for 0.9V.

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 32 33

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