Dual-Output Power-Supply

Controller for Notebook Computers

______________________________________________________________________________________

15

Be sure to select output capacitors that satisfy both

the minimum capacitance and maximum ESR require-

ments. To achieve the low ESR required, it may be

appropriate to use a capacitance value 2 or 3 times

larger than the calculated minimum.

The output ripple in continuous-current mode is:

In idle-mode, the ripple has a capacitive and resistive

component:

1

1

follows:

Diodes D1 and D3

Use 1N5819s or similar Schottky diodes. D1 and D3

conduct only about 3% of the time, so the 1N5819’s

1A current rating is conservative. The voltage rating

of D1 and D3 must exceed the maximum input supply

voltage from the battery. These diodes must

be Schottky diodes to prevent the lossy MOSFET

body diodes from turning on, and they must be

placed physically close to their associated synchro-

nous rectifier MOSFETs.

Soft-Start Capacitors (C8, C9)

A capacitor connected from GND to either SS pin

causes that supply to ramp up slowly. The ramp time to

of capacitance on SS_, with a minimum value of 10µs.

Typical capacitor values are in the 10nF to 100nF

range; a 5V rating is sufficient.

Because this ramp is applied to the current-limit circuit,

the actual time for the output voltage to ramp up

depends on the load current and output capacitor

value. Using Figure 1’s circuit with a 2A load and no SS

capacitor, full output voltage is reached about 600µs

after ON_ is driven high.

Boost Capacitors (C4, C5)

Capacitors C4 and C5 store the boost voltage and pro-

vide the supply for the DH3 and DH5 drivers. Use 0.1µF

and place each within 10mm of the BST_ and LX_ pins.

Boost Diodes (D1A, D1B)

Use high-speed signal diodes; e.g., 1N4148 or

equivalent.

Bypass Capacitors

Input Filter Capacitors (C1, C10)

Use at least 3µF/W of output power for the input filter

capacitors, C1 and C10. They should have less than

150m

Ω ESR, and should be located no further than

10mm from N1 and N2 to prevent ringing. Connect the

negative terminals directly to PGND. Do not exceed the

surge current ratings of input bypass capacitors.

Shutdown Mode

Shutdown (SHDN = low) forces both PWMs off and dis-

ables the REF output and both comparators (Q1 = Q2

= 0V). Supply current in shutdown mode is typically

25µA. The VL supply remains active and can source

25mA for external loads. Note that the VL load capabili-

ty is higher in shutdown and standby modes than when

the PWMs are operating (25mA vs. 5mA). Standby

mode is achieved by holding ON3 and ON5 low while

SHDN is high. This disables both PWMs, but keeps VL,

REF, and the precision comparators alive. Supply current

in standby mode is typically 70µA.

MAX786

KELVIN SENSE TRACES

SENSE RESISTOR

MAIN CURRENT PATH

FAT, HIGH-CURRENT TRACES

Figure 5. Kelvin Connections for the Current-Sense Resistors