MAX783CSLASHD datasheet(17/28 Pages) MAXIM

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Diode D3

The voltage rating of D3 should be at least 4 x VIN +

5V plus a safety margin. A rating of at least 100V is

necessary for the maximum 20V supply. Use a high-

speed silicon diode (with a higher breakdown voltage

and low capacitance) rather than a Schottky diode.

D3’s current rating should exceed twice the maximum

current load on VDD.

Diodes D2 and D5

Use 1N5819s or similar Schottky diodes. D2 and D5

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

current rating is conservative. The voltage rating of D2

and D5 must exceed the maximum input supply volt-

age 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 synchronous rectifier MOSFETs.

Soft-Start Capacitors (C13, C14)

A capacitor connected from GND to either SS pin caus-

es that supply to ramp up slowly. The ramp time to full

current limit, tSS, is approximately 1ms for every nF of

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

Typical capacitor values are in the 10nF to 100nF range.

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 in less than

1ms after ON_ is driven high.

Bypass Capacitors

Input Filter Capacitors (C1, C2)

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

capacitors, C1 and C2. 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. Be careful not to

exceed the surge current ratings of the bypass capaci-

tors. If the battery pack or AC adapter has very low

output impedance, tantalum capacitors may be dam-

aged when initial connection is made. In this situation,

electrolytic capacitors such as Sanyo OS-CON may be

necessary. Also, take care that the RMS input current

of the MAX783 circuit does not exceed the bypass

capacitor ripple current rating. The RMS input current

(IRMS) can be calculated as shown below:

IRMS = RMS AC input current

————————

√V

OUT (VIN - VOUT)

= ILOAD

———————–——

VIN

Low-Voltage Operation

Low input voltages, such as the 6V end-of-life voltage

of a 6-cell NiCd battery, place extra demands on the

+5V buck regulator because of the very low input-out-

put differential voltage. The standard application cir-

cuit works well with supply voltages down to 6V; at

input voltages less than 6V, the +5V filter capacitor

values must be increased. If the minimum battery

voltage is 6.5V or higher, the 660µF total 5V filter

capacitance can be reduced to 330µF.

The +5V supply’s load-transient response is

impaired due to reduced inductor-current slew rate,

which is in turn caused by reduced voltage applied

across the buck inductor during the high-side

switch-on time. So, the +5V output sags when hit

with an abrupt load current change, unless the +5V

filter capacitor value is increased. Only the capaci-

tance is affected and ESR requirements don’t

change. Therefore, the added capacitance can be

supplied by an additional low-cost bulk capacitor in

parallel with the normal low-ESR switching-regulator

capacitor. The equation for voltage sag under a

step-load change follows:

(ISTEP2)(L)

VSAG = —————————————————

(2)(CF) (VIN(MIN) x DMAX - VOUT)

where DMAX is the maximum duty cycle. Higher duty

cycles are possible when the oscillator frequency is

reduced to 200kHz, due to fixed propagation delays

through the PWM comparator becoming a lesser part of

the whole period. The tested worst-case limit for DMAX

is 92% at 200kHz. Lower inductance values can reduce

the filter capacitance requirement, but only at the

expense of increased noise at high input voltages

(resulting from higher peak currents).

Triple-Output Power-Supply Controller

for Notebook Computers

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