Multi-Output, Low-Noise Power-Supply

Controllers for Notebook Computers

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21

where:

(1A typical)

20ns = DH driver inherent rise/fall time

Under output short circuit, the MAX1633A/MAX1634A/

MAX1635As’ synchronous rectifier MOSFET suffers

extra stress because its duty factor can increase to

greater than 0.9. It may need to be oversized to tolerate

a continuous DC short circuit. During short circuit, the

MAX1630A/MAX1631A/MAX1632As’ output undervolt-

age shutdown protects the synchronous rectifier under

output short-circuit conditions.

To reduce EMI, add a 0.1µF ceramic capacitor from the

high-side switch drain to the low-side switch source.

Rectifier Clamp Diode

The rectifier is a clamp across the low-side MOSFET

that catches the negative inductor swing during the

60ns dead time between turning one MOSFET off and

each low-side MOSFET on. The latest generations of

MOSFETs incorporate a high-speed silicon body diode,

which serves as an adequate clamp diode if efficiency

is not of primary importance. A Schottky diode can be

placed in parallel with the body diode to reduce the for-

ward voltage drop, typically improving efficiency 1% to

2%. Use a diode with a DC current rating equal to one-

third of the load current; for example, use an MBR0530

(500mA-rated) type for loads up to 1.5A, a 1N5819 type

for loads up to 3A, or a 1N5822 type for loads up to

10A. The rectifier’s rated reverse breakdown voltage

must be at least equal to the maximum input voltage,

preferably with a 20% derating factor.

Boost-Supply Diode D2

A signal diode such as a 1N4148 works well in most

applications. If the input voltage can go below +6V, use

a small (20mA) Schottky diode for slightly improved

efficiency and dropout characteristics. Do not use large

power diodes, such as 1N5817 or 1N4001, since high

junction capacitance can pump up VL to excessive

voltages.

Rectifier Diode D3

(Transformer Secondary Diode)

The secondary diode in coupled-inductor applications

must withstand flyback voltages greater than 60V,

which usually rules out most Schottky rectifiers.

Common silicon rectifiers, such as the 1N4001, are also

prohibited because they are too slow. This often makes

fast silicon rectifiers such as the MURS120 the only

choice. The flyback voltage across the rectifier is relat-

former turns ratio:

where:

N = the transformer turns ratio SEC/PRI

in this equation if the secondary winding is returned to

rating must also accommodate any ringing due to leak-

age inductance. D3’s current rating should be at least

twice the DC load current on the secondary output.

Low-Voltage Operation

Low input voltages and low input-output differential

voltages each require extra care in their design. Low

absolute input voltages can cause the VL linear regula-

tor to enter dropout and eventually shut itself off. Low

ferential) can cause bad load regulation in multi-output

flyback applications (see the design equations in the

entials can also cause the output voltage to sag when

the load current changes abruptly. The amplitude of the

sag is a function of inductor value and maximum duty

factor (an Electrical Characteristics parameter, 98%

guaranteed over temperature at f = 200kHz), as follows:

The cure for low-voltage sag is to increase the output

tance of 660µF keeps the sag less than 200mV. Note

that only the capacitance requirement increases, and

the ESR requirements do not change. Therefore, the

added capacitance can be supplied by a low-cost bulk

capacitor in parallel with the normal low-ESR capacitor.

V=

(I

) x L

2 x C

x (V

x D

- V

)

SAG

STEP

2

OUT

IN(MAX)

MAX

OUT

V

= V

+ (V

- V

) x N

FLYBACK

SEC

IN

OUT