capacitor, even when the switch is off. This will usually not

be a problem for a 5V output at room temperature, since

the diode’s reverse leakage current and the feedback

resistors’ current typically drain the excess. However, if

the diode leakage is very low (which can occur at low

temperatures and/or small output voltages), charge may

its set point. If this happens, add a small load resistor

(typically 1M

Ω) to the output to pull a few extra

microamps of current from the output capacitor.

Layout

Several of the external components in a MAX639/

MAX640/MAX653 circuit experience peak currents up

to 600mA. Wherever one of these components con-

nects to ground, there is a potential for ground bounce.

Ground bounce occurs when high currents flow

through the parasitic resistances of PC board traces.

What one component interprets as ground can differ

from the IC’s ground by several millivolts. This may

increase the MAX639/MAX640/MAX653’s output ripple,

since the error comparator (which is referenced to

ground) will generate extra switching pulses when they

are not needed. It is essential that the input filter capac-

itor’s ground lead, the MAX639/MAX640/MAX653’s

GND pin, the diode’s anode, and the output filter

capacitor’s ground lead are as close together as possi-

ble, preferably at the same point. Figure 5 shows a

suggested through-hole printed circuit layout that mini-

mizes ground bounce.

Inverter Configuration

Figure 6 shows the MAX639/MAX640/MAX653 in a

floating ground configuration. By tying what would nor-

mally be the output to the supply-voltage ground, the

IC’s GND pin is forced to a regulated -5V (MAX639),

-3.3V (MAX640), or -3V (MAX653). Avoid exceeding the

Other negative voltages can be generated by placing a

point to VFB in the same manner as the normal step-

down configuration.

Two AA Batteries to 5V, 3.3V, or 3V

For battery-powered applications, where the signal

ground does not have to correspond to the power-supply

ground, the circuit in Figure 6 generates 5V (MAX639),

3.3V (MAX640), or 3V (MAX653) from a pair of AA batter-

and connect the output to your system’s ground input.

This configuration has the added advantage of reduced

5V/3.3V/3V/Adjustable, High-Efficiency,

______________________________________________________________________________________

11

GND

MAX639

MAX640

MAX653

SHDN

LX

VOUT

-5V

-3.3V

OR -3V

4

V+

VFB

L = 100

µH

7

1

5

100

µF

100

µF

8

6

1N5817

Figure 6. Inverting Configuration

0

V+ (V)

L = 100

µH

MAX639

0

20

40

60

80

100

160

1

2

34

5

120

140

Figure 7. Maximum Current Capability of Figure 6 Circuit

84.0

V+ (V)

L = 470

µH

1.5

84.5

85.0

85.5

86.0

86.5

87.0

2.0 2.5 3.0 3.5 4.0 4.5 5.0

5.5 6.0

Figure 8. Efficiency of Figure 6 Circuit