To choose the proper current-sense resistor, simply fol-

low the two-step procedure outlined below:

1) Determine:

• Input voltage range, V+

For example, let V+ range from 4V to 6V, and

2) Next, referring to Figure 9, find the curve with the

lowest current limit whose output current (with the

lowest input voltage) meets your requirements.

4V input and a -24V output is optimal.

Ω (Figure 9) shows only approxi-

mately 125mA of output current with a 4V input, so we

will be 0.210V / 0.070

Ω = 3A. These curves take into

account worst-case inductor (±10%) and current-

sense trip levels, but not sense-resistor tolerance. The

switch on resistance is 70m

Ω.

Standard wire-wound and metal-film resistors have

an inductance high enough to degrade performance.

Metal-film resistors are usually deposited on a ceramic

rod in a spiral, making their inductances relatively high.

Surface-mount (or chip) resistors have very little induc-

tance and are well suited for use as current-sense

resistors. To use through-hole resistors, IRC has a wire

resistor that is simply a band of metal shaped as a “U”

so that inductance is less than 10nH (an order of mag-

nitude less than metal-film resistors). These are avail-

able in resistance values between 5m

Ω and 0.1Ω.

External Switching Transistor

The MAX774/MAX775/MAX776 are capable of driving

P-channel enhancement-mode MOSFET transistors only.

The choice of power transistor is dictated by input and

output voltage, peak current rating, on-resistance, gate-

source threshold, and gate capacitance. The drain-to-

input-to-output voltage differential. The gate-to-source

rating must be greater than V+ (the source voltage) plus

the absolute value of the most negative swing of EXT.

For bootstrapped operation, the most negative swing of

ground or some other negative voltage. Gate capaci-

tance is not normally a limiting factor, but values should

be less than 1nF for best efficiency. For maximum effi-

ciency, the MOSFET should have a very-low on-resis-

tance at the peak current and be capable of handling

that current. The transistor chosen for the typical operat-

ing circuit has a 30V drain-source voltage limit and a

0.07

Table 1 lists suppliers of switching transistors suitable

for use with the MAX774/MAX775/MAX776.

-5V/-12V/-15V or Adjustable, High-Efficiency,

______________________________________________________________________________________

13

Figure 8. MAX776 Maximum Output Current vs. Input Voltage

Figure 9. MAX774/MAX775/MAX776 Maximum Output Current

100

INPUT VOLTAGE (V)

200

300

400

500

600

700

34

5

6

7

Ω

Ω

Ω

Ω

Ω

INPUT VOLTAGE (V)

0

3 4

5

678

9 10 11 12 13 14 15

200

400

600

800

Ω

Ω

Ω

Ω

Ω