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

Low-Dropout, Step-Down DC-DC Controllers

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9

To maximize efficiency and reduce the size and cost

of external components, minimize the peak current.

However, since the available output current is a func-

tion of the peak current, the peak current must not be

too low.

To choose the proper current-sense resistor for a par-

ticular output voltage, determine the minimum input

voltage and the maximum load current. Next, refer-

ring to Figures 5a or 5b, using the minimum input volt-

age, find the curve with the largest sense resistor that

provides sufficient output current. It is not necessary

to perform worst-case calculations. These curves take

into account the sense-resistor (±5%) and inductor

(47µH ±10%) values, the diode drop (0.4), and the

IC’s current-sense trip level (85mV); an external MOS-

FET on-resistance of 0.07

Standard wire-wound and metal-film resistors have an

inductance high enough to degrade performance.

Surface-mount (chip) resistors have very little inductance

and are well suited for use as current-sense resistors.

A U-shaped wire resistor made by IRC works well in

through-hole applications. Because this resistor is a

band of metal shaped as a “U”, its inductance is less

than 10nH (an order of magnitude less than metal film

resistors). Resistance values between 5m

Ω and 0.1Ω

are available (see Table 1).

Inductor Selection

The MAX1649/MAX1651 operate with a wide range of

inductor values, although for most applications coils

between 10µH and 68µH take best advantage of the con-

trollers’ high switching frequency. With a high inductor

value, the MAX1649/MAX1651 will begin continuous-cur-

rent operation (see

Detailed Description) at a lower frac-

tion of full-load current. In general, smaller values pro-

duce higher ripple (see below) while larger values require

larger size for a given current rating.

In both the continuous and discontinuous modes, the

lower limit of the inductor is important. With a too-small

inductor value, the current rises faster and overshoots the

desired peak current limit because the current-limit com-

parator has a finite response time (300ns). This reduces

efficiency and, more importantly, could cause the current

rating of the external components to be exceeded.

Calculate the minimum inductor value as follows:

L(min) = ——————————––——

where

∆I is the inductor-current overshoot factor,

parator to switch. Set

∆I = 0.1 for an overshoot of 10%.

For highest efficiency, use a coil with low DC resis-

minimize radiated noise, use a toroid, pot core, or

shielded-bobbin inductor. Inductors with a ferrite core

or equivalent are recommended. Make sure the induc-

However, it is generally acceptable to bias the inductor

into saturation by about 20% (the point where the

inductance is 20% below its nominal value).

5.0

5.4

5.8

6.2

6.6

16.0

INPUT VOLTAGE (V)

3.0

2.5

2.0

1.5

1.0

0

0.5

Figure 5a. MAX1649 Current-Sense Resistor Graph

3.0

3.4

3.8

4.2

4.6

16.0

INPUT VOLTAGE (V)

3.0

2.5

2.0

1.5

1.0

0

0.5

Figure 5b. MAX1651 Current-Sense Resistor Graph