Fixed Output Voltages

tion of common voltages without requiring external com-

ponents (Figure 6). Connect FB to GND for a fixed 2.5V

to OUT for a fixed 1V output.

The output voltage can be adjusted from 1V to 5.5V with

a resistor-divider if desired (Figure 7). The equation for

adjusting the output voltage is:

Design Procedure

Component selection for the MAX1844 is primarily dictat-

ed by the following four criteria:

must accommodate the worst-case high AC-adapter

for the lowest battery voltage after drops due to con-

nectors, fuses, and battery selector switches. Lower

input voltages result in better efficiency.

2) Maximum load current. There are two values to con-

the instantaneous component stresses and filtering

requirements and thus drives output capacitor selec-

tion, inductor saturation rating, and the design of the

current-limit circuit. The continuous load current

ves the selection of input capacitors, MOSFETs, and

other critical heat-contributing components.

3) Switching frequency. This choice determines the

basic trade-off between size and efficiency. The opti-

mal frequency is largely a function of maximum input

voltage, due to MOSFET switching losses that are

quency is also a moving target, due to rapid improve-

ments in MOSFET technology that are making higher

frequencies more practical (Table 4).

4) Inductor operating point. This choice provides

trade-offs between size vs. efficiency. Low inductor

values cause large ripple currents, resulting in the

smallest size, but poor efficiency and high output rip-

ple. The minimum practical inductor value is one that

causes the circuit to operate at the edge of critical

conduction (where the inductor current just touches

zero with every cycle at maximum load). Inductor val-

ues lower than this grant no further size-reduction

benefit.

The MAX1844’s pulse-skipping algorithm initiates skip

mode at the critical conduction point. So, the inductor

operating point also determines the load-current value

at which PFM/PWM switchover occurs.

These four factors impact the component selection

process. Selecting components and calculating their

effect on the MAX1844’s operation is best done with a

spreadsheet. Using the formulas provided, calculate the

LIR (the ratio of the inductor ripple current to the

designed maximum load current) for both the minimum

and maximum input voltages. Maintaining an LIR within a

20% to 50% range is recommended. The use of a

spreadsheet allows quick evaluation of component

selection.

Inductor Selection

The switching frequency and inductor operating point

determine the inductor value as follows:

f = 300kHz, 33% ripple current or LIR = 0.33.

Find a low-loss inductor having the lowest possible DC

resistance that fits in the allotted dimensions. Ferrite

cores are often the best choice, although powdered iron

is inexpensive and can work well at 200kHz. The core

must be large enough not to saturate at the peak induc-

Most inductor manufacturers provide inductors in stan-

dard values, such as 1.0µH, 1.5µH, 2.2µH, 3.3µH, etc.

Also look for nonstandard values, which can provide a

better compromise in LIR across the input voltage range.

If using a swinging inductor (where the no-load induc-

tance decreases linearly with increasing current), evalu-

ate the LIR with properly scaled inductance values.

Transient Response

The inductor ripple current also impacts transient-

ferentials. Low inductor values allow the inductor

current to slew faster, replenishing charge removed

from the output filter capacitors by a sudden load step.

L

1.5V (7V -1.5V)

7V 300kHz 0.33 8A

1.49 H

=

××

×

=µ

L =

V(V

- V

)

V

f

LIR

I

OUT

IN

OUT

IN

LOAD(MAX)

××

×

V

V

1

R1

R2

OUT

FB

=+









High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

16

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Dual Mode is a trademark of Maxim Integrated Products.