pation limits often limits how small the MOSFET can be.

Again, the optimum occurs when the switching (AC)

switching losses do not usually become an issue until

the input is greater than approximately 15V.

Switching losses in the high-side MOSFET can become

an insidious heat problem when maximum AC adapter

voltages are applied, due to the squared term in the

becomes extraordinarily hot when subjected to

Calculating the power dissipation in Q1 due to switching

losses is difficult, since it must allow for difficult-to-quanti-

fy factors that influence the turn-on and turn-off times.

These factors include the internal gate resistance, gate

charge, threshold voltage, source inductance, and PC

board layout characteristics. The following switching loss

calculation provides only a very rough estimate and is no

substitute for breadboard evaluation, preferably including

a sanity check using a thermocouple mounted on Q1.

typ).

For the low-side MOSFET, Q2, the worst-case power dis-

sipation always occurs at maximum battery voltage:

The absolute worst case for MOSFET power dissipation

occurs under heavy overloads that are greater than

current limit. To protect against this possibility, you must

mum valley current allowed by the current-limit circuit,

including threshold tolerance and sense-resistance vari-

ation. If short-circuit protection without overload protec-

tion is adequate, enable undervoltage protection, and

Choose a Schottky diode D1 having a forward voltage

drop low enough to prevent the Q2 MOSFET body diode

from turning on during the dead time. As a general rule,

a diode having a DC current rating equal to 1/3 of the

load current is sufficient. This diode is optional, and if

efficiency isn’t critical it can be removed.

Applications Information

Dropout Performance

The output voltage adjust range for continuous-conduc-

tion operation is restricted by the nonadjustable 500ns

(max) minimum off-time one-shot. For best dropout per-

formance, use the slower (200kHz) on-time settings.

When working with low input voltages, the duty-factor

limit must be calculated using worst-case values for on-

and off-times. Manufacturing tolerances and internal

propagation delays introduce an error to the TON K-

factor. This error is greater at higher frequencies (Table

5). Also, keep in mind that transient response perfor-

mance of buck regulators operated close to dropout is

poor, and bulk output capacitance must often be

Response section).

The absolute point of dropout is when the inductor cur-

rent ramps down during the minimum off-time (

as much as it ramps up during the on-time (

ratio h =

slew the inductor current higher in response to

increased load, and must always be greater than 1. As

h approaches 1, the absolute minimum dropout point,

the inductor current will be less able to increase during

unless additional output capacitance is used.

A reasonable minimum value for h is 1.5, but this may

be adjusted up or down to allow trade-offs between

voltage. For a given value of h, the minimum operating

voltage can be calculated as:

from the Electrical Characteristics table, and K is taken-

from Table 5. The absolute minimum input voltage is cal-

culated with h = 1.

minimum input voltage, then operating frequency must

be reduced or output capacitance added to obtain an

response.

V

VV

th

K

VV

IN MIN

OUT

DROP

OFF MIN

DROP

DROP

()

()

=

+

()

×













+

1

21

1-

-

PD(Q1 switching)

CV

f

I

I

RSS

IN(MAX)

2

LOAD

GATE

=

×× ×

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

______________________________________________________________________________________

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