width is inversely proportional to input voltage and directly

proportional to output voltage. Another one-shot sets a

minimum off-time (400ns typ). The on-time one-shot is trig-

gered if the error comparator is low, the low-side switch

current is below the current-limit threshold, and the mini-

mum off-time one-shot has timed out.

On-Time One-Shot (TON)

The heart of the PWM core is the one-shot that sets the

high-side switch on-time. This fast, low-jitter, adjustable

one-shot includes circuitry that varies the on-time in

response to battery and output voltage. The high-side

switch on-time is inversely proportional to the battery

voltage as measured by the V+ input, and proportional

to the output voltage. This algorithm results in a nearly

constant switching frequency despite the lack of a fixed-

frequency clock generator. The benefits of a constant

switching frequency are twofold: first, the frequency can

be selected to avoid noise-sensitive regions such as the

455kHz IF band; second, the inductor ripple-current

operating point remains relatively constant, resulting in

easy design methodology and predictable output volt-

age ripple. The on-time is given by:

where K (switching period) is set by the TON pin-strap

connection (Table 4), and 0.075V is an approximation to

accommodate for the expected drop across the low-side

MOSFET switch. One-shot timing error increases for the

shorter on-time settings due to fixed propagation delays;

it is approximately ±12.5% at 600kHz and 450kHz, and

±10% at the two slower settings. This translates to

reduced switching-frequency accuracy at higher frequen-

cies (Table 5). Switching frequency increases as a func-

tion of load current due to the increasing drop across the

low-side MOSFET, which causes a faster inductor-current

discharge ramp. The on-times guaranteed in the

Electrical Characteristics are influenced by switching

delays in the external high-side power MOSFET.

Two external factors that influence switching-frequency

accuracy are resistive drops in the two conduction loops

(including inductor and PC board resistance) and the

dead-time effect. These effects are the largest contribu-

tors to the change of frequency with changing load cur-

rent. The dead-time effect increases the effective

on-time, reducing the switching frequency as one or

both dead times are added to the effective on-time. It

occurs only in PWM mode (SKIP = high) when the induc-

tor current reverses at light or negative load currents.

With reversed inductor current, the inductor’s EMF caus-

es LX to go high earlier than normal, extending the on-

time by a period equal to the low-to-high dead time.

For loads above the critical conduction point, the actual

switching frequency is:

f

VV

t(V

V

)

OUT

DROP1

ON

IN

DROP2

=

+

+

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

12

______________________________________________________________________________________

Table 3. Operating Mode Truth Table

Normal operation with automatic PWM/PFM switchover for pulse skipping at light loads.

Best light-load efficiency.

Run

(PFM/PWM)

Switching

GND

1

Low-noise operation with no automatic switchover. Fixed-frequency PWM action is

forced regardless of load. Inductor current reverses at light load levels. Low noise,

Run (PWM),

Low Noise

Switching

1

Fault latch has been set by overvoltage protection, output UVLO, or thermal shutdown.

Fault

High

X

1

Shutdown

High or

Low

X

0

COMMENTS

MODE

DL

SKIP

SHDN

Good operating point for

compound buck designs

or desktop circuits.

+5V input

600

TON = GND

450

TON = REF

3-cell Li+ notebook

Useful in 3-cell systems

for lighter loads than the

CPU core or where size is

key.

Considered mainstream

by current standards.

4-cell Li+ notebook

300

TON = Float

200

4-cell Li+ notebook

Use for absolute best

efficiency.

COMMENTS

TYPICAL

APPLICATION

FREQUENCY

(kHz)

Table 4. Frequency Selection Guidelines