Simple SOT23 Boost Controllers

10

______________________________________________________________________________________

Optional Feed-Forward

Capacitor Selection

For proper control of peak inductor current during soft-

start and for stable switching, the ripple at FB should

be greater than 25mV. Without a feed-forward capaci-

tor connected between the output and FB, the output’s

requirement. Alternatively, if a low-ESR output capacitor

is chosen to obtain small output ripple, then a feed-for-

ward capacitor should be used, and the output ripple

may be as low as 25mV. The approximate value of the

feed-forward capacitor is given by:

Do not use a feed-forward capacitor that is much larger

than this because line-transient performance will

degrade. Do not use a feed-forward capacitor at all if

the output ripple is large enough without it to provide

stable switching because load regulation will degrade.

Optional Feedback Capacitor Selection

When using a feed-forward capacitor, it is possible to

achieve too much ripple at FB. The symptoms of this

include excessive line and load regulation and possibly

high output ripple at light loads in the form of pulse

groupings or “bursts.” Fortunately, this is easy to cor-

rect by either choosing a lower-ESR output capacitor or

by adding a feedback capacitor between FB and

feed-forward capacitor, form an AC-coupled ripple volt-

age-divider from the output to FB:

ideal ripple at FB is from 25mV to 40mV, which will pro-

vide stable switching, low output ripple at light and

medium loads, and reasonable line and load regula-

tion. Never use a feedback capacitor without also using

a feed-forward capacitor.

Input Capacitor Selection

current peaks drawn from the input supply, increases

efficiency, and reduces noise injection. The source

impedance of the input supply largely determines the

input capacitance, particularly as the input voltage

falls. Since step-up DC-DC converters act as “constant-

power” loads to their input supply, input current rises

as input voltage falls. Consequently, in low-input-volt-

can add as many as five percentage points to conver-

sion efficiency. A good starting point is to use the same

capacitor must also meet the ripple current requirement

imposed by the switching currents, which is about 30%

designs.

In addition to the bulk input capacitor, a ceramic 0.1µF

sible. In bootstrapped configuration, it is recommended

to isolate the bypass capacitor from the output capaci-

tor with a series 10

Ω resistor between the output and

Power MOSFET Selection

The MAX1522/MAX1523/MAX1524 drive a wide variety

of N-channel power MOSFETs (NFETs). Since the out-

less than 4.5V, is achieved with low-threshold NFETs

that specify on-resistance with a gate-source voltage

parameters include:

1) Total gate charge (Qg)

At high switching rates, dynamic characteristics (para-

meters 1 and 2 above) that predict switching losses

which predicts I

associated with charging the gate. In addition, this

parameter helps predict the current needed to drive the

gate when switching at high frequency. The continuous

Use the FET manufacturer’s typical value for Qg (see

manufacturer’s graph of Qg vs. Vgs) in the above

equation since a maximum value (if supplied) is usually

IQg

GATE

SWITCHING

=× ƒ

Ripple

Ripple

C

CC

FB

OUTPUT

FF

FB

FF

=

+









×

C

RR

+









−

310

1

1

1

2

6