Output Capacitor Stability Considerations

Stability is determined by the value of the ESR zero rela-

tive to the switching frequency. The point of instability is

given by the following equation:

where:

For a typical 300kHz application, the ESR zero frequency

must be well below 95kHz, preferably below 50kHz.

Tantalum and OS-CON capacitors in widespread use at

the time of publication have typical ESR zero frequencies

of 25kHz. In the design example used for inductor selec-

60mV/2.7A = 22m

Ω. Two 470µF/4V Kemet T510 low-ESR

tantalum capacitors in parallel provide 22m

Ω (max) ESR.

Their typical combined ESR results in a zero at 27kHz,

well within the bounds of stability.

Do not put high-value ceramic capacitors directly across

the feedback sense point without taking precautions to

ensure stability. Large ceramic capacitors can have a

high ESR zero frequency and cause erratic, unstable

operation. However, it’s easy to add enough series resis-

tance by placing the capacitors a couple of inches

downstream from the feedback sense point, which

should be as close as possible to the inductor.

Unstable operation manifests itself in two related but dis-

tinctly different ways: double-pulsing and fast-feedback

loop instability.

Double-pulsing occurs due to noise on the output or

because the ESR is so low that there isn’t enough volt-

age ramp in the output voltage signal. This “fools” the

error comparator into triggering a new cycle immediately

after the 400ns minimum off-time period has expired.

Double-pulsing is more annoying than harmful, resulting

in nothing worse than increased output ripple. However,

it can indicate the possible presence of loop instability,

which is caused by insufficient ESR.

Loop instability can result in oscillations at the output

after line or load perturbations that can trip the overvolt-

age protection latch or cause the output voltage to fall

below the tolerance limit.

The easiest method for checking stability is to apply a

very fast zero-to-max load transient and carefully

observe the output voltage ripple envelope for over-

shoot and ringing. It can help to monitor simultaneously

the inductor current with an AC current probe. Don’t

allow more than one cycle of ringing after the initial

step-response under- or overshoot.

Input Capacitor Selection

The input capacitor must meet the ripple current

Nontantalum chemistries (ceramic, aluminum, or OS-

CON) are preferred due to their resistance to power-up

surge currents.

For optimal circuit reliability, choose a capacitor that

has less than 10°C temperature rise at the peak ripple

current.

Power MOSFET Selection

Most of the following MOSFET guidelines focus on the

challenge of obtaining high load-current capability (>5A)

when using high-voltage (>20V) AC adapters. Low-cur-

rent applications usually require less attention.

For maximum efficiency, choose a high-side MOSFET

(Q1) that has conduction losses equal to the switching

losses at the optimum battery voltage (15V). Check to

ensure that the conduction losses at minimum input

voltage do not exceed the package thermal limits or

violate the overall thermal budget. Check to ensure that

conduction losses plus switching losses at the maxi-

mum input voltage do not exceed the package ratings

or violate the overall thermal budget.

Choose a low-side MOSFET (Q2) that has the lowest

age (i.e., SO-8), and is reasonably priced. Ensure that

the MAX1844 DL gate driver can drive Q2; in other

words, check that the gate is not pulled up by the high-

side switch turn on, due to parasitic drain-to-gate capac-

itance, causing cross-conduction problems. Switching

losses are not an issue for the low-side MOSFET since it

is a zero-voltage switched device when used in the buck

topology.

MOSFET Power Dissipation

Worst-case conduction losses occur at the duty factor

extremes. For the high-side MOSFET, the worst-case

power dissipation due to resistance occurs at minimum

battery voltage:

Generally, a small high-side MOSFET is desired to

reduce switching losses at high input voltages. However,

I

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High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

18

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