Miniature, Low-Voltage,

Precision Step-Down Controller

16

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Use only specialized low-ESR capacitors intended for

switching-regulator applications, such as AVX TPS,

Sprague 595D, Sanyo OS-CON, or Nichicon PL series.

To ensure stability, the capacitor must meet both mini-

mum capacitance and maximum ESR values as given

in the following equations:

ƒ

below.

These equations are worst case, with 45 degrees of

phase margin to ensure jitter-free, fixed-frequency

operation, and provide a nicely damped output

response for zero to full-load step changes. Some cost-

conscious designers may wish to bend these rules with

less-expensive capacitors, particularly if the load lacks

large step changes. This practice is tolerable if some

bench testing over temperature is done to verify

acceptable noise and transient response.

No well-defined boundary exists between stable and

unstable operation. As phase margin is reduced, the first

symptom is timing jitter, which shows up as blurred edges

in the switching waveforms where the scope does not quite

sync up. Technically speaking, this jitter (usually harmless)

is unstable operation since the duty factor varies slightly.

As capacitors with higher ESRs are used, the jitter

becomes more pronounced, and the load-transient output

voltage waveform starts looking ragged at the edges.

Eventually, the load-transient waveform has enough ringing

on it that the peak noise levels exceed the allowable output

voltage tolerance. Note that even with zero phase margin

and gross instability, the output voltage seldom declines

Designers of RF communicators or other noise-sensi-

tive analog equipment should be conservative and stay

within the guidelines. Designers of notebook computers

and similar commercial-temperature-range digital sys-

without affecting stability or transient response.

The output voltage ripple, which is usually dominated by

the filter capacitor’s ESR, can be approximated as

full equation for ripple in continuous-conduction mode is

idle mode, the inductor current becomes discontinuous,

with high peaks and widely spaced pulses, so the noise

can actually be higher at light load (compared to full

load). In idle mode, calculate the output ripple as follows:

Selecting Other Components

MOSFET Switches

The high-current N-channel MOSFETs must be logic-

level types with guaranteed on-resistance specifications

better (i.e., 2V max rather than 3V max). Drain-source

breakdown voltage ratings must at least equal the maxi-

mum input voltage, preferably with a 20% margin. The

best MOSFETs have the lowest on-resistance per

Qg provides a good figure of merit for comparing vari-

ous MOSFETs. Newer MOSFET process technologies

with dense cell structures generally perform best. The

internal gate drivers tolerate >100nC total gate charge,

but 70nC is a more practical upper limit to maintain best

switching times.

In high-current applications, MOSFET package power

dissipation often becomes a dominant design factor.

distributed between Q1 and Q2 according to duty fac-

tor, as shown in the following equations. Generally,

switching losses affect only the upper MOSFET since

the Schottky rectifier usually clamps the switching node

before the synchronous rectifier turns on. Gate-charge

losses are dissipated by the driver and do not heat the

MOSFET. Calculate the temperature rise according to

package thermal-resistance specifications to ensure

that both MOSFETs are within their maximum junction

temperature at high ambient temperature. The worst-

case dissipation for the high-side MOSFET occurs at

both extremes of input voltage, and the worst-case dis-

sipation for the low-side MOSFET occurs at maximum

input voltage.

bility (1A typ), and the DH driver inherent rise/fall time is

20ns. The MAX1637’s output undervoltage shutdown

function protects the synchronous rectifier under output

short-circuit conditions. To reduce EMI, add a 0.1µF

ceramic capacitor from the high-side switch drain to

the low-side switch source.