Capacitor ESR Losses

Note that losses in the output filter capacitors are small

when the circuit is heavily loaded, because the current

into the capacitor is not chopped. The output capacitor

sees only the small AC sawtooth ripple current. Ensure

that the input bypass capacitor has a ripple current rat-

IC Supply-Current Losses

5V times the quiescent supply current (a data sheet

parameter), or about 5mW.

Light-Load Efficiency

Under light loads, the PWM will operate in discontinu-

ous-conduction mode, where the inductor current dis-

charges to zero at some point during each switching

cycle. New loss mechanisms, insignificant at heavy

loads, begin to become important. The basic difference

is that in discontinuous mode, the AC component of the

inductor current is large compared to the load current.

This increases losses in the core and in the output filter

capacitors. Ferrite cores are recommended over pow-

dered-material types for best light-load efficiency.

At light loads, the inductor delivers triangular current

pulses rather than the nearly square waves found in

continuous-conduction mode. These pulses ramp up to

a point set by the idle-mode current comparator, which

is internally fixed at approximately 25% of the full-scale

current-limit level. This 25% threshold provides an opti-

mum balance between low-current efficiency and out-

put voltage noise (the efficiency curve would actually

look better with this threshold set at about 45%, but the

output noise would be too high).

____Additional Application Circuits

High-Accuracy Power Supplies

The standard application circuit’s accuracy is dominat-

ed by reference voltage error (±1.8%) and load regula-

tion error (-2.5%). Both of these parameters can be

improved as shown in Figures 5 and 6. Both circuits

rely on an external integrator amplifier to increase the

DC loop gain in order to reduce the load regulation

error to 0.1%. Reference error is improved in the first

circuit by employing a version of the MAX767 (“T”

grade) which has a ±1.2% reference voltage tolerance.

Reference error of the second circuit is further

improved by substituting a highly accurate external ref-

erence chip (MAX872), which contributes ±0.38% total

error over temperature.

These two circuits were designed with the latest gener-

ation of dynamic-clock µPs in mind, which place great

demands on the transient-response performance of the

power supply. As the µP clock starts and stops, the

load current can change by several amps in less than

100ns. This tremendous

∆i/∆t can cause output voltage

overshoot or sag that results in the CPU VCC going out

of tolerance unless the power supply is carefully

designed and located close to the CPU. These circuits

have excellent dynamic response and low ripple, with

transient excursions of less than 40mV under zero to

full-load step change. In particular, these two circuits

support the “VR” (voltage regulator) version of the Intel

P54C Pentium™ CPU, which requires that its supply

voltage, including noise and transient errors, be within

the 3.30V to 3.45V range.

To configure these circuits for a given load current

requirement, substitute standard components from

Table 1 for the power switching elements (N1, N2, L1,

C1, C2) or use the

Design Procedure. R1 can also be

taken from Table 1, but must be adjusted approximate-

ly 10% higher in order to maintain the correct current-

limit threshold. This increased value is due to the 0.9

gain factor introduced by the H-bridge resistor divider

(R3–R6).

If the remote sense line must sense the output voltage

on the far side of a connector or jumper that has the

possibility of becoming disconnected while the power

supply is operating, an additional 10k

Ω resistor should

connect the sense line to the output voltage in the con-

nector’s power-supply side in order to prevent acciden-

tal overvoltage at the CPU.

For applications that are powered from a fixed +12V or

battery input rather than from +5V, use a MAX797 IC

instead of the MAX767. The MAX797 is capable of

accepting inputs up to 30V. See the MAX796–MAX799

data sheet for a high-accuracy circuit schematic.

5V-to-3.3V, Synchronous, Step-Down

Power-Supply Controller

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