ML4873CR データシートの表示（PDF） - Micro Linear Corporation

部品番号

コンポーネント説明

メーカー

ML4873CR

Battery Power Control IC

Micro Linear Corporation 

ML4873

Core losses, which contribute significantly to overall

efficiency losses, should be minimized by using an

inductor designed for minimum losses at the chosen

operating frequency. This is a function of the core

material, and is lowest in “Kool Mu” and molyperm cores.

Of course, efficiency and cost are often inversely related

when it comes to magnetic materials.

FREQUENCY SELECTION

Frequency is set by the resistor RT, which establishes the

charge current for the internal capacitor. Since the

discharge current is a constant, the dead time of the

oscillator is constant, the maximum duty cycle increases

as the oscillator frequency decreases. For low input

voltage applications, a lower switching frequency may be

required to maintain regulation at minimum input voltage.

Losses are heavily comprised of AC losses from the

switching characteristic of the power MOSFETs and

inductor core losses. Hence, reducing the switching

frequency may result in higher efficiencies. As inductor

conduction losses will increase at lower frequencies (size

goes up, hence there are more copper losses), there will

be a point at which this effect cancels the beneficial effect

on the AC losses and further reductions no longer increase

efficiencies. Also, reductions in operating frequency will

result in larger magnetics, and a larger overall supply.

COMPENSATION

Proper compensation is the most critical part of designing

a working supply. The compensation network must ensure

stability over the full range of input voltage and load

conditions, as well as maximize the available bandwidth

for good transient response.

If an appreciable ESR exists such that ResrC > LC/5, then

we can get away with adding one additional zero to the

error amplifier‘s feedback network, and make use of the

other zero created by the combination of the ESR and the

output capacitance. We must also add an additional

resistance in parallel with the zero we have added, this

will give us increased bandwidth and lower the DC gain.

Its size is determined by the gain necessary to bring the

system to 0dB at the desired crossover point. As a rule of

thumb, this point should be no more than 1/5 the

switching frequency.

In cases where the ESR of the output capacitors is

minimal, we no longer have a zero for free.

Now, we must use a zero on the input of the error

amplifier in addition to the zero in the feedback network.

The parallel feedback resistor is also still required; the

gain is now the parallel combination of the feedback zero

resistor and this resistor.

The internal error amplifier has an open loop gain of 90

dB, and a single pole at 31Hz. These must be taken into

account in order to adequately compensate the supply.

8

Share Link: