AN557 datasheet(3/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

Figure 3. The Basic Step-down Switching Regulator Configuration

Fig. 4 shows the behaviour of the most significant waveforms, in different points of the circuit, which help

to understand better the operation of the power section of the switching regulator. For the sake of simplic-

ity, the series resistance of the coil has been neglected. Fig. 2a shows the behaviour of the emitter voltage

(which is practically the voltage across the recirculation diode), where the power saturation and the for-

ward VF drop across the diode era taken into account.

The ON and OFF times are established by the following expression :

Fig. 4b shows the current across the switching transistor. The current shape is trapezoidal and the oper-

ation is in continuous mode. At this stage, the phenomena due to the catch diode, that we consider as

dynamically ideal, are neglected. Fig. 4c shows the current circulating in the recirculation diode. The sum

of the currents circulating in the power and in the diode is the current circulating in the coil as shown in

Fig. 4e. In balanced conditions the

∆IL+ current increase occuring during TON has to be equal to the ∆IL–

decrease occurring during TOFF. The mean value of IL corresponds to the charge current. The current rip-

ple is given by the following formula :

It is a good rule to respect to IoMIN ≥ IL/2 relationship, that implies good operation in continuous mode.

When this is not done, the regulator starts operating in discontinuous mode. This operation is still safe but

variations of the switching frequency may occur and the output regulation decreases.

Fig. 4d shows the behaviour of the voltage across coil L. In balanced conditions, the mean value of the

voltage across the coil is zero. Fig. 4f shows the current flowing through the capacitor, which is the differ-

ence between IL and ILOAD.

In balanced conditions, the mean current is equal to zero, and

∆IC = ∆IL. The current IC through the ca-

pacitor gives rise to the voltage ripple.

This ripple consists of two components : a capacitive component,

∆VC, and a resistive component, ∆VESR,

due to the ESR equivalent series resistance of the capacitor. Fig. 4g shows the capacitive component

∆VC

of the voltage ripple, which is the integral of a triangular-shaped current as a function of time. Moreover,

it should be observed that vC (t) is in quadrature with iC(t) and therefore with the voltage VESR. The quantity

of charge

∆Q+ supplied to the capacitor is given by the area enclosed by the ABC triangle in Fig. 4f :

sa t

–

()

OFF

--------------------------------

⋅

∆

sat

–

() V

–

---------------------------------------- T

-------------------- T

OFF

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