LM3477 datasheet(17/23 Pages) NSC

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Output Capacitor Selection

(Continued)

How the output voltage recovers after that initial excursion

depends on how fast the inductor current falls and how large

the output capacitance is. See Figure 10.

The ESR and the capacitance of the output capacitor must

be carefully chosen so that the output voltage overshoot is

within the design’s specification V

OS(MAX). If the total com-

bined ESR of the output capacitors is not low enough, the

initial output voltage excursion will violate the specification,

see

∆V

C1. If the ESR is low enough, but there is not enough

output capacitance, the output voltage will travel outside the

specification window due to the extra charge being dumped

into the capacitor, see

∆V

C2. The LM3477/A has output over

voltage protection (OVP) which could trigger if the transient

overshoot is high enough. If this happens, the controller will

operate in hysteretic mode (see OVER VOLTAGE PROTEC-

TION section) for a few cycles before the output voltage

settles to its steady state. If this behavior is not desired,

substitute V

OVP (referred to the output) for VOS(MAX) (VOVP is

found in the ELECTRICAL CHARACTERISTICS table) to

find the minimum capacitance and maximum ESR of the

output capacitor.

Calculations for the Output Capacitor

During a loading transient, the delta output voltage

∆V

c has

two changing components. One is the voltage difference

across the ESR (

∆V

r), the other is the voltage difference

caused by the gained charge (

∆V

q). This gives:

∆V

c =

∆V

r +

∆V

The design objective is to keep

∆V

c lower than some maxi-

mum overshoot (V

OS(MAX)). VOS(MAX) is chosen based on

the output load requirements.

Both voltages

∆V

r and

∆V

qwill change with time. For

∆V

r the

equation is:

where,

ESR = the output capacitor ESR

∆I

OUT = the difference between the load current change

OUT(MAX) −IOUT(MIN)

MIN = Minimum duty cycle of device (0.165 typical)

Evaluating this equation at t = 0 gives

∆V

r(max). Substituting

OS(MAX) for

∆V

r(MAX) and solving for RESR gives:

The expression for

∆V

q is:

From Figure 11 it can be told that

∆V

C will reach its peak

value at some point in time and then decrease. The larger

the output capacitance is, the earlier the peak will occur. To

find the peak position, let the derivative of

∆V

C go to zero,

and the result is:

The intention is to find the capacitance value that will yield, at

peak,a

∆V

C that equals VOS(max). Substituting tpeak for t and

equating

∆V

C to VOS(max) gives the following solution for

OUT(MIN):

The chosen output capacitance should not be less than

47µF, even if the solution for C

OUT(MIN) is less than 47µF.

Notice it is already assumed that the total ESR is no greater

than R

ESR(MAX), otherwise the term under the square root

will be a negative number.

Power MOSFET Selection

The drive pin of LM3477/A must be connected to the gate of

an external MOSFET. In a buck topology, the drain of the

external N-Channel MOSFET is connected to the input and

the source is connected to the inductor. The C

B pin voltage

provides the gate drive needed for an external N-Channel

MOSFET. The gate drive voltage depends on the input volt-

age (see TYPICAL PERFORMANCE CHARACTERISTICS).

In most applications, a logic level MOSFET can be used. For

very low input voltages, a sub-logic level MOSFET should be

used.

The selected MOSFET directly controls the efficiency. The

critical parameters for selection of a MOSFET are:

Minimum threshold voltage, V

TH(MIN)

On-resistance, R

DS(ON)

200033B5

FIGURE 10. Output Voltage Overshoot Violation

200033B6

FIGURE 11. Output Voltage Overshoot Peak

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