VIN(MIN)

VOUT

Dmax

|

PCOND(MAX) = I

2D

MAX 1+

'

i(PK-PK)

>

@

1

12

I

2

RDS(ON)

VOS(MAX)-

VOS(MAX)

2 - ('IOUT(MAX) x RESR)2

COUT(MIN)=

(VOUT) RESR

2

(F)

L

'V

q

'V

r

'V

c

Time

0

t

peak

LM3477

www.ti.com

SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

Figure 30. Output Voltage Overshoot Peak

The intention is to find the capacitance value that will yield, at tpeak, a ΔVC that equals VOS(max). Substituting tpeak

for t and equating

ΔVC to VOS(max) gives the following solution for COUT(MIN):

(29)

The chosen output capacitance should not be less than 47µF, even if the solution for COUT(MIN) is less than 47µF.

Notice it is already assumed that the total ESR is no greater than RESR(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

CB pin voltage provides the gate drive needed for an external N-Channel MOSFET. The gate drive voltage

depends on the input voltage (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:

1. Minimum threshold voltage, VTH(MIN)

2. On-resistance, RDS(ON)

3. Total gate charge, Qg

4. Reverse transfer capacitance, CRSS

5. Maximum drain to source voltage, VDS(MAX)

The off-state voltage of the MOSFET is approximately equal to the input voltage. VDS(MAX) of the MOSFET must

be greater than the input voltage. The power losses in the MOSFET can be categorized into conduction losses

and ac switching or transition losses. RDS(ON) is needed to estimate the conduction losses. The conduction loss,

PCOND, is the I

2R loss across the MOSFET. The maximum conduction loss is given by:

(30)

where DMAX is the maximum operating duty cycle:

(31)

The turn-on and turn-off transition times of a MOSFET from the MOSFET specifications require tens of nano-

seconds. CRSS and Qg are needed from the MOSFET specifications to estimate the large instantaneous power

loss that occurs during these transitions.

The average amount of gate current required to turn the MOSFET on can be calculated using the formula:

IG = Qg.FS

(32)

The required gate drive power to turn the MOSFET on is equal to the switching frequency times the energy

required to deliver the charge to bring the gate charge voltage to VDR (see Electrical Characteristics and Typical

Performance Characteristics for the drive voltage specification).

PDrive = FS.Qg.VDR

(33)

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