MIC4451BM(1998) 데이터 시트보기 (PDF) - Micrel

부품명

상세내역

제조사

MIC4451BM
(Rev.:1998)

12A-Peak Low-Side MOSFET Driver

Micrel 

MIC4451/4452

driver. The energy stored in a capacitor is described by the

equation:

E = 1/2 C V2

As this energy is lost in the driver each time the load is

charged or discharged, for power dissipation calculations the

1/2 is removed. This equation also shows that it is good

practice not to place more voltage on the capacitor than is

necessary, as dissipation increases as the square of the

voltage applied to the capacitor. For a driver with a capacitive

load:

PL = f C (VS)2

where:

f = Operating Frequency

C = Load Capacitance

VS = Driver Supply Voltage

Inductive Load Power Dissipation

For inductive loads the situation is more complicated. For the

part of the cycle in which the driver is actively forcing current

into the inductor, the situation is the same as it is in the

resistive case:

PL1 = I2 RO D

However, in this instance the RO required may be either the

on resistance of the driver when its output is in the high state,

or its on resistance when the driver is in the low state,

depending on how the inductor is connected, and this is still

only half the story. For the part of the cycle when the inductor

is forcing current through the driver, dissipation is best

described as

PL2 = I VD (1 – D)

where VD is the forward drop of the clamp diode in the driver

(generally around 0.7V). The two parts of the load dissipation

must be summed in to produce PL

PL = PL1 + PL2

Quiescent Power Dissipation

Quiescent power dissipation (PQ, as described in the input

section) depends on whether the input is high or low. A low

input will result in a maximum current drain (per driver) of

≤ 0.2mA; a logic high will result in a current drain of ≤ 3.0mA.

Quiescent power can therefore be found from:

PQ = VS [D IH + (1 – D) IL]

where:

IH = quiescent current with input high

IL = quiescent current with input low

D = fraction of time input is high (duty cycle)

Micrel

VS = power supply voltage

Transition Power Dissipation

Transition power is dissipated in the driver each time its

output changes state, because during the transition, for a

very brief interval, both the N- and P-channel MOSFETs in

the output totem-pole are ON simultaneously, and a current

is conducted through them from VS to ground. The transition

power dissipation is approximately:

PT = 2 f VS (A•s)

where (A•s) is a time-current factor derived from the typical

characteristic curve “Crossover Energy vs. Supply Voltage.”

Total power (PD) then, as previously described is:

PD = PL + PQ + PT

Definitions

CL = Load Capacitance in Farads.

D = Duty Cycle expressed as the fraction of time the

input to the driver is high.

f = Operating Frequency of the driver in Hertz

IH = Power supply current drawn by a driver when both

inputs are high and neither output is loaded.

IL = Power supply current drawn by a driver when both

inputs are low and neither output is loaded.

ID = Output current from a driver in Amps.

PD = Total power dissipated in a driver in Watts.

PL = Power dissipated in the driver due to the driver’s

load in Watts.

PQ = Power dissipated in a quiescent driver in Watts.

PT =

Power dissipated in a driver when the output

changes states (“shoot-through current”) in Watts.

NOTE: The “shoot-through” current from a dual

transition (once up, once down) for both drivers is

stated in Figure 7 in ampere-nanoseconds. This

figure must be multiplied by the number of repeti-

tions per second (frequency) to find Watts.

RO = Output resistance of a driver in Ohms.

VS = Power supply voltage to the IC in Volts.

5-78

April 1998

Share Link: