KB3426B-1.8 datasheet(10/16 Pages) KINGBOR

10 / 16 page

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of the

losses in KB3426 circuits: V IN quiescent current and I2R

losses. The VIN quiescent current loss dominates the

efficiency loss at very low load currents whereas the I2R

loss dominates the efficiency loss at medium to high load

currents. In a typical efficiency plot, the efficiency curve at

very low load currents can be misleading since the actual

power lost is of no consequence as illustrated in Figure 4.

2. I2R losses are calculated from the resistances of the

internal switches, RSW, and external inductor RL. In

continuous mode, the average output current flowing

through inductor L is “chopped” between the main

switch and the synchronous switch. Thus, the series

resistance looking into the SW pin is a function of both

top and bottom MOSFET RDS(ON) and the duty cycle

(DC) as follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

The RDS(ON) for both the top and bottom MOSFETs can

be obtained from the Typical Performance Charateristics

curves. Thus, to obtain I2R losses, simply add RSW to

RL and multiply the result by the square of the average

output current.

Other losses including CIN and COUT ESR dissipative

losses and inductor core losses generally account for less

than 2% total additional loss.

Thermal Considerations

In most applications the KB3426 does not dissipate

much heat due to its high efficiency. But, in applications

where the KB3426 is running at high ambient tempera-

ture with low supply voltage and high duty cycles, such

as in dropout, the heat dissipated may exceed the maxi-

mum junction temperature of the part. If the junction

temperature reaches approximately 150

°C, both power

switches will be turned off and the SW node will become

high impedance.

To avoid the KB3426 from exceeding the maximum

junction temperature, the user will need to do some

thermal analysis. The goal of the thermal analysis is to

determine whether the power dissipated exceeds the

maximum junction temperature of the part. The tempera-

ture rise is given by:

TR = (PD)( JA)

where PD is the power dissipated by the regulator and JA

is the thermal resistance from the junction of the die to the

ambient temperature.

Figure 4. Power Lost vs Load Current

1. The VIN quiescent current is due to two components:

the DC bias current as given in the electrical character-

istics and the internal main switch and synchronous

switch gate charge currents. The gate charge current

results from switching the gate capacitance of the

internal power MOSFET switches. Each time the gate is

switched from high to low to high again, a packet of

charge, dQ, moves from VIN to ground. The resulting

dQ/dt is the current out of VINthat is typically larger than

the DC bias current. In continuous mode, IGATECHG =

f(QT + QB) where QT and QB are the gate charges of the

internal top and bottom switches. Both the DC bias and

gate charge losses are proportional to VIN and thus

their effects will be more pronounced at higher supply

voltages.

LOAD CURRENT (mA)

0.1

0.00001

0.001

100

1000

0.0001

0.01

0.1

VOUT = 1.2V

VOUT = 1.5V

VOUT = 1.8V

VOUT = 2.5V

KB3426

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