KB3511 datasheet(11/14 Pages) KINGBOR

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KB3511

Kingbor Technology Co.,Ltd

TEL:(86)0755-26508846 FAX:(86)0755-26509052

APPLICATIONS INFORMATION

produce the most improvement. Percent efficiency can be

expressed as:

%Efficiency = 100% - (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, 4 main sources usually account for most of the

losses in KB3511 circuits: 1)VIN quiescent current, 2)

switching losses, 3) I2R losses, 4) other losses.

1) The VIN current is the DC supply current given in the

Electrical Characteristics which excludes MOSFET driver

and control currents. VIN current results in a small (<0.1%)

loss that increases with VIN, even at no load.

2) The switching current is the sum of the MOSFET driver

and control currents. The MOSFET driver current results

from switching the gate capacitance of the power MOSFETs.

Each time a MOSFET gate is switched from low to high to

low again, a packet of charge dQ moves from VIN to

ground. The resulting dQ/dt is a current out of VIN that is

typically much larger than the DC bias current. In continu-

ous mode, IGATECHG = fO(QT + QB), where QT and QB are the

gate charges of the internal top and bottom MOSFET

switches. The gate charge losses are proportional to VIN

and thus their effects will be more pronounced at higher

supply voltages.

3) I2R losses are calculated from the DC resistances of the

internal switches, RSW, and external inductor, RL. In

continuous mode, the average output current flowing

through inductor L, but is “chopped” between the internal

top and bottom switches. 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 Characteristics

curves. Thus, to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

4) Other ‘hidden’ losses such as copper trace and internal

battery resistances can account for additional efficiency

degradations in portable systems. It is very important to

include these “system” level losses in the design of a

system. The internal battery and fuse resistance losses

can be minimized by making sure that CIN has adequate

charge storage and very low ESR at the switching fre-

quency. Other losses including diode conduction losses

during dead-time and inductor core losses generally ac-

count for less than 2% total additional loss.

Thermal Considerations

In a majority of applications, the KB3511 does not

dissipate much heat due to its high efficiency. However, in

applications where the KB3511 is running at high ambi-

ent temperature with low supply voltage and high duty

cycles, such as in dropout, the heat dissipated may exceed

the maximum 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 prevent the KB3511 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:

TRISE = PD • θJA

where PD is the power dissipated by the regulator and θJA

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

ambient temperature.

The junction temperature, TJ, is given by:

TJ = TRISE + TAMBIENT

As an example, consider the case when the KB3511 is in

dropout on both channels at an input voltage of 2.7V with

a load current of 800mA and an ambient temperature of

°C.FromtheTypicalPerformanceCharacteristicsgraph

of Switch Resistance, the RDS(ON) resistance of the main

switch is 0.425

Ω. Therefore, power dissipated by each

channel is:

PD = I2 • RDS(ON) = 272mW

The MS package junction-to-ambient thermal resistance,

θJA, is 45°C/W. Therefore, the junction temperature of the

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