Kingbor Technology Co.,Ltd
TEL:(86)0755-26508846 FAX:(86)0755-26509052
KB3511
APPLICATIONS INFORMATION
produce the most improvement. Percent efficiency can be
expressed as:
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.
%Efficiency = 100% - (L1 + L2 + L3 + ...)
whereL1, L2, etc. aretheindividuallossesasapercentage
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.
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, suchasindropout, theheatdissipatedmayexceed
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.
1) The VIN current is the DC supply current given in the
Electrical Characteristics which excludes MOSFET driver
andcontrolcurrents.VIN currentresultsinasmall(<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
fromswitchingthegatecapacitanceofthepowerMOSFETs.
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-
ousmode, IGATECHG =fO(QT +QB), whereQT andQB arethe
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.
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
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:
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
70°C.FromtheTypicalPerformanceCharacteristicsgraph
of Switch Resistance, the RDS(ON) resistance of the main
switch is 0.425Ω. Therefore, power dissipated by each
channel is:
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)
PD = I2 • RDS(ON) = 272mW
4) Other ‘hidden’ losses such as copper trace and internal
battery resistances can account for additional efficiency
The MS package junction-to-ambient thermal resistance,
θJA, is 45°C/W. Therefore, the junction temperature of the
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