LTC3607
applicaTions inForMaTion
Efficiency Considerations
The R
for both the top and bottom MOSFETs can
DS(ON)
be obtained from the Typical Performance Characteristics
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can
be expressed as:
2
curves. Thus, to obtain I R losses:
2
2
I R losses = I
(R + R )
SW L
OUT
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
includethesesystemlevellossesinthedesignofasystem.
The internal battery and fuse resistance losses can be
%Efficiency = 100% - (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power. Although all dissipative elements in
the circuit produce losses, four main sources usually
minimized by making sure that C has adequate charge
IN
storage and very low ESR at the switching frequency.
Other losses including diode conduction losses during
dead-time and inductor core losses generally account for
less than 2% total additional loss.
account for most of the losses in LTC3607 circuits: 1)
2
V quiescent current, 2) switching losses, 3) I R losses,
IN
4) other losses.
1) The V current is the DC supply current given in the
Thermal Considerations
IN
Electrical Characteristics which excludes MOSFET driver
In a majority of applications, the LTC3607 does not dis-
sipate much heat due to its high efficiency. However, in
applicationswheretheLTC3607isrunningathighambient
temperaturewithlowsupplyvoltageandhighdutycycles,
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 for each channel will be turned off and the SW
nodes will become high impedance.
and control currents. V current results in a small loss
IN
that increases with V , even at no load.
IN
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 V to
IN
ground. The resulting dQ/dt is a current out of V that is
IN
typically much larger than the DC bias current. In continu-
To prevent the LTC3607 from exceeding the maximum
junctiontemperature,theuserwillneedtodosomethermal
analysis. The goal of the thermal analysis is to determine
whether the power dissipated exceeds the maximum
junction temperature of the part. The temperature rise
is given by:
ous mode, I
= f (Q + Q ), where Q and Q are
GATECHG
O T B T B
the gate charges of the internal top and bottom MOSFET
switches. The gate charge losses are proportional to V
IN
and thus their effects will be more pronounced at higher
supply voltages.
2
3) I R losses are calculated from the DC resistances of
T
= P • θ
D JA
RISE
the internal switches, R , and external inductor, R . In
SW
L
where P is the power dissipated by the regulator and θ
continuousmode,theaverageoutputcurrentflowsthrough
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
D
JA
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, T , is given by:
J
R
and the duty cycle (D) as follows:
DS(ON)
T = T
J
+ T
AMBIENT
RISE
R
SW
= (R
)(D) + (R
)(1 – D)
DS(ON)TOP
DS(ON)BOT
3607fb
12
For more information www.linear.com/LTC3607
Linear Systems [ Linear Systems ]
