LTC3605A
14
3605afg
For more information www.linear.com/LTC3605A
operaTion
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 percent
-
age of input power.
Although all
dissipative elements in the circuit produce
losses, three main sources usually account for most of the
losses in LTC3605A circuits: 1) I
2
R losses, 2) switching
and biasing losses, 3) other losses.
1. I
2
R losses are calculated from the DC resistances of
the internal switches, R
SW
, and external inductor, R
L
.
In continuous mode, the average output current flows
through inductor L but is “chopped” between the
internal top and bottom power MOSFETs. Thus, the
series resistance looking into the SW pin is a function
of both top and bottom MOSFET R
DS(ON)
and the duty
cycle (DC) as follows:
R
SW
= (R
DS(ON)
TOP)(DC) + (R
DS(ON)
BOT)(1-DC)
The R
DS(ON)
for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
curves. Thus to obtain I
2
R losses:
I
2
R losses = I
OUT
2
(R
SW
+ R
L
)
2. The INTV
CC
current is the sum of the power MOSFET
driver and control currents. The power MOSFET driver
current results from switching the gate capacitance of
the power MOSFETs. Each time a power MOSFET gate is
switched from low to high to low again, a packet of charge
dQ moves from INTV
CC
to ground. The resulting dQ/dt
is a current out of INTV
CC
that is typically much larger
than the DC control bias current. In continuous mode,
I
GATECHG
= f(Q
T
+ Q
B
), where Q
T
and Q
B
are the gate
charges of the internal top and bottom power MOSFETs
and f is the switching frequency. Since INTV
CC
is a low
dropout regulator output powered by V
IN
, its power
loss equals:
P
LDO
= V
IN
• I
INTVCC
Refer to the I
INTVCC
vs Frequency curve in the Typical
Performance Characteristics for typical INTV
CC
current
at various frequencies.
3. Other “hidden” losses such as transition loss and cop-
per trace and internal load resistances can account for
additional
efficiency
degradations in the overall power
system. It is very important to include these “system”
level losses in the design of a system. Transition loss
arises from the brief amount of time the top power
MOSFET spends in the saturated region during switch
node transitions. The LTC3605A internal power devices
switch quickly enough that these losses are not signifi
-
cant compared to other sources. Other losses including
diode conduction losses during dead-time and inductor
core losses which generally account for less than
2%
total additional loss.
Thermal Considerations
In a majority of applications
, the LTC3605A does not dis
-
sipate much heat due to its high efficiency and low thermal
resistance of its exposed-back QFN package. However, in
applications where the LTC3605A is running at high ambi
-
ent temperature, high V
IN
, high switching frequency and
maximum output current load, the heat dissipated may
exceed the maximum junction temperature of the part.
If the junction temperature reaches approximately 160°C,
both power switches will be turned off until the temperature
drops about 15°C cooler.
To avoid the LTC3605A from exceeding the maximum junc
-
tion 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 junc
-
tion temperature of the part. The temperature rise is given by:
T
RISE
= P
D
• θ
JA
As an example, consider the case when the LTC3605A is
used in applications where V
IN
= 12V, I
OUT
= 5A, f = 1MHz,
V
OUT
= 1.8V. The equivalent power MOSFET resistance
R
SW
is:
R
SW
= R
DS(ON)
Top •
V
OUT
V
IN
+R
DS(ON)
Bot 1–
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
= 70mW •
1.8
12
+ 35mW •
10.2
12