Datasheet

LTC3411A
16
3411afc
For more information www.linear.com/LTC3411A
applicaTions inForMaTion
3) 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 flowing
through inductor L is “chopped” between the internal top
and bottom switches. Thus, the series resistance look-
ing 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
)
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 C
IN
has adequate charge
storage and very low ESR at the switching frequency. 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 LTC3411A does not
dissipate much heat due to its high efficiency. However,
in applications where the LTC3411A is running at high
ambient 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 avoid the LTC3411A 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 junction
temperature of the part. The temperature rise is given by:
T
RISE
= P
D
• θ
JA
where P
D
is the power dissipated by the regulator and θ
JA
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, T
J
, is given by:
T
J
= T
RISE
+ T
AMBIENT
As an example, consider the case when the LTC3411A
is in dropout at an input voltage of 3.3V with a load cur-
rent of 1A. From the Typical Performance Characteristics
graph of Switch Resistance, the R
DS(ON)
resistance of the
P-channel switch is 0.15Ω. Therefore, power dissipated
by the part is:
P
D
= I
2
• R
DS(ON)
= 150mW
The MS10 package junction-to-ambient thermal resistance,
θ
JA
, will be in the range of 100°C/W to 120°C/W. Therefore,
the junction temperature of the regulator operating in a
70°C ambient temperature is approximately:
T
J
= 0.15 • 120 + 70 = 88°C
Remembering that the above junction temperature is
obtained from an R
DS(ON)
at 25°C, we might recalculate
the junction temperature based on a higher R
DS(ON)
since
it increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
Design Example
As a design example, consider using the L
TC3411A in a
portable application with a Li-Ion batter
y. The battery pro
-
vides a V
IN
= 2.5V to 4.2V. The load requires a maximum
of 1.25A in active mode and 10mA in standby mode. The
output voltage is V
OUT
= 2.5V. Since the load still needs
power in standby, Burst Mode operation is selected for
good low load efficiency.
First, calculate the timing resistor for 1MHz operation:
R
T
= 5 • 10
7
(10
3
)
–1.6508
= 557.9k
Use a standard value of 549k. Next, calculate the inductor
value for about 40% ripple current at maximum V
IN
:
L =
2.5V
1MHz • 500mA
• 1−
2.5V
4.2V






= 2µH
Choosing the closest standard inductor value from a vendor
of 2.2µH, results in a maximum ripple current of:
ΔI
L
=
2.5V
1MHz • 2.2µ
• 1−
2.5V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 460mA