LTC3407EMSE Linear Technology, LTC3407EMSE Datasheet - Page 11

• Manufacturer Part Number: LTC3407EMSE
• Description: IC REG DC/DC DUAL 1.5MHZ 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3407EMSE

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 5 V
• Current - Output: 1A
• Frequency - Switching: 1.5MHz
• Voltage - Input: 2.5 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP Exposed Pad, 10-HMSOP, 10-eMSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS INFORMATION
produce the most improvement. Percent effi ciency can
be expressed as:
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, 4 main sources usually account for most of the
losses in LTC3407 circuits: 1)V
switching losses, 3) I
1) The V
Electrical Characteristics which excludes MOSFET driver
and control currents. V
loss that increases with V
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 V
ground. The resulting dQ/dt is a current out of V
typically much larger than the DC bias current. In continu-
ous mode, I
the gate charges of the internal top and bottom MOSFET
switches. The gate charge losses are proportional to V
and thus their effects will be more pronounced at higher
supply voltages.
3) I
internal switches, R
tinuous mode, the average output current fl owing 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
R
The R
be obtained from the Typical Performance Characteristics
curves. Thus, to obtain I
4) Other ‘hidden’ losses such as copper trace and internal
battery resistances can account for additional effi ciency
degradations in portable systems. It is very important
DS(ON)
%Effi ciency = 100% - (L1 + L2 + L3 + ...)
R
I
2
2
R losses are calculated from the DC resistances of the
SW
R losses = (I
DS(ON)
= (R
and the duty cycle (DC) as follows:
IN
current is the DC supply current given in the
GATECHG
DS(ON)TOP
for both the top and bottom MOSFETs can
OUT
SW
= f
)
2
2
)(DC) + (R
IN
, and external inductor, R
R losses, 4) other losses.
(R
O
current results in a small (<0.1%)
2
(Q
R losses:
SW
IN
T
, even at no load.
+ Q
+ R
DS(ON)BOT
IN
B
L
), where Q
)
quiescent current, 2)
)(1 – DC)
T
and Q
L
IN
. In con-
that is
B
IN
are
to
IN
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
charge 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.
Thermal Considerations
In a majority of applications, the LTC3407 does not dis-
sipate much heat due to its high effi ciency. However, in
applications where the LTC3407 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 prevent the LTC3407 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 temperature rise is
given by:
where P
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, T
As an example, consider the case when the LTC3407 is
in dropout on both channels at an input voltage of 2.7V
with a load current of 600mA and an ambient temperature
of 70°C. From the Typical Performance Characteristics
graph of Switch Resistance, the R
the main switch is 0.425Ω. Therefore, power dissipated
by each channel is:
The MS package junction-to-ambient thermal resistance,
θ
JA
T
T
P
, is 45°C/W. Therefore, the junction temperature of
RISE
J
D
= T
= I
D
= P
OUT
RISE
is the power dissipated by the regulator and θ
D
2
+ T
• θ
• R
AMBIENT
JA
DS(ON)
= 153mW
J
, is given by:
DS(ON)
LTC3407
IN
has adequate
resistance of
11
3407fa
JA