LTC3414 LINER [Linear Technology], LTC3414 Datasheet - Page 11

• Manufacturer Part Number: LTC3414
• Description: 4A, 4MHz, Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
The V
loss at very low load currents whereas the I
dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve at
very low load currents can be misleading since the actual
power lost is of no consequence.
1. The V
the DC bias current as given in the electrical characteristics
and the internal main switch and synchronous switch gate
charge currents. The gate charge current results from
switching the gate capacitance of the internal power
MOSFET switches. Each time the gate is switched from
high to low to high again, a packet of charge dQ moves
from V
of V
continuous mode, I
are the gate charges of the internal top and bottom
switches. Both the DC bias and gate charge losses are
proportional to V
nounced at higher supply voltages.
2. I
internal switches, R
tinuous mode the average output current flowing through
inductor L is “chopped” between the main switch and the
synchronous switch. Thus, the series resistance looking
into the SW pin is a function of both top and bottom
MOSFET R
The R
obtained from the Typical Performance Characteristics
curves. To obtain I
multiply the result by the square of the average output
current.
Other losses including C
losses and inductor core losses generally account for less
than 2% of the total loss.
Thermal Considerations
In most applications, the LTC3414 does not dissipate
much heat due to its high efficiency.
R
2
IN
SW
R losses are calculated from the resistances of the
DS(ON)
IN
that is typically larger than the DC bias current. In
IN
= (R
quiescent current loss dominates the efficiency
IN
to ground. The resulting dQ/dt is the current out
DS(ON)
quiescent current is due to two components:
DS(ON)
for both the top and bottom MOSFETs can be
IN
and the duty cycle (DC) as follows:
TOP)(DC) + (R
GATECHG
2
; thus, their effects will be more pro-
U
SW
R losses, simply add R
, and external inductor R
U
IN
= f(QT + QB) where QT and QB
and C
DS(ON)
W
OUT
BOT)(1 – DC)
ESR dissipative
SW
U
to R
L
. In con-
2
R loss
L
and
However, in applications where the LTC3414 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 LTC3414 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:
where P
is the thermal resistance from the junction of the die to the
ambient temperature. For the 20-lead exposed TSSOP
package, the
The junction temperature, T
where T
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (R
To maximize the thermal performance of the LTC3414, the
exposed pad should be soldered to a ground plane.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current.
When a load step occurs, V
amount equal to I
series resistance of C
discharge C
the regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability prob-
lem. The I
tor shown in Figure 1 will provide adequate compensation
for most applications.
t
T
r
J
= (P
= T
D
A
A
D
is the power dissipated by the regulator and
is the ambient temperature.
TH
)(
+ t
OUT
pin external components and output capaci-
r
JA
JA
)
generating a feedback error signal used by
is 38 C/W.
LOAD(ESR)
OUT
. I
OUT
J
OUT
LOAD
, is given by:
, where ESR is the effective
OUT
to its steady-state value.
immediately shifts by an
also begins to charge or
can be monitored for
LTC3414
DS(ON)
11
3414f
JA
).