LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 38

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3838
APPLICATIONS INFORMATION
3. DRV
4. C
“Hidden” copper trace, fuse and battery resistance, even
at DC current, can cause a significant amount of efficiency
degradation, so it is important to consider them during
the design phase. Other losses, which include the C
ESR loss, bottom MOSFET ’s body diode reverse-recovery
loss, and inductor core loss generally account for less
than 2% additional loss.
38
and INTV
rents result 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 DRV
current out of DRV
the controller I
where Qg
top and bottom MOSFETs, respectively.
Supplying DRV
several percents of efficiency, especially for high V
applications. Connecting EXTV
source will scale the V
and controller circuits by a factor of (Duty Cycle)/(Ef-
ficiency). For example, in a 20V to 5V application, 10mA
of DRV
current. This reduces the mid-current loss from 10%
or more (if the driver was powered directly from V
to only a few percent.
input current drawn by the regulator into an averaged
DC current from the supply. The capacitor itself has
a zero average DC current, but square-wave-like AC
current flows through it. Therefore the input capacitor
must have a very low ESR to minimize the RMS current
loss on ESR. It must also have sufficient capacitance
to filter out the AC component of the input current to
prevent additional RMS losses in upstream cabling,
fuses or batteries. The LTC3838 2-phase architecture
improves the ESR loss.
IN
I
GATECHG
loss. The input capacitor filters large square-wave
CC
CC
current. This is the sum of the MOSFET driver
CC
(TOP)
current results in approximately 2.5mA of V
= f • (Qg
control currents. The MOSFET driver cur-
Q
and Qg
CC
current. In continuous mode,
CC
power through EXTV
CC
to ground. The resulting dQ/dt is a
(TOP)
that is typically much larger than
(BOT)
IN
current required for the driver
+ Qg
are the gate charges of the
(BOT)
CC
to an output-derived
),
CC
could save
OUT
IN
IN
IN
)
Power losses in the switching regulator will reflect as
a higher than ideal duty cycle, or a longer on-time for a
constant frequency. This efficiency accounted on-time
can be calculated as:
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased.
Design Example
Consider a channel of step-down converter from V
4.5V to 26V to V
f = 350kHz (see Figure 13, Channel 1.
The regulated output voltage is determined by:
Using a 10k resistor for R
The frequency is programmed by:
Use the nearest 1% resistor standard value of 115k.
The minimum on-time occurs for maximum V
t
as references, make sure that the t
is greater than that the LTC3838 can achieve, and allow
sufficient margin to account for the extension of effective
on-time at light load due to the dead times (t
t
on-time for this application is:
ON(MIN)
D(TG/BG)
t
R
V
t
ON
ON(MIN)
OUT
T
[ ]
≈ t
kΩ
curves in the Typical Performance Characteristics
= 0.6V • 1+
ON(IDEAL)
in the Electrical Characteristics). The minimum
=
=
41550
f kHz
V
[
IN(MAX)
V
⎛
⎝ ⎜
OUT
/Efficiency
OUT
]
– 2.2 =
R
= 1.2V, with I
R
• f
FB2
FB1
=
FB1
⎞
⎠ ⎟
24V • 350kHz
41550
, R
350
FB2
1.2V
ON(MIN)
OUT(MAX)
is also 10k.
– 2.2 ≈ 116.5
at maximum V
= 143ns
IN
= 15A, and
. Using the
D(TG/BG) +
3838fa
IN
IN
=