LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 24

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

LTC3838
APPLICATIONS INFORMATION
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
R
higher with a DCR network than with a sense resistor due
to the extra switching losses incurred through R1. However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher efficiency at heavy loads.
Peak efficiency is about the same with either method.
To maintain a good signal-to-noise ratio for the current
sense signal, start with a ∆V
sensing application, the actual ripple voltage will be de-
termined by:
Power MOSFET Selection
Two external N-channel power MOSFETs must be selected
for each channel of the LTC3838 controller: one for the
top (main) switch and one for the bottom (synchronous)
switch. The gate drive levels are set by the DRV
This voltage is typically 5.3V. Pay close attention to the
BV
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the on-
resistance, R
voltage and maximum output current. Miller capacitance,
C
usually provided on the MOSFET manufacturers’ data sheet.
C
24
MILLER
MILLER
SENSE
P
DSS
ΔV
LOSS
SENSE
specification for the MOSFETs as well; most of the
sensing. Light load power loss can be modestly
, can be approximated from the gate charge curve
is equal to the increase in gate charge along the
( )
R1
=
DS(ON)
=
V
(
IN
R1• C1
V
IN(MAX)
– V
, Miller capacitance, C
OUT
– V
•
R1
V
V
IN
OUT
OUT
SENSE
• f
)
• V
of 10mV. For a DCR
OUT
MILLER
CC
voltage.
, input
horizontal axis while the curve is approximately flat (or
the parameter Q
sheet), divided by the specified V
When the IC is operating in continuous mode, the duty
cycles for the top and bottom MOSFETs are given by:
The MOSFET power dissipations at maximum output
current are given by:
where δ is the temperature dependency of R
is the TG pull-up resistance, and R
down resistance. V
and is taken graphically from the MOSFET ’s data sheet.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
the high current efficiency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
short-circuit when the synchronous switch is on close to
100% of the period.
P
•
P
TOP
⎛
⎝ ⎜
BOT
I
C
D
D
OUT(MAX)
MILLER
BOT
TOP
= D
= D
2
TOP
= 1–
=
BOT
V
≅
V
MILLER
⎞
⎠ ⎟
OUT
•I
IN
•C
OUT(MAX)
V
• I
V
V
OUT
DS(TEST)
MILLER
IN
OUT(MAX)
GD
Q
GD
actually provides higher efficiency. The
MILLER
if specified on a manufacturer’s data
2
IN
R losses while the topside N-channel
2
⎡
⎢
⎣
> 20V, the transition losses rapidly
•R
V
2
DRVCC
DS(ON)(MAX)
is the Miller effect V
• R
R
DS(ON)(MAX)
TG(UP)
– V
DS
TG(DOWN)
MILLER
test voltage:
(
1+ δ
+
• (1 + δ )
)
DS(ON)
R
+ V
is the TG pull-
DS(ON)
TG(DOWN)
V
MILLER
IN
GS
2
IN
, R
voltage
device
< 20V,
TG(UP)
3838fa
⎤
⎥ • f
⎦