Datasheet

LTC3838
24
3838fa
APPLICATIONS INFORMATION
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
P
LOSS
R1
()
=
V
IN(MAX)
–V
OUT
()
•V
OUT
R1
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
SENSE
sensing. Light load power loss can be modestly
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
SENSE
of 10mV. For a DCR
sensing application, the actual ripple voltage will be de-
termined by:
ΔV
SENSE
=
V
IN
–V
OUT
R1• C1
•
V
OUT
V
IN
•f
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
CC
voltage.
This voltage is typically 5.3V. Pay close attention to the
BV
DSS
specification for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the on-
resistance, R
DS(ON)
, Miller capacitance, C
MILLER
, input
voltage and maximum output current. Miller capacitance,
C
MILLER
, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data sheet.
C
MILLER
is equal to the increase in gate charge along the
horizontal axis while the curve is approximately flat (or
the parameter Q
GD
if specified on a manufacturer’s data
sheet), divided by the specified V
DS
test voltage:
C
MILLER
≅
Q
GD
V
DS(TEST)
When the IC is operating in continuous mode, the duty
cycles for the top and bottom MOSFETs are given by:
D
TOP
=
V
OUT
V
IN
D
BOT
=1–
V
OUT
V
IN
The MOSFET power dissipations at maximum output
current are given by:
P
TOP
=D
TOP
•I
OUT(MAX)
2
•R
DS(ON)(MAX)
1+δ
()
+ V
IN
2
•
I
OUT(MAX)
2
⎛
⎝
⎜
⎞
⎠
⎟
•C
MILLER
R
TG(UP)
V
DRVCC
–V
MILLER
+
R
TG(DOWN)
V
MILLER
⎡
⎣
⎢
⎤
⎦
⎥
•f
P
BOT
= D
BOT
• I
OUT(MAX)
2
• R
DS(ON)(MAX)
• (1 + δ )
where δ is the temperature dependency of R
DS(ON)
, R
TG(UP)
is the TG pull-up resistance, and R
TG(DOWN)
is the TG pull-
down resistance. V
MILLER
is the Miller effect V
GS
voltage
and is taken graphically from the MOSFET ’s data sheet.
Both MOSFETs have I
2
R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
IN
< 20V,
the high current efficiency generally improves with larger
MOSFETs, while for V
IN
> 20V, the transition losses rapidly
increase to the point that the use of a higher R
DS(ON)
device
with lower C
MILLER
actually provides higher efficiency. The
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.