LTC3878 Linear Technology Corporation, LTC3878 Datasheet - Page 11

• Manufacturer Part Number: LTC3878
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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The resulting power dissipation in the MOSFETs at maxi-
mum output current are:
DR
TG driver pull-down resistance. V
fect V
MOSFET data sheet.
MOSFET input capacitance is a combination of several
components but can be taken from the typical “gate charge”
curve included on the most data sheets (Figure 2). The
curve is generated by forcing a constant input current
into the gate of a common source, current source, loaded
stage and then plotting the gate versus time. The initial
slope is the effect of the gate-to-source and gate-to-drain
capacitance. The fl at portion of the curve is the result of the
Miller multiplication effect of the drain-to-gate capacitance
as the drain drops the voltage across the current source
load. The upper sloping line is due to the drain-to-gate
accumulation capacitance and the gate-to-source capaci-
tance. The Miller charge (the increase in coulombs on the
horizontal axis from a to b while the curve is fl at) is speci-
fi ed from a given V
for different V
the application V
way to estimate the C
gate charge from points a and b or the parameter Q
a manufacturers data sheet and divide by the specifi ed
V
C
mining the transition loss term in the top MOSFET but is
not directly specifi ed on MOSFET data sheets.
APPLICATIONS INFORMATION
DS
MILLER
P
P
TGHIGH
C
BOT
TOP
test voltage, V
MILLER
GS
=
=
is the most important selection criteria for deter-
voltage and is taken graphically from the power
+
D
D
⎡
⎢
⎣
is pull-up driver resistance and DR
V
BO
V
TOP
INTVC
V
I I N
T T
DS TEST
2
DR
DS
Q
•
•
⎛
⎜
⎝
I
(
I
OUT MAX
GD
OUT MAX
I
C C
OUT MAX
DS
TGHIGH
voltages by multiplying by the ratio of
–
DS(TEST)
DS
(
(
V
to the curve specifi ed V
(
2
)
MILLER
MILLER
drain voltage, but can be adjusted
)
)
2
2
)
⎞
⎟
⎠
•
•
.
(
ρ
ρ
C
+
τ
term is to take the change in
τ
MILLER
(
(
DR
BOT
TOP
V
MILLER
TGLOW
)
)
MILLER
•
•
R
)
R
DS ON MAX
DS ON MAX
(
⎤
⎥
⎦
(
f
OSC
is the Miller ef-
)(
)(
DS
TGLOW
)
)
values. A
GD
is the
on
Both MOSFETs have I
includes an additional term for transition loss, which are
highest at high input voltages. For V
rent effi ciency generally improves with larger MOSFETs,
while for V
to the point that the use of a higher R
lower C
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
Operating Frequency
The choice of operating frequency is a tradeoff between
effi ciency and component size. Lowering the operating fre-
quency improves effi ciency by reducing MOSFET switching
losses but requires larger inductance and/or capacitance
to maintain low output ripple voltage. Conversely, raising
the operating frequency degrades effi ciency but reduces
component size.
The operating frequency of LTC3878 applications is de-
termined implicitly by the one-shot timer that controls the
on-time, t
set by the current into the I
Tying a resistor R
on-time inversely proportional to V
converter, this results in pseudo fi xed frequency operation
as the input supply varies.
t
f
OP
V
ON
GS
=
=
MILLER
0 7
0 7
C
ON
I
.
MILLER
IN
ION
.
a
Figure 2. Gate Charge Characteristic
, of the top MOSFET switch. The on-time is
MILLER EFFECT
V R
V
> 20V, the transition losses rapidly increase
•
(
= (Q
actually provides higher effi ciency. The
10
V
Q
OUT
IN
ON
B
pF
ON
– Q
(
2
A
)
10
)/V
from V
R power loss, and the top MOSFET
b
DS
pF
)
ON
[ ]
IN
Hz
pin according to:
to the I
IN
V
IN
+
GS
–
< 20V, the high cur-
. For a step-down
DS(ON)
V
LTC3878
ON
pin yields an
device with
+
–
3878 F02
V
DS
11
V
IN
3878f