Datasheet

LTC3863
16
3863f
For more information www.linear.com/3863
APPLICATIONS INFORMATION
The LTC3863 has internal filtering of the current sense
voltage which should be adequate in most applications.
However, adding a provision for an external filter offers
added flexibility and noise immunity, should it be neces-
sary. The filter can be created by placing a resistor from the
R
SENSE
resistor to the SENSE pin and a capacitor across
the V
IN
and SENSE pins.
Power MOSFET Selection
The LTC3863 drives a P-channel power MOSFET that
serves as the main switch for the nonsynchronous
inverting converter. Important P-channel power MOSFET
parameters include drain-to-source breakdown voltage
BV
DSS
, threshold voltage V
GS(TH)
, on-resistance R
DS(ON)
,
gate-to-drain reverse transfer capacitance C
RSS
, maximum
drain current I
D(MAX)
, and the MOSFET’s thermal resistance
θ
JC(MOSFET)
and θ
JA(MOSFET)
.
The drain-to-source breakdown voltage must meet the
following condition:
BV
DSS
> V
IN(MAX)
+ |V
OUT
| + V
D
The gate driver bias voltage V
IN
-V
CAP
is set by an internal
LDO regulator. In normal operation, the CAP pin will be
regulated to 8V below V
IN
. A minimum 0.1µF capacitor
is required across the V
IN
and CAP pins to ensure LDO
stability. If required, additional capacitance can be added
to accommodate higher gate currents without voltage
droop. In shutdown and Burst Mode operation, the CAP
LDO is turned off. In the event of CAP leakage to ground,
the CAP voltage is limited to 9V by a weak internal clamp
from V
IN
to CAP. As a result, a minimum 10V V
GS
rated
MOSFET is required.
The power dissipated by the P-channel MOSFET when the
LTC3863 is in continuous conduction mode is given by:
P
PMOS
≈D•
I
OUT
1–D






2
• ρ
T
•R
DS(ON)
+
f • C
MILLER
• V
IN
+| V
OUT
|+V
D
( )
2
2
•
I
OUT
1–D
•
R
DN
V
IN
– V
CAP
– V
MILLER
( )
+
R
UP
V
MILLER






where D is duty factor, R
DS(ON)
is on-resistance of
P-channel MOSFET, ρ
T
is temperature coefficient of on-
resistance, R
DN
is the pull-down driver resistance specified
at 0.9Ω typical and R
UP
is the pull-up driver resistance
specified at 2Ω typical. V
MILLER
is the Miller effective V
GS
voltage and is taken graphically from the power MOSFET
data sheet.
The power MOSFET input capacitance, C
MILLER
, is the most
important selection criteria for determining the transition
loss term in the P-channel MOSFET but is not directly speci-
fied on MOSFET data sheets. C
MILLER
is a combination of
several components, but it can be derived from the typical
gate charge curve included on most data sheets (Figure4).
The curve is generated by forcing a constant current out
of the gate of a common-source connected P-channel
MOSFET that is loaded with a resistor, and then plotting
the gate voltage versus time. The initial slope is the effect
of the gate-to-source and gate-to-drain capacitances. The
flat portion of the curve is the result of the Miller multipli-
cation effect of the drain-to-gate capacitance as the drain
Figure 4. (4a) Typical P-Channel MOSFET Gate Charge
Characteristics and (4b) Test Set-Up to Generate Gate
Charge Curve
S
D
G
V
SD(TEST)
R
LOAD
I
GATE
3863 F04
MILLER EFFECT
Q
IN
(4a) (4b)
a b
C
MILLER
= (Q
B
– Q
A
)/V
SD(TEST)
V
SG
–
+