Datasheet

LTC3864
14
3864f
APPLICATIONS INFORMATION
The peak inductor current limit is equal to:
I
L(PEAK)
≅
95mV
R
SENSE






This inductor current limit would translate to an output
current limit based on the inductor ripple:
I
LIMIT
≅
95mV
R
SENSE
–
∆I
L
2
The SENSE pin is a high impedance input with a maximum
leakage of ±2µA. Since the LTC3864 is a peak current
mode controller, noise on the SENSE pin can create pulse
width jitter. Careful attention must be paid to the layout of
R
SENSE
. To ensure the integrity of the current sense signal,
V
SENSE
, the traces from V
IN
and SENSE pins should be
short and run together as a differential pair and Kelvin
(4-wire) connected across R
SENSE
(Figure 3).
drain current I
D(MAX)
, and the MOSFET’s thermal resistance
θ
JC(MOSFET)
and θ
JA(MOSFET)
.
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
LTC3864 is in continuous conduction mode is given by:
P
MOSFET
≅ D • I
OUT
2
• ρ
τ
• R
DS(ON)
+
V
IN
2
•
I
OUT
2






• C
MILLER
( )
•
R
DN
V
IN
– V
CAP
( )
– V
MILLER
+
R
UP
V
MILLER






• f
where D is duty factor, R
DS(ON)
is on-resistance of
P-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 determin-
ing the transition loss term in the P-channel MOSFET
but is not directly specified 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
The LTC3864 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 LTC3864 drives a P-channel power MOSFET that
serves as the main switch for the asynchronous step-
down converter. Important P-channel power MOSFET
parameters include drain-to-source breakdown voltage
V
BR(DSS)
, threshold voltage V
GS(TH)
, on-resistance R
DS(ON)
,
gate-to-drain reverse transfer capacitance C
RSS
, maximum
Figure 3. Inductor Current Sensing
Figure 4. (a) Typical P-MOSFET Gate Charge Characteristics
and (b) Test Set-Up to Generate Gate Charge Curve
S
D
G
V
SD(TEST)
R
LOAD
I
GATE
3864 F04
MILLER EFFECT
Q
IN
a b
C
MILLER
= (Q
B
– Q
A
)/V
SD(TEST)
V
SG
–
+
(a)
(b)
V
IN
R
SENSE
LTC3864
V
IN
SENSE
R
F
MP
OPTIONAL
FILTERING
3864 F03
C
F