LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 21

• Manufacturer Part Number: LTC3858-1
• Description: Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology Corporation

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1. EXTV
2. EXTV
3. EXTV
4. EXTV
Topside MOSFET Driver Supply (C
External bootstrap capacitors, C
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
external diode D
When one of the topside MOSFETs is to be turned on, the
driver places the C
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V
APPLICATIONS INFORMATION
IN
to be powered from the internal 5.1V regulator result-
ing in an effi ciency penalty of up to 10% at high input
voltages.
connection for a 5V to 14V regulator and provides the
highest effi ciency.
supply is available in the 5V to 14V range, it may be
used to power EXTV
the MOSFET gate drive requirements. Ensure that
EXTV
For 3.3V and other low voltage regulators, effi ciency
gains can still be realized by connecting EXTV
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with the capacitive charge
pump shown in Figure 8. Ensure that EXTV
and the BOOST pin follows. With the topside MOSFET
EXTV
1/2 LTC3858-1
CC
CC
CC
CC
CC
Figure 8. Capacitive Charge Pump for EXTV
CC
Left Open (or Grounded). This will cause INTV
Connected to an Output-Derived Boost Network.
Connected Directly to V
Connected to an External Supply. If an external
< V
B
in the Functional Diagram is charged though
PGND
BG1
IN
TG1
SW
V
C
IN
.
IN
B
MTOP
MBOT
B
from INTV
voltage across the gate-source of the
CC
providing it is compatible with
V
IN
BAT85
CC
D
B
L
when the SW pin is low.
, connected to the BOOST
OUT
VN2222LL
B
R
, D
SENSE
. This is the normal
B
)
38581 F08
CC
CC
BAT85
BAT85
C
V
OUT
OUT
CC
< V
to an
IN
.
CC
on, the boost voltage is above the input supply: V
V
to be 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of the external
Schottky diode must be greater than V
When adjusting the gate drive level, the fi nal arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the effi ciency has
improved. If there is no change in input current, then there
is no change in effi ciency.
Fault Conditions: Current Limit and Current Foldback
When the output current hits the current limit, the output
voltage begins to drop. If the output falls below 70% of its
nominal output level, then the maximum sense voltage is
progressively lowered from 100% to 30% of its maximum
selected value. Under short-circuit conditions with very
low duty cycles, the LTC3858-1 will begin cycle skipping
in order to limit the short-circuit current. In this situation
the bottom MOSFET will be dissipating most of the power
but less than in normal operation. The short-circuit ripple
current is determined by the minimum on-time, t
of the LTC3858-1 (≈90ns), the input voltage and inductor
value:
The resulting average short-circuit current is:
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
much higher than nominal levels. The crowbar causes huge
currents to fl ow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the control-
ler is operating.
A comparator monitors the output for overvoltage condi-
tions. The comparator detects faults greater than 10%
above the nominal output voltage. When this condition
IN
I
ΔI
SC
+ V
L SC
(
=
INTVCC
30
)
=
% •
R
. The value of the boost capacitor, C
t
ON MIN
SENSE
I
LIM MAX
(
(
)
⎛
⎝ ⎜
V
L
)
IN
–
⎞
⎠ ⎟
2
1
Δ
I
L SC
(
LTC3858-1
)
IN(MAX)
.
B
ON(MIN)
BOOST
21
, needs
38581f
=
,