LTC3858 Linear Technology, LTC3858 Datasheet - Page 22

• Manufacturer Part Number: LTC3858
• Description: Synchronous Step-Down Controller
• Manufacturer: Linear Technology

LTC3858
driver places the C
The following list summarizes the four possible connec-
tions for EXTV
1. E XTV
2. E XTV
3. E XTV
4. E XTV
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 turned on, the
desired MOSFET. This enhances the top MOSFET switch

applicaTions inForMaTion
to be powered from the internal 5.1V regulator result-
ing in an efficiency penalty of up to 10% at high input
voltages.
connection for a 5V to 14V regulator and provides the
highest efficiency.
supply is available in the 5V to 14V range, it may be
used to power EXTV
For 3.3V and other low voltage regulators, efficiency
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
EXTV
1/2 LTC3858
CC
CC
CC
CC
Figure 8. Capacitive Charge Pump for EXTV
Connected to an Output-Derived Boost Network.
CC
Left Open (or Grounded). This will cause INTV
Connected Directly to V
Connected to an External Supply. If an external
B
in the Functional Diagram is charged though
PGND
BG1
TG1
CC
V
SW
C
IN
IN
B
:
from INTV
MTOP
MBOT
B
voltage across the gate-source of the
CC
. Ensure that EXTV
V
IN
BAT85
D
CC
L
B
when the SW pin is low.
, connected to the BOOST
OUT
VN2222LL
B
R
, D
SENSE
. This is the normal
B
)
CC
3858 F08
CC
CC
BAT85
BAT85
C
< V
V
OUT
OUT
< V
CC
to an
IN
IN
.
.
CC
and turns it on. The switch node voltage, SW, rises to V
and the BOOST pin follows. With the topside MOSFET
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 final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
Fault Conditions: Current Limit and Current Foldback
When the output current hits the current limit, the output
voltage begins to drop. If the output voltage falls below
70% of its nominal output level, then the maximum
sense voltage is progressively lowered to about half of
its maximum selected value. Under short-circuit condi-
tions with very low duty cycles, the LTC3858 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
age 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 flow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the control-
ler is operating.
IN
I
∆I
+ V
SC
L SC
(
=
INTVCC
50
)
ON(MIN)
=
% •
R
. The value of the boost capacitor, C
t
ON MIN
SENSE
I
LIM MAX
(
, of the LTC3858 (≈95ns), the input volt-
(
)

 
V
L
)
IN
–

 
2
1
∆
I
L SC
www.DataSheet4U.com
(
)
IN(MAX)
.
B
BOOST
, needs
3858fa
IN
=