ltc3890 Linear Technology Corporation, ltc3890 Datasheet - Page 22

• Manufacturer Part Number: ltc3890
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

LTC3890
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 top MOSFET switch
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 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
The LTC3890 includes current foldback to help limit
load current when the output is shorted to ground. If
APPLICATIONS INFORMATION
22
IN
+ V
EXTV
INTVCC
1/2 LTC3890
Figure 9. Capacitive Charge Pump for EXTV
CC
B
PGND
in the Functional Diagram is charged though
. The value of the boost capacitor, C
BG1
TG1
SW
V
C
IN
IN
B
MBOT
MTOP
B
from INTV
voltage across the gate-source of the
BAT85
D
CC
L
B
when the SW pin is low.
, connected to the BOOST
NDS7002
R
B
SENSE
, D
IN(MAX)
B
)
3890 F09
CC
BAT85
.
BAT85
C
V
OUT
OUT
B
BOOST
, needs
IN
=
the output voltage falls below 70% of its nominal output
level, then the maximum sense voltage is progressively
lowered from 100% to 45% of its maximum selected
value. Under short-circuit conditions with very low duty
cycles, the LTC3890 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
LTC3890 (≈95ns), 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
is sensed, the top MOSFET is turned off and the bottom
MOSFET is turned on until the overvoltage condition is
cleared. The bottom MOSFET remains on continuously
for as long as the overvoltage condition persists; if V
returns to a safe level, normal operation automatically
resumes.
A shorted top MOSFET will result in a high current condition
which will open the system fuse. The switching regulator
will regulate properly with a leaky top MOSFET by altering
the duty cycle to accommodate the leakage.
I
SC
I
L(SC)
= 45% • I
= t
ON(MIN)
LIM(MAX)
V
L
IN
–
2
1
ΔI
L(SC)
ON(MIN)
, of the
OUT
3890f