LTC3890EGN-1#TRPBF Linear Technology, LTC3890EGN-1#TRPBF Datasheet - Page 22

• Manufacturer Part Number: LTC3890EGN-1#TRPBF
• Description: IC BUCK SYNC ADJ DUAL 28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3890EGN-1#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 350kHz ~ 535kHz
• Voltage - Input: 4 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

LTC3890-1
APPLICATIONS INFORMATION
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
The external diode D
diode, but in either case it should have low-leakage and
fast recovery. Pay close attention to the reverse leakage
current specification for this diode, especially at high
temperatures where it generally increases substantially.
For applications with output voltages greater than ~5V
that are switching infrequently, a leaky diode D
discharge the bootstrap capacitor C
path from the output voltage to the BOOST pin to INTV
Not only does this increase the quiescent current of the
converter, but it can cause INTV
levels if the leakage exceeds the current consumption on
INTV
Particularly, this is a concern in Burst Mode operation at
no load or very light loads, where the part is switching
very infrequently and the current draw on INTV
low (typically about 35μA). Generally, pulse-skipping and
forced continuous modes are less sensitive to leakage,
since the more frequent switching keeps the bootstrap
capacitor C
output voltage to INTV
However, in cases where the converter has been operat-
ing (in any mode) and then is shut down, if the leakage
of diode D
before the output voltage discharges to below ~5V, then
the leakage current path can be created from the output
voltage to INTV
sink about 30μA. To accommodate diode leakage greater
22
IN
+ V
CC
INTVCC
.
B
B
B
in the Functional Diagram is charged though
charged, preventing a current path from the
. The value of the boost capacitor, C
fully discharges the bootstrap capacitor C
CC
B
. In shutdown, the INTV
from INTV
B
voltage across the gate-source of the
B
can be a Schottky diode or silicon
CC
.
CC
B
when the SW pin is low.
, connected to the BOOST
CC
B
to rise to dangerous
B
, D
, creating a current
IN(MAX)
B
)
CC
pin is able to
.
B
CC
B
can fully
BOOST
, needs
is very
CC
IN
=
B
.
than this amount in shutdown, INTV
with an external resistor or clamped with a Zener diode.
Alternatively, the PGOOD resistor can be used to sink the
current (assuming the resistor pulls up to INTV
PGOOD is pulled low when the converter is shut down.
Nonetheless, using a low-leakage diode is the best choice
to maintain low quiescent current under all conditions.
Fault Conditions: Current Limit and Current Foldback
The LTC3890-1 includes current foldback to help limit
load current when the output is shorted to ground. If
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 LTC3890-1 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
LTC3890-1 (≈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 flow, 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
to a safe level, normal operation automatically resumes.
I
ΔI
SC
L(SC)
= 45% • I
= t
ON(MIN)
LIM(MAX)
⎛
⎜
⎝
V
L
IN
–
⎞
⎟
⎠
2
1
ΔI
L(SC)
CC
can be loaded
ON(MIN)
OUT
CC
, of the
returns
) since
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