LTC3857IUH#PBF Linear Technology, LTC3857IUH#PBF Datasheet - Page 22

• Manufacturer Part Number: LTC3857IUH#PBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3857IUH#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

LTC3857
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 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 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.
APPLICATIONS INFORMATION
22
IN
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 9. Ensure that EXTV
+ V
INTVCC
CC
EXTV
Figure 9. Capacitive Charge Pump for EXTV
1/2 LTC3857
Connected to an Output-Derived Boost Network.
CC
B
in the Functional Diagram is charged though
. The value of the boost capacitor, C
PGND
BG1
TG1
SW
V
C
B
IN
IN
B
from INTV
MBOT
MTOP
voltage across the gate-source of the
BAT85
CC
L
B
when the SW pin is low.
, connected to the BOOST
VN2222LL
R
B
SENSE
, D
IN(MAX)
B
)
3857 F09
BAT85
CC
CC
.
BAT85
C
V
OUT
OUT
B
CC
< V
BOOST
, needs
to an
IN
.
IN
=
Fault Conditions: Current Limit and Current Foldback
The LTC3857 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 to
about half of its maximum selected value. Under short-
circuit conditions with very low duty cycles, the LTC3857
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
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
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
ΔI
SC
L(SC)
≈50%•I
=t
ON(MIN)
LIM(MAX)
⎛
⎝ ⎜
V
–
L
IN
ON(MIN)
2
1
⎞
⎠ ⎟
ΔI
L(SC)
, of the LTC3857 (≈90ns),
3857fc
OUT