LTC3857 Datasheet, PDF (22/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857
Applications Information
C_IN
V_IN
MTOP
TG1
1/2 LTC3857
EXTV_CC
SW
MBOT
BG1
BAT85
VN2222LL
L
R_SENSE
BAT85
BAT85
V_OUT
C_OUT
PGND
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Figure 9. Capacitive Charge Pump for EXTV_CC
4. EXTV_CC Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV_CC to an
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_CC < V_IN.
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitors, C_B, connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C_B in the Functional Diagram is charged though
external diode D_B from INTV_CC when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the C_B voltage across the gate-source of the
desired MOSFET. This enhances the top MOSFET switch
and turns it on. The switch node voltage, SW, rises to V_IN
and the BOOST pin follows. With the topside MOSFET
on, the boost voltage is above the input supply: V_BOOST =
V_IN + V_INTVCC. The value of the boost capacitor, C_B, needs
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_IN(MAX).
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
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_ON(MIN), of the LTC3857 (≈90ns),
the input voltage and inductor value:
∆I_L(SC)
=
t_ON(MIN)


V_IN
L


The resulting average short-circuit current is:
I_SC
≈
50%
• I_LIM(MAX)
–
1
2
∆I_L(SC)
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_OUT
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.
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