LTC3834-1 Datasheet, PDF (16/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

LTC3834-1
APPLICATIO S I FOR ATIO
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3834-1 to be
exceeded. The INTV_CC current, which is dominated by the
gate charge current, is supplied by the 5.25V V_IN LDO.
Power dissipation for the IC in this case is equal to V_IN •
I_INTVCC. The gate charge current is dependent on operating
frequency as discussed in the Efficiency Considerations
section. The junction temperature can be estimated by
using the equations given in Note 2 of the Electrical
Characteristics. For example, the LTC3834-1 INTV_CC cur-
rent is limited to less than 25mA from a 24V supply when
in the GN package:
T_J = 70°C + (25mA)(24V)(90°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV_CC) at maximum 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
MOSFET. 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 MOSFET is to be turned on,
the driver places the C_B voltage across the gate-source of
the desired MOSFET. This enhances the
MOSFET and turns on the topside switch. 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 LTC3834-1 includes current foldback to help limit load
current when the output is shorted to ground. If the output
falls below 70% of its nominal output level, then the
maximum sense voltage is progressively lowered from
100mV to 30mV. Under short-circuit conditions with very
low duty cycles, the LTC3834-1 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_ON(MIN) of
the LTC3834-1 ( 200ns), the input voltage and induct-or
value:
I _L(SC) = t_ON(MIN) (V_IN/L)
The resulting short-circuit current is:
I_SC
=
30mV
R_SENSE
–
1
2
ΔI_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
controller is operating.
A comparator monitors the output for overvoltage condi-
tions. The comparator (OV) detects overvoltage 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 OV condition persists; if
V_OUT returns to a safe level, normal operation automati-
cally 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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