LTC3826
22
3826fc
APPLICATIONS INFORMATION
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 top-
side 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 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
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 LTC3826 includes current foldback to help limit load
current when the output is shorted to ground. If the out-
put 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 LTC3826 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 cur-
rent is determined by the minimum on-time t
ON(MIN)
of the
LTC3826 (≈230ns), the input voltage and inductor value:
ΔI
L(SC)
= t
ON(MIN)
(V
IN
/L)
The resulting short-circuit current is:
I
SC
=
30mV
R
SENSE
–
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 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 con-
ditions. 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.
Phase-Locked Loop and Frequency Synchronization
The LTC3826 has a phase-locked loop (PLL) comprised of
an internal voltage-controlled oscillator (VCO) and a phase
detector. This allows the turn-on of the top MOSFET of
controller 1 to be locked to the rising edge of an external
clock signal applied to the PLLIN/MODE pin. The turn-on
of controller 2’s top MOSFET is thus 180 degrees out of
phase with the external clock. The phase detector is an
edge sensitive digital type that provides zero degrees
phase shift between the external and internal oscillators.
This type of phase detector does not exhibit false lock to
harmonics of the external clock.
The output of the phase detector is a pair of comple-
mentary current sources that charge or discharge the
external fi lter network connected to the PLLLPF pin. The
relationship between the voltage on the PLLLPF pin and
operating frequency, when there is a clock signal applied
