Datasheet

LTC3827-1
19
38271fe
Figure 8. Capacitive Charge Pump for EXTV
CC
EXTV
CC
V
IN
TG1
SW
BG1
PGND
LTC3827-1
R
SENSE
V
OUT
VN2222LL
+
C
OUT
38271 F08
N-CH
N-CH
+
C
IN
1µF
V
IN
L1
BAT85 BAT85
BAT85
0.22µF
APPLICATIONS INFORMATION
4. EXTV
CC
Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, effi ciency
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 8.
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 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 LTC3827-1 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 LTC3827-1 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 LTC3827-1 (≈180ns), the input voltage and induc-
tor 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.