ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 18

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3827-1
INTV
ply when in the G package and not using the EXTV
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
When the voltage applied to EXTV
V
EXTV
EXTV
regulate the INTV
less than 7.5V, the LDO is in dropout and the INTV
voltage is approximately equal to EXTV
is greater than 7.5V up to an absolute maximum of 10V,
INTV
Using the EXTV
control power to be derived from one of the LTC3827-1’s
switching regulator outputs (4.7V ≤ V
normal operation and from the V
is out of regulation (e.g., startup, short-circuit). If more
current is required through the EXTV
fied, an external Schottky diode can be added between
the EXTV
to the EXTV
Significant efficiency and thermal gains can be realized by
powering INTV
resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 10V regulator outputs, this means connecting
the EXTV
a 5V supply reduces the junction temperature in the
previous example from 125°C to:
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV
the output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
18
IN
T
T
INTV
LDO is turned off and the EXTV
J
J
CC
CC
CC
CC
= 70°C + (24mA)(24V)(95°C/W) = 125°C
= 70°C + (24mA)(5V)(95°C/W) = 81°C
CC
current is limited to less than 24mA from a 24V sup-
CC
is regulated to 7.5V.
remains above 4.5V. The EXTV
LDO remains on as long as the voltage applied to
CC
CC
) at maximum V
CC
to be powered from the internal 5.25V regulator
Left Open (or Grounded). This will cause
and INTV
CC
pin directly to V
CC
pin and make sure than EXTV
CC
CC
:
CC
from the output, since the V
U
LDO allows the MOSFET driver and
voltage to 7.5V, so while EXTV
CC
pins. Do not apply more than 10V
IN
U
.
OUT
. Tying the EXTV
IN
CC
CC
W
LDO when the output
CC
rises above 4.7V, the
LDO is enabled. The
CC
OUT
LDO than is speci-
CC
LDO attempts to
. When EXTV
CC
≤ 10V) during
CC
power from
U
IN
CC
≤ V
CC
current
supply:
IN
pin to
CC
.
CC
CC
is
2. EXTV
3. EXTV
4. EXTV
Topside MOSFET Driver Supply (C
External bootstrap capacitors C
pins supply the gate drive voltages for the topside
MOSFETs. Capacitor C
charged though external diode D
SW pin is low. When one of the topside MOSFETs is to be
turned on, the driver places the C
source of the desired MOSFET. This enhances the
MOSFET and turns on the topside switch. The switch node
voltage, SW, rises to V
With the topside MOSFET on, the boost voltage is above
resulting in an efficiency penalty of up to 10% at high
input voltages.
connection for a 5V to 10V regulator and provides the
highest efficiency.
supply is available in the 5V to 10V range, it may be used
to power EXTV
MOSFET gate drive requirements.
work. For 3.3V and other low voltage regulators, effi-
ciency gains can still be realized by connecting EXTV
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.
EXTV
LTC3827-1
CC
CC
CC
CC
Figure 8. Capacitive Charge Pump for EXTV
Connected directly to V
Connected to an External supply. If an external
Connected to an Output-Derived Boost Net-
PGND
BG1
TG1
SW
V
C
IN
IN
CC
+
N-CH
N-CH
providing it is compatible with the
V
IN
B
IN
in the Functional Diagram is
and the BOOST pin follows.
BAT85
B
L1
B
connected to the BOOST
B
voltage across the gate-
OUT
from INTV
VN2222LL
B
, D
R
. This is the normal
SENSE
0.22μF
B
)
CC
+
CC
when the
BAT85
1μF
BAT85
C
V
38271 F08
OUT
OUT
38271fd
CC