LTC3788 Linear Technology, LTC3788 Datasheet - Page 22

• Manufacturer Part Number: LTC3788
• Description: Dual Output Synchronous Boost Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC3788
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
to regulate the INTV
is less than 5.4V, the LDO is in dropout and the INTV
voltage is approximately equal to EXTV
is greater than 5.4V, up to an absolute maximum of 6V,
INTV
The following list summarizes possible connections for
EXTV
Topside MOSFET Driver Supply (C
External bootstrap capacitors C
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C
external diode D
When one of the topside MOSFETs is to be turned on, the
driver places the C
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V
on, the boost voltage is above the input supply: V
V
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
22
IN
IN
IN
EXTV
INTV
lator resulting in an effi ciency penalty at high input
voltages.
EXTV
nal supply is available in the 5.4V to 6V range, it may
be used to power EXTV
with the MOSFET gate drive requirements. Ensure that
EXTV
LDO is turned off and the EXTV
and the BOOST pin follows. With the topside MOSFET
+ V
CC
CC
CC
CC
CC
INTVCC
:
CC
is regulated to 5.4V.
CC
LDO remains on as long as the voltage applied to
remains above 4.55V. The EXTV
CC
CC
) at maximum V
to be powered from the internal 5.4V regu-
< VBIAS.
Connected to an External Supply. If an exter-
Left Open (or Grounded). This will cause
. The value of the boost capacitor C
B
B
B
in the Block Diagram is charged though
from INTV
voltage across the gate-source of the
CC
voltage to 5.4V, so while EXTV
IN
.
CC
CC
providing it is compatible
B
when the SW pin is low.
connected to the BOOST
CC
CC
B
rises above 4.7V, the
, D
LDO is enabled. The
IN(MAX)
CC
B
CC
)
. When EXTV
LDO attempts
.
B
BOOST
needs
CC
CC
CC
=
Fault Conditions: Overtemperature Protection
At higher temperatures, or in cases where the internal
power dissipation causes excessive self heating on-chip
(such as an INTV
shutdown circuitry will shut down the LTC3788. When the
junction temperature exceeds approximately 170°C, the
overtemperature circuitry disables the INTV
the INTV
the entire LTC3788 chip. Once the junction temperature
drops back to approximately 155°C, the INTV
back on. Long term overstress (T
avoided as it can degrade the performance or shorten the
life of the part.
Phase-Locked Loop and Frequency Synchronization
The LTC3788 has an internal phase-locked loop (PLL)
comprised of a phase frequency detector, a low pass fi lter
and a voltage-controlled oscillator (VCO). 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.
If the external clock frequency is greater than the internal
oscillator’s frequency, f
ously from the phase detector output, pulling up the VCO
input. When the external clock frequency is less than f
current is sunk continuously, pulling down the VCO input.
If the external and internal frequencies are the same but
exhibit a phase difference, the current sources turn on for
an amount of time corresponding to the phase difference.
The voltage at the VCO input is adjusted until the phase
and frequency of the internal and external oscillators are
identical. At the stable operating point, the phase detector
output is high impedance and the internal fi lter capacitor,
C
LP
, holds the voltage at the VCO input.
CC
supply to collapse and effectively shut down
CC
short to ground), the overtemperature
OSC
, then current is sourced continu-
J
> 125°C) should be
CC
LDO, causing
CC
LDO turns
OSC
3788f
,