LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 19

• Manufacturer Part Number: LTC3785
• Description: High Efficiency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
where C
ers. The constant k, which accounts for the loss caused by
reverse recovery current, is inversely proportional to the
gate drive current and has an empirical value of 1.7.
For switch D, the maximum power dissipation happens
in the boost region, when its duty cycle is higher than
50%. Its maximum power dissipation at maximum output
current is given by:
For the same output voltage and current, switch A has the
highest power dissipation and switch B has the lowest
power dissipation unless a short occurs at the output.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
The R
the R
the case to the ambient temperature (R
of T
used in the iterative calculation process.
Schottky Diode (D1, D2) Selection
The Schottky diodes, D1 and D2, shown in Figure 13,
conduct during the dead time between the conduction
of the power MOSFET switches. They are intended to
prevent the body diode of synchronous switches B and D
from turning on and storing charge during the dead time.
In particular, D2 significantly reduces reverse recovery
current between switch D turn-off and switch C turn-on,
which improves converter efficiency and reduces switch
C voltage stress. In order for the diode to be effective, the
inductance between it and the synchronous switch must
be as small as possible, mandating that these components
be placed adjacently.
INTV
The LTC3789 features a true PMOS LDO that supplies
power to INTV
T
P
J
D,BOOST
J
CC
TH(JC)
can then be compared to the original, assumed value
= T
TH(JA)
RSS
Regulators and EXTV
A
+ P • R
for the device plus the thermal resistance from
is usually specified by the MOSFET manufactur-
to be used in the equation normally includes
=
CC
V
V
OUT
IN
from the V
TH(JA)
•

 
V
V
OUT
IN
IN
• I
CC
supply. INTV
OUT(MAX)
TH(JC)

 
2
CC
• ρ
). This value
powers the
t
• R
DS(ON)
gate drivers and much of the LTC3789’s internal circuitry.
The linear regulator regulates the voltage at the INTV
to 5.5V when V
the needed power when its voltage is higher than 4.8V
through another on-chip PMOS LDO. Each of these can
supply a peak current of 100mA and must be bypassed
to ground with a minimum of 1µF ceramic capacitor or
low ESR electrolytic capacitor. No matter what type of bulk
capacitor is used, an additional 0.1µF ceramic capacitor
placed directly adjacent to the INTV
highly recommended. Good bypassing is needed to supply
the high transient current required by the MOSFET gate
drivers and to prevent interaction between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3789 to be
exceeded. The INTV
gate charge current, may be supplied by either the 5.5V
linear regulator from V
When the voltage on the EXTV
linear regulator from V
the IC in this case is highest and is equal to V
gate charge current is dependent on operating frequency,
as discussed in the Efficiency Considerations section. The
junction temperature can be estimated by using the equa-
tions given in Note 3 of the Electrical Characteristics. For
example, the LTC3789 INTV
than 24mA from a 24V supply in the SSOP 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 (MODE/PLLIN
= SGND) at maximum V
EXTV
V
turned on and remains on as long as the voltage applied
to EXTV
MOSFET driver and control power to be derived from
the LTC3789’s switching regulator output during normal
operation and from the V
regulation (e.g., start-up, short-circuit). Do not apply
more than 14V to EXTV
IN
T
J
is turned off and the linear regulator from EXTV
CC
= 70°C + (28mA)(24V)(80°C/W) = 125°C
rises above 4.8V, the INTV
CC
remains above 4.5V. Using EXTV
IN
is greater than 6.5V. EXTV
CC
CC
supply:
IN
current, which is dominated by the
IN
CC
is enabled. Power dissipation for
IN
or the 5.5V LDO from EXTV
.
. When the voltage applied to
IN
CC
when the output is out of
CC
current is limited to less
pin is less than 4.5V, the
CC
CC
linear regulator from
and PGND pins is
LTC3789
IN
CC
CC
• I
can supply
INTVCC
allows the
19
CC
. The
CC
CC
3789f
pin
is
.