Datasheet

LTC3789
20
3789fa
applicaTions inForMaTion
where C
RSS
is usually specified by the MOSFET manufac-
turers. 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:
P
D,BOOST
=
V
IN
V
OUT
•
V
OUT
V
IN
• I
OUT(MAX)






2
• ρ
τ
• R
DS(ON)
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:
T
J
= T
A
+ P • R
TH(JA)
The R
TH(JA)
to be used in the equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(JC)
). This value
of T
J
can then be compared to the original, assumed value
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
CC
Regulators and EXTV
CC
The LTC3789 features a true PMOS LDO that supplies
power to INTV
CC
from the V
IN
supply. INTV
CC
powers the
gate drivers and much of the LTC3789’s internal circuitry.
The linear regulator regulates the voltage at the INTV
CC
pin
to 5.5V when V
IN
is greater than 6.5V. EXTV
CC
can supply
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
CC
and PGND pins is
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
CC
current, which is dominated by the
gate charge current, may be supplied by either the 5.5V
linear regulator from V
IN
or the 5.5V LDO from EXTV
CC
.
When the voltage on the EXTV
CC
pin is less than 4.5V, the
linear regulator from V
IN
is enabled. Power dissipation for
the IC in this case is highest and is equal to V
IN
• I
INTVCC
. The
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
CC
current is limited to less
than 24mA from a 24V supply in the SSOP package and
not using the EXTV
CC
supply:
T
J
= 70°C + (28mA)(24V)(80°C/W) = 125°C
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
IN
. When the voltage applied to
EXTV
CC
rises above 4.8V, the INTV
CC
linear regulator from
V
IN
is turned off and the linear regulator from EXTV
CC
is
turned on and remains on as long as the voltage applied
to
EXTV
CC
remains above 4.5V. Using EXTV
CC
allows the
MOSFET driver and control power to be derived from the
LTC3789’s switching regulator output during normal
operation and from the
V
IN
when the output is out of
regulation (e.g., start-up, short-circuit). Do not apply
more than 14V to
EXTV
CC
.