Datasheet

LTC3883/LTC3883-1
44
3883fa
For more information www.linear.com/LTC3883
APPLICATIONS INFORMATION
sufficient current to assure the output is a zero volts by
the end of the fall time interval. If the TOFF_FALL time is
set shorter than the time required to discharge the load
capacitance, the output will not reach the desired zero volt
state. At the end of TOFF_FALL, the controller will cease
to sink current and V
OUT
will decay at the natural rate
determined by the load impedance. If the controller is in
discontinuous mode, the controller will not pull negative
current and the output will be pulled low by the load, not
the power stage. The maximum fall time is limited to 1.3
seconds. The shorter TOFF_FALL time is set, the more
jagged the TOFF_FALL ramp will appear. The number of
steps in the ramp is equal to TOFF_FALL/0.1ms.
INTV
CC
REGULATOR
The LTC3883 features an NPN linear regulator that sup-
plies power
to INTV
CC
from the V
IN
supply. INTV
CC
powers
the gate drivers, V
DD33
and much of the LTC3883 internal
circuitry. The linear regulator produces 5V at the INTV
CC
pin when V
IN
is greater than 6.5V. The regulator can sup-
ply 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 currents required by the MOSFET gate
drivers. The NPN linear regulator on the LTC3883-1 is not
present and an external 5V supply is needed.
H
igh input voltage application in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction
temperature rating for the LTC3883 to be
exceeded. The INTV
CC
current, of which a large percent-
age is due to the gate charge current, may be supplied by
either
the internal 5V linear regulator or from an external
5V regulator on the LTC3883-1. If the LTC3883 is used
with the internal regulator activated, the power through
the IC is equal to V
IN
• I
INTVCC
. The gate charge current is
dependent on operating frequency as discussed in the Ef-
ficiency Considerations
section.
The junction temperature
can be estimated by using the equations in Note 2 of the
Electrical Characteristics. For example, at 70°C ambient,
the
LTC3883 INTV
CC
current is limited to less than 52mA
from a 24V supply:
T
J
= 70°C + 52mA • 24V • 44°C/W = 125°C
To prevent the maximum junction temperature from being
exceeded, a LTC3883-1 can be used. In the LTC3883-1,
the INTV
CC
linear regulator is disabled and approximately
2mA of current is supplied internally from V
IN
. Significant
system efficiency and thermal gains can be realized by
powering the EXTV
CC
pin from a switching 5V regulator.
The V
IN
current resulting from the gate driver and control
circuitry will be scaled by a factor of:
V
EXTVCC
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
1
Efficiency
⎛
⎝
⎜
⎞
⎠
⎟
Tying the EXTV
CC
pin to a 5V supply (LTC3883-1 only)
reduces the junction temperature in the previous example
from 125°C to:
T
J
= 70°C + 52mA • 5V • 44°C/W + 2mA • 24V • 44°C/W
= 103°C
Do not tie INTV
CC
on the LTC3883 to an external supply
because INTV
CC
will attempt to pull the external supply
high and hit current limit, significantly increasing the die
temperature.
For applications where V
IN
is 5V, tie the V
IN
and INTV
CC
pins together and tie the combined pins to the 5V input
with a 1Ω or 2.2Ω resistor as shown in Figure 23. To mini
-
mize the voltage drop caused by the gate charge current a
low ESR capacitor must be connected to the V
IN
/INTV
CC
(EXTV
CC
) pins. This configuration will override the INTV
CC
(EXTV
CC
) linear regulator and will prevent INTV
CC
(EXTV
CC
)
from dropping too low. Make sure the INTV
CC
(EXTV
CC
)
Figure 23. Setup for a 5V Input
R
VIN
1Ω
C
IN
3883 F23
5V
C
INTVCC
4.7µF
+
INTV
CC
/EXTV
CC
LTC3883
LTC3883-1
V
IN