LTC3883 Datasheet, PDF (44/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
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.
High 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   ₁ 
 V_IN   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)
LTC3883 ^VIN
LTC3883-1
INTV_CC/EXTV_CC
R_VIN
1Ω
5V
+ _CINTVCC
4.7µF
C_IN
3883 F23
Figure 23. Setup for a 5V Input
3883f
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