ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 23

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

INTV
The LTC3858-2 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTV
V
INTV
circuitry. The V
TV
50mA and must be bypassed to ground with a minimum
of 4.7μF low ESR capacitor. Regardless of what type of
bulk capacitor is used, an additional 1μF ceramic capacitor
placed directly adjacent to the INTV
highly recommended. Good bypassing is needed to supply
the high transient currents 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 LTC3858-2 to be
exceeded. The INTV
gate charge current, may be supplied by either the V
or the EXTV
is less than 4.7V, the V
tion for the IC in this case is highest and is equal to V
I
frequency as discussed in the Efficiency Considerations
section. The junction temperature can be estimated by
using the equations given in Note 3 of the Electrical Char-
acteristics. For example, the LTC3858-2 INTV
is limited to less than 32mA from a 40V supply when not
using the EXTV
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in forced continuous mode (PLLIN/MODE
= INTV
When the voltage applied to EXTV
V
EXTV
EXTV
to regulate the INTV
is less than 5.1V, the LDO is in dropout and the INTV
APPLICATIONS INFORMATION
INTVCC
CC
IN
CC
T
LDO is turned off and the EXTV
J
pin depending on the connection of the EXTV
CC
CC
CC
CC
to 5.1V. Each of these can supply a peak current of
= 70°C + (32mA)(40V)(43°C/W) = 125°C
CC
. The gate charge current is dependent on operating
Regulators
powers the gate drivers and much of the internal
LDO remains on as long as the voltage applied to
remains above 4.5V. The EXTV
CC
) at maximum V
pin from either the V
CC
LDO. When the voltage on the EXTV
IN
CC
LDO and the EXTV
supply at 70°C ambient temperature:
CC
CC
current, which is dominated by the
IN
voltage to 5.1V, so while EXTV
LDO is enabled. Power dissipa-
IN
.
IN
CC
CC
supply pin or the EXT-
CC
rises above 4.7V, the
CC
LDO is enabled. The
and PGND pins is
LDO regulate IN-
CC
LDO attempts
CC
current
CC
IN
CC
LDO
pin.
IN
pin
CC
CC
•
voltage is approximately equal to EXTV
is greater than 5.1V, up to an absolute maximum of 14V,
INTV
Using the EXTV
control power to be derived from one of the switching
regulator outputs (4.7V ≤ V
operation and from the V
of regulation (e.g., start-up, short-circuit). If more current
is required through the EXTV
external Schottky diode can be added between the EXTV
and INTV
6V to the EXTV
Significant efficiency and thermal gains can be realized
by powering INTV
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 14V regulator outputs, this means connecting
the EXTV
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
However, for 3.3V and other low voltage outputs, additional
circuitry is required to derive INTV
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
4. EXTV
T
from the internal 5.1V regulator resulting in an efficiency
penalty of up to 10% at high input voltages.
connection for a 5V to 14V regulator and provides the
highest efficiency.
supply is available in the 5V to 14V range, it may be
used to power EXTV
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with the capacitive charge
pump shown in Figure 9. Ensure that EXTV
J
CC
= 70°C + (32mA)(8.5V)(43°C/W) = 82°C
is regulated to 5.1V.
CC
CC
CC
CC
CC
CC
Connected to an Output-Derived Boost Network.
Grounded. This will cause INTV
Connected Directly to V
Connected to an External Supply. If an external
pin directly to V
pins. In this case, do not apply more than
CC
CC
CC
:
pin and make sure that EXTV
CC
LDO allows the MOSFET driver and
from the output, since the V
CC
IN
. Ensure that EXTV
OUT
LDO when the output is out
CC
OUT
. Tying the EXTV
LDO than is specified, an
CC
≤ 14V) during normal
OUT
power from the output.
LTC3858-2
. This is the normal
CC
CC
. When EXTV
to be powered
CC
CC
CC
< V
CC
CC
< V
23
IN
≤ V
pin to
IN
to an
IN
38582f
cur-
.
IN
.
CC
CC
.