LTC3857IUH#TRPBF Linear Technology, LTC3857IUH#TRPBF Datasheet - Page 21

• Manufacturer Part Number: LTC3857IUH#TRPBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3857IUH#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

INTV
The LTC3857 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTV
pin depending on the connection of the EXTV
INTV
internal circuitry. The V
INTV
50mA and must be bypassed to ground with a minimum
of 4.7μF ceramic capacitor. No matter 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
maximum junction temperature rating for the LTC3857
to be exceeded. The INTV
by the gate charge current, may be supplied by either the
V
EXTV
dissipation for the IC in this case is highest and is equal
to V
on operating 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 Characteristics. For example, the LTC3857
INTV
supply when not using the EXTV
ent temperature:
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
APPLICATIONS INFORMATION
IN
IN
T
LDO is turned off and the EXTV
J
LDO or the EXTV
IN
CC
CC
CC
CC
CC
CC
CC
CC
= 70°C + (32mA)(40V)(43°C/W) = 125°C
CC
• I
powers the gate drivers and much of the LTC3857’s
to 5.1V. Each of these can supply a peak current of
Regulators
pin from either the V
pin is less than 4.7V, the V
current is limited to less than 32mA from a 40V
LDO remains on as long as the voltage applied to
remains above 4.5V. The EXTV
) at maximum V
INTVCC
. The gate charge current is dependent
CC
IN
CC
voltage to 5.1V, so while EXTV
LDO and the EXTV
LDO. When the voltage on the
IN
CC
.
current, which is dominated
IN
supply pin or the EXTV
IN
CC
CC
CC
LDO is enabled. Power
supply at 70°C ambi-
CC
rises above 4.7V, the
LDO is enabled. The
and PGND pins is
CC
CC
LDO attempts
LDO regulate
CC
pin.
CC
CC
is less than 5.1V, the LDO is in dropout and the INTV
voltage is approximately equal to EXTV
is greater than 5.1V, up to an absolute maximum of 14V,
INTV
Using the EXTV
trol power to be derived from one of the LTC3857’s switch-
ing regulator outputs (4.7V ≤ V
operation and from the V
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
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, addi-
tional circuitry is required to derive INTV
the output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
T
to be powered from the internal 5.1V regulator result-
ing 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
MOSFET gate drive requirements. Ensure that EXTV
< V
J
CC
= 70°C + (32mA)(8.5V)(43°C/W) = 82°C
IN
is regulated to 5.1V.
.
CC
CC
CC
CC
CC
Left Open (or Grounded). This will cause INTV
Connected to an External Supply. If an external
Connected Directly to V
CC
pins. In this case, do not apply more than 6V
pin directly to V
pin and make sure that EXTV
CC
CC
:
LDO allows the MOSFET driver and con-
CC
from the output, since the V
CC
providing it is compatible with the
IN
LDO when the output is out of
OUT
CC
OUT
. Tying the EXTV
LDO than is specified, an
OUT
≤ 14V) during normal
. This is the normal
CC
LTC3857
. When EXTV
CC
CC
power from
≤ V
CC
21
IN
IN
pin to
.
3857fc
cur-
CC
CC
CC
CC
CC