LTC3834 Linear Technology, LTC3834 Datasheet - Page 17

• Manufacturer Part Number: LTC3834
• Description: Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
INTV
The LTC3834 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTV
pin, respectively, depending on the connection of the
EXTV
the LTC3834’s internal circuitry. The V
the voltage at the INTV
LDO regulates it to 7.5V. Each of these can supply a peak
current of 50mA and must be bypassed to ground with a
minimum of 4.7μF ceramic capacitor. The 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 LTC3834 to be exceeded.
The INTV
current, may be supplied by either the 5V V
7.5V 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 Effi ciency Considerations
section. The junction temperature can be estimated by
using the equations given in Note 3 of the Electrical Char-
acteristics. For example, the LTC3834 INTV
limited to less than 41mA from a 24V supply when in the
G package and not using the EXTV
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV
When the voltage applied to EXTV
V
EXTV
EXTV
to regulate the INTV
is less than 7.5V, the LDO is in dropout and the INTV
voltage is approximately equal to EXTV
INTVCC
IN
T
LDO is turned off and the EXTV
J
CC
CC
CC
CC
CC
= 70°C + (41mA)(36V)(95°C/W) = 125°C
CC
. The gate charge current is dependent on operating
Regulators
pin from either the V
pin. INTV
LDO remains on as long as the voltage applied to
remains above 4.5V. The EXTV
CC
) at maximum V
CC
current, which is dominated by the gate charge
LDO. When the voltage on the EXTV
CC
powers the gate drivers and much of
CC
IN
voltage to 7.5V, so while EXTV
CC
LDO is enabled. Power dissipa-
IN
pin to 5.25V and the EXTV
.
IN
supply pin or the EXTV
CC
CC
CC
CC
rises above 4.7V, the
LDO is enabled. The
and PGND IC pins is
supply:
CC
IN
CC
. When EXTV
LDO regulates
LDO attempts
IN
CC
LDO or the
current is
CC
IN
pin
CC
CC
CC
CC
CC
•
is greater than 7.5V up to an absolute maximum of 10V,
INTV
Using the EXTV
control power to be derived from the LTC3834 switch-
ing regulator output (4.7V ≤ V
operation and from the V
of regulation (e.g., start-up, short circuit). If more current
is required through the EXTV
an external Schottky diode can be added between the
EXTV
the EXTV
Signifi cant effi ciency 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 Effi ciency). For
4.7V to 10V regulator outputs, this means connecting the
EXTV
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
4. EXTV
T
INTV
resulting in an effi ciency penalty of up to 10% at high
input voltages.
connection for a 5V regulator and provides the highest
effi ciency.
supply is available in the 5V to 7V range, it may be used
to power EXTV
MOSFET gate drive requirements.
For 3.3V and other low voltage regulators, effi ciency
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 6.
J
CC
CC
CC
= 70°C + (24mA)(5V)(95°C/W) = 81°C
CC
is regulated to 7.5V.
CC
CC
CC
pin directly to V
CC
and INTV
CC
to be powered from the internal 5.25V regulator
Connected to an Output-Derived Boost Network.
Connected Directly to V
Connected to an External supply. If an external
Left Open (or Grounded). This will cause
pin and make sure that EXTV
CC
CC
:
CC
CC
CC
LDO allows the MOSFET driver and
pins. Do not apply more than 10V to
providing it is compatible with the
from the output, since the V
OUT
IN
. Tying the EXTV
LDO when the output is out
OUT
CC
LDO than is specifi ed,
OUT
≤ 10V) during normal
. This is the normal
LTC3834
CC
CC
CC
≤ V
power from
pin to a 5V
IN
CC
17
.
IN
to an
3834fb
cur-