LTC3835 Linear Technology, LTC3835 Datasheet - Page 17

• Manufacturer Part Number: LTC3835
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
INTVCC Regulators
The LTC3835 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 LTC3835’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 tantalum, 10μF special polymer, or
low ESR electrolytic capacitor. A ceramic capacitor with a
minimum value of 4.7μF can also be used if a 1Ω resistor
is added in series with the 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 LTC3835 to be exceeded.
The INTV
current, may be supplied by either the 5V V
7.5V EXTV
less than 4.7V, the V
for the IC in this case is highest and is equal to V
The gate charge current is dependent on operating frequency
as discussed in the Effi ciency Considerations section.
The junction temperature can be estimated by using the
equations given in Note 2 of the Electrical Characteristics.
For example, the LTC3835 INTV
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
IN
T
LDO is turned off and the EXTV
J
CC
CC
CC
= 70°C + (41mA)(36V)(95°C/W) = 125°C
CC
pin from either the V
pin. INTV
LDO remains on as long as the voltage applied to
CC
) at maximum V
CC
current, which is dominated by the gate charge
LDO. When the voltage on the EXTV
CC
CC
powers the gate drivers and much of
IN
supply:
LDO is enabled. Power dissipation
CC
IN
pin to 5.25V and the EXTV
.
IN
CC
supply pin or the EXTV
current is limited to less
CC
CC
CC
rises above 4.7V, the
LDO is enabled. The
and PGND IC pins is
IN
LDO regulates
IN
LDO or the
IN
• I
CC
INTVCC
pin is
CC
CC
.
EXTV
to regulate the INTV
is less than 7.5V, the LDO is in dropout and the INTV
voltage is approximately equal to EXTV
is greater than 7.5V up to an absolute maximum of 10V,
INTV
Using the EXTV
control power to be derived from the LTC3835 switching
regulator output (4.7V ≤ V
operation and from the V
of regulation (e.g., startup, short-circuit). If more cur-rent
is required through the EXTV
external Schottky diode can be added between the EXTV
and INTV
pin and make sure than 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
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.
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
remains above 4.5V. The EXTV
CC
to be powered from the internal 5.25V regulator
Connected Directly to V
Connected to an External supply. If an external
pins. Do not apply more than 10V to the EXTV
Left Open (or Grounded). This will cause
CC
CC
:
CC
CC
LDO allows the MOSFET driver and
CC
providing it is compatible with the
from the output, since the V
voltage to 7.5V, so while EXTV
OUT
IN
. Tying the EXTV
LDO when the output is out
OUT
CC
CC
LDO than is specifi ed, an
≤ V
≤ 10V) during normal
OUT
IN
. This is the normal
.
CC
LTC3835
CC
. When EXTV
CC
LDO attempts
CC
power from
pin to a 5V
17
IN
3835fc
cur-
CC
CC
CC
CC
CC