LTC3834 Datasheet, PDF (17/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

LTC3834
APPLICATIONS INFORMATION
INTV_CC Regulators
The LTC3834 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTV_CC pin from either the V_IN supply pin or the EXTV_CC
pin, respectively, depending on the connection of the
EXTV_CC pin. INTV_CC powers the gate drivers and much of
the LTC3834’s internal circuitry. The V_IN LDO regulates
the voltage at the INTV_CC pin to 5.25V and the EXTV_CC
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_CC and PGND IC pins is
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_CC current, which is dominated by the gate charge
current, may be supplied by either the 5V V_IN LDO or the
7.5V EXTV_CC LDO. When the voltage on the EXTV_CC pin
is less than 4.7V, the V_IN LDO is enabled. Power dissipa-
tion for the IC in this case is highest and is equal to V_IN •
I_INTVCC. The gate charge current is dependent 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 Char-
acteristics. For example, the LTC3834 INTV_CC current is
limited to less than 41mA from a 24V supply when in the
G package and not using the EXTV_CC supply:
T_J = 70°C + (41mA)(36V)(95°C/W) = 125°C
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_CC) at maximum V_IN.
When the voltage applied to EXTV_CC rises above 4.7V, the
V_IN LDO is turned off and the EXTV_CC LDO is enabled. The
EXTV_CC LDO remains on as long as the voltage applied to
EXTV_CC remains above 4.5V. The EXTV_CC LDO attempts
to regulate the INTV_CC voltage to 7.5V, so while EXTV_CC
is less than 7.5V, the LDO is in dropout and the INTV_CC
voltage is approximately equal to EXTV_CC. When EXTV_CC
is greater than 7.5V up to an absolute maximum of 10V,
INTV_CC is regulated to 7.5V.
Using the EXTV_CC LDO allows the MOSFET driver and
control power to be derived from the LTC3834 switch-
ing regulator output (4.7V ≤ V_OUT ≤ 10V) during normal
operation and from the V_IN LDO when the output is out
of regulation (e.g., start-up, short circuit). If more current
is required through the EXTV_CC LDO than is specified,
an external Schottky diode can be added between the
EXTV_CC and INTV_CC pins. Do not apply more than 10V to
the EXTV_CC pin and make sure that EXTV_CC ≤ V_IN.
Significant efficiency and thermal gains can be realized
by powering INTV_CC from the output, since the V_IN cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency). For
4.7V to 10V regulator outputs, this means connecting the
EXTV_CC pin directly to V_OUT. Tying the EXTV_CC pin to a 5V
supply reduces the junction temperature in the previous
example from 125°C to:
T_J = 70°C + (24mA)(5V)(95°C/W) = 81°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV_CC power from
the output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC Left Open (or Grounded). This will cause
INTV_CC to be powered from the internal 5.25V regulator
resulting in an efficiency penalty of up to 10% at high
input voltages.
2. EXTV_CC Connected Directly to V_OUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV_CC Connected to an External supply. If an external
supply is available in the 5V to 7V range, it may be used
to power EXTV_CC providing it is compatible with the
MOSFET gate drive requirements.
4. EXTV_CC Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV_CC to an
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.
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