LTC3827 Datasheet, PDF (21/36 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3827
APPLICATIONS INFORMATION
gate charge current, may be supplied by either the 5.25V
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 dissipation for the IC in this case is highest and is
equal to V_IN • I_NTVCC. 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 2 of the
Electrical Characteristics. For example, the LTC3827 INTV_CC
current is limited to less than 24mA from a 24V supply
when in the G package and not using the EXTV_CC supply:
T_J = 70°C + (24mA)(24V)(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 one of the LTC3827’s
switching regulator outputs (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 spec-
ified, 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 than 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
5V 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 to 10V regulator and provides the
highest efficiency.
3. EXTV_CC Connected to an External supply. If an external
supply is available in the 5V to 10V 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 8.
V_IN
+
C_IN
V_IN
LTC3827
EXTV_CC
TG1
N-CH
SW
BG1
N-CH
PGND
1μF
BAT85
0.22μF
BAT85
VN2222LL
BAT85
R_SENSE
V_OUT
L1
+
C_OUT
3827 F08
Figure 8. Capacitive Charge Pump for EXTV_CC
3827ff
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