LTC3868 Datasheet, PDF (21/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3868
Applications Information
Soft-start is enabled by simply connecting a capacitor from
the SS pin to ground, as shown in Figure 7. An internal 1µA
current source charges the capacitor, providing a linear
ramping voltage at the SS pin. The LTC3868 will regulate
the V_FB pin (and hence V_OUT) according to the voltage on
the SS pin, allowing V_OUT to rise smoothly from 0V to
its final regulated value. The total soft-start time will be
approximately:
t_SS
= C_SS
•
0.8V
1µA
1/2 LTC3868
SS
C_SS
SGND
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Figure 7. Using the TRACK/SS Pin to Program Soft-Start
INTV_CC Regulators
The LTC3868 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 depending on the connection of the EXTV_CC
pin. INTV_CC powers the gate drivers and much of the
LTC3868’s internal circuitry. The V_IN LDO and the EXTV_CC
LDO regulate INTV_CC to 5.1V. Each of these can supply a
peak current of 50mA and must be bypassed to ground
with a minimum of 4.7µF low ESR capacitor. No matter
what type of bulk capacitor is used, an additional 1µF
ceramic capacitor placed directly adjacent to the INTV_CC
and PGND 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 LTC3868 to be exceeded.
The INTV_CC current, which is dominated by the gate charge
current, may be supplied by either the V_IN LDO or the
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_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 Characteristics.
For example, the LTC3868 INTV_CC current is limited to less
than 45mA from a 28V supply when not using the EXTV_CC
supply at 70°C ambient temperature:
T_J = 70°C + (45mA)(28V)(43°C/W) = 125°C
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_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 5.1V, so while EXTV_CC
is less than 5.1V, the LDO is in dropout and the INTV_CC
voltage is approximately equal to EXTV_CC. When EXTV_CC
is greater than 5.1V, up to an absolute maximum of 14V,
INTV_CC is regulated to 5.1V.
Using the EXTV_CC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3868’s
switching regulator outputs (4.7V ≤ V_OUT ≤ 14V) during
normal operation and from the V_IN LDO when the out-
put 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. In this case, do
not apply more than 6V 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 5V to 14V regulator outputs, this means connecting
the EXTV_CC pin directly to V_OUT . Tying the EXTV_CC pin to
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T_J = 70°C + (45mA)(8.5V)(43°C/W) = 86°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV_CC power from
the output.
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