LTC3858-1 Datasheet, PDF (20/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3858-1
Applications Information
linear ramping voltage at the SS pin. The LTC3858-1 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
INTV_CC Regulators
The LTC3858-1 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 EXT-
V_CC pin depending on the connection of the EXTV_CC pin.
INTV_CC powers the gate drivers and much of the internal
circuitry. The V_IN LDO and the EXTV_CC LDO regulate IN-
TV_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. Regardless of what type of
bulk capacitor is used, an additional 1µF 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 maxi-
mum junction temperature rating for the LTC3858-1 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 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 2 of the Electrical Char-
acteristics. For example, the LTC3858-1 INTV_CC current
is limited to less than 15mA from a 40V supply when not
using the EXTV_CC supply at 70°C ambient temperature in
the SSOP package:
T_J = 70°C + (15mA)(40V)(90°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 switching
regulator outputs (4.7V ≤ V_OUT ≤ 14V) 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. 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
a 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T_J = 70°C + (15mA)(8.5V)(90°C/W) = 82°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:
38581fb
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