LTC3856_15 Datasheet, PDF (27/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

Applications Information
V_OUT1
V_OUT2
LTC3856
V_OUT1
V_OUT2
TIME
3856 F11a
(11a) Coincident Tracking
TIME
3856 F11b
(11b) Ratiometric Tracking
Figure 11. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TK/SS2
PIN
R3 R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R4 R2
V_OUT2
R3
R4
(12a) Coincident Tracking Set-Up
V_OUT1
TO
TK/SS2
PIN
R1
TO
TO
V_FB1
V_FB2
PIN
PIN
R2
V_OUT2
R3
R4
3856 F12
(12b) Ratiometric Tracking Set-Up
Figure 12. Set-Up and Coincident and Ratiometric Tracking
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 LTC3856 to be
exceeded. The INTV_CC current, which is dominated by the
gate charge current, may be supplied by either the 5V LDO
or EXTV_CC. When the voltage on the EXTV_CC pin is less
than 4.7V, the 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
table. For example, the LTC3856 INTV_CC current is limited
to less than 42mA from a 38V supply in the UH package
and not using the EXTV_CC supply:
T_J = 70°C + (42mA)(38V)(34°C/W) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in continuous conduction mode (MODE
= SGND) at maximum V_IN. When the voltage applied to
EXTV_CC rises above 4.7V, the INTV_CC LDO is turned off
and the EXTV_CC is connected to the INTV_CC. The EXTV_CC
remains on as long as the voltage applied to EXTV_CC remains
above 4.5V. Using the EXTV_CC allows the MOSFET driver
and control power to be derived from one of switching
regulator outputs during normal operation and from the
INTV_CC when the output is out of regulation (e.g., start-up,
3856f
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