LTC3859_15 Datasheet, PDF (26/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3859
APPLICATIONS INFORMATION
V_X(MASTER)
V_OUT(SLAVE)
V_X(MASTER)
V_OUT(SLAVE)
TIME
3859 F07a
TIME
3859 F07b
7a. Coincident Tracking
7b. Radiometric Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
V_OUT
R_B
LTC3859
V_FB1,2
V_X
R_A
R_TRACKB
R_TRACKA
TRACK/SS1,2
3859 F08
Figure 8. Using the TRACK/SS Pin for Tracking
Alternatively, the TRACK/SS1 and TRACK/SS2 pins for the
two buck controllers can be used to track two (or more) sup-
plies during start-up, as shown qualitatively in Figures 7a
and 7b. To do this, a resistor divider should be connected
from the master supply (V_X) to the TRACK/SS pin of the
slave supply (V_OUT), as shown in Figure 8. During start-up
V_OUT will track V_X according to the ratio set by the resis-
tor divider:
V_{X =} R_{A •} R_TRACKA + R_TRACKB
V_OUT R_TRACKA
R_A + R_B
For coincident tracking (V_OUT = V_X during start-up),
R_A = R_TRACKA
R_B = R_TRACKB
INTV_CC Regulators
The LTC3859 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
26
at the INTV_CC pin from either the V_BIAS 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
LTC3859’s internal circuitry. The V_BIAS LDO and the EXTV_CC
LDO regulate INTV_CC to 5.4V. Each of these can supply a
peak current of 50mA and must be bypassed to ground
with a minimum of 4.7μF ceramic capacitor. No matter
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
maximum junction temperature rating for the LTC3859 to
be exceeded. The INTV_CC current, which is dominated by
the gate charge current, may be supplied by either the V_BIAS
LDO or the EXTV_CC LDO. When the voltage on the EXTV_CC
pin is less than 4.7V, the V_BIAS LDO is enabled. Power
dissipation for the IC in this case is highest and is equal to
V_BIAS • 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 Characteristics. For example, the LTC3859 INTV_CC
current is limited to less than 40mA from a 40V supply
when not using the EXTV_CC supply at a 70°C ambient
temperature in the QFN package:
T_J = 70°C + (40mA)(40V)(34°C/W) = 125°C
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