LTC3890-2 Datasheet, PDF (22/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3890-2
APPLICATIONS INFORMATION
1/2 LTC3890-2
TRACK/SS
C_SS
SGND
38902 F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies 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 resistor 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 LTC3890-2 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 LTC3890-
2’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 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 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.
V_X(MASTER)
V_OUT(SLAVE)
TIME
38902 F07a
(7a) Coincident Tracking
V_X(MASTER)
V_OUT(SLAVE)
TIME
38902 F07b
(7b) Ratiometric Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
V_x V_OUT
R_TRACKB
R_TRACKA
R_B 1/2 LTC3890-2
V_FB
R_A
TRACK/SS
38902 F08
Figure 8. Using the TRACK/SS Pin for Tracking
38902f
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