LTC3890-3 Datasheet, PDF (21/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

Applications Information
V_X(MASTER)
V_OUT(SLAVE)
TIME
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(7a) Coincident Tracking
V_X(MASTER)
V_OUT(SLAVE)
TIME
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(7b) Ratiometric Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
V_x V_OUT
R_TRACKB
R_TRACKA
R_B 1/2 LTC3890-3
V_FB
R_A
TRACK/SS
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Figure 8. Using the TRACK/SS Pin for Tracking
LTC3890-3
For coincident tracking (V_OUT = V_X during start-up):
R_A = R_TRACKA
R_B = R_TRACKB
INTV_CC Regulators
The LTC3890-3 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 LTC3890-3’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 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 LTC3890-3 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 dis-
sipation 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 Con-
siderations section. The junction temperature can be
estimated by using the equations given in Note 3 of the
Electrical Characteristics. For example, the LTC3890E-3
INTV_CC current is limited to less than 15mA from a 40V
supply when not using the EXTV_CC supply at 70°C ambi-
ent temperature:
T_J = 70°C + (15mA)(40V)(90°C/W) = 125°C
For more information www.linear.com/3890-3
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