LTC3827 Datasheet, PDF (20/36 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3827
APPLICATIONS INFORMATION
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3827 will regulate the V_FB pin (and hence V_OUT)
according to the voltage on the TRACK/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
V_x V_OUT
R_TRACKB
R_TRACKA
R_B 1/2 LTC3827
V_FB
R_A
TRACK/SS
3827 F07
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies during start-up, as shown qualitatively
in Figures 6a and 6b. 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 7.
During start-up V_OUT will track V_X according to the ratio
V_X (MASTER)
Figure 7. Using the TRACK/SS Pin for Tracking
set by the resistor divider:
V_X
V_OUT
=
R_A
R_TRACKA
•
R_TRACKA
R_A
+ R_TRACKB
+ R_B
For coincident tracking (V_OUT = V_X during start-up),
R_A = R_TRACKA
R_B = R_TRACKB
V_OUT (SLAVE)
TIME
3827 F06a
(6a) Coincident Tracking
V_X (MASTER)
V_OUT (SLAVE)
TIME
3827 F06b
(6b) Ratiometric Tracking
Figure 6. Two Different Modes of Output
Voltage Tracking
20
INTV_CC Regulators
The LTC3827 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, respectively, depending on the connection of the
EXTV_CC pin. INTV_CC powers the gate drivers and much of
the LTC3827’s internal circuitry. The V_IN LDO regulates
the voltage at the INTV_CC pin to 5.25V and the EXTV_CC
LDO regulates it to 7.5V. Each of these can supply a peak
current of 50mA and must be bypassed to ground with
a minimum of 4.7μF tantalum, 10μF special polymer, or
low ESR electrolytic capacitor. A ceramic capacitor with a
minimum value of 4.7μF can also be used if a 1Ω resistor
is added in series with the 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 LTC3827 to be
exceeded. The INTV_CC current, which is dominated by the
3827ff