参数资料
型号: LTC3890EGN-1#PBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: SWITCHING CONTROLLER, PDSO28
封装: LEAD FREE, PLASTIC, SSOP-28
文件页数: 13/36页
文件大小: 498K
代理商: LTC3890EGN-1#PBF
LTC3890-1
20
38901f
APPLICATIONS INFORMATION
(7a) Coincident Tracking
(7b) Ratiometric Tracking
Figure 8. Using the TRACK/SS Pin for Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
Tracking and Soft-Start (TRACK/SS Pins)
The start-up of each VOUT is controlled by the voltage on
the respective TRACK/SS pin. When the voltage on the
TRACK/SS pin is less than the internal 0.8V reference, the
LTC3890-1 regulates the VFB pin voltage to the voltage on
the TRACK/SS pin instead of 0.8V. The TRACK/SS pin can
be used to program an external soft-start function or to
allow VOUT to track another supply during start-up.
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 6.
An internal 1μA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3890-1 will regulate the VFB pin (and hence VOUT)
according to the voltage on the TRACK/SS pin, allowing
VOUT to rise smoothly from 0V to its nal regulated value.
The total soft-start time will be approximately:
tSS = CSS
0.8V
1μA
TIME
VX(MASTER)
VOUT(SLAVE)
OUTPUT
VOLTAGE
38901 F07a
TIME
38901 F07b
VX(MASTER)
VOUT(SLAVE)
OUTPUT
VOLTAGE
1/2 LTC3890-1
VOUT
Vx
VFB
TRACK/SS
38901 F08
RB
RA
RTRACKA
RTRACKB
INTVCC Regulators
The LTC3890-1 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the INTVCC pin from either the VIN supply pin or the
EXTVCC pin depending on the connection of the EXTVCC
pin. INTVCC powers the gate drivers and much of the
LTC3890-1’s internal circuitry. The VINLDOandtheEXTVCC
1/2 LTC3890-1
TRACK/SS
CSS
SGND
38901 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 (VX) to the TRACK/
SS pin of the slave supply (VOUT), as shown in Figure 8.
During start-up VOUT will track VX according to the ratio
set by the resistor divider:
V
R
RR
X
OUT
A
TRACKA
TRACKB
AB
=
+
For coincident tracking (VOUT = VX during start-up):
RA = RTRACKA
RB = RTRACKB
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