Rainbow Electronics DS4426 - Manual (Page 8)

DS4426
THR Inputs for Enabling
Power-Supply Tracking
Comparators are used to individually enable/disable
power-supply tracking based on the voltage presented
to the corresponding THR pin relative to a fixed internal
reference (V
REF /2 = +0.62V). Figure 4 shows a typical
startup and shutdown plot based on the voltage pre-
sented to the THR pin. Tracking can be disabled by
connecting the corresponding THR pin to a voltage
greater than V
REF
/2. Below this threshold, the tracking
circuit is active.
Power-Supply Tracking in DC-DC
Power Applications
The DS4426 provides several options for power-supply
tracking control of DC-DC power supplies. In many
cases, it is desirable to prevent certain DC-DC supplies
from exceeding the voltage of other supplies. This is
often the case with the voltages applied to a digital
core and I/O. Each DS4426 supports one master with
three slave DC-DCs. See Figure 5 for more information.
Loop Bandwidth Consideration
Power-supply tracking is used to override each slave
DC-DC’s feedback loop during power-up and power-
down. Power-supply tracking is capable of slewing at a
much faster rate than most DC-DC converters. Care
must be exercised when selecting the loop bandwidth
of the master DC-DC, slave DC-DC, and power-supply
tracking control loop such that oscillations and over-
shoot are minimized.
While the slave DC-DC supplies are tracking the master
DC-DC supply, there are three time constants of concern:
1) Master BW. The master DC-DC control loop band-
width, power-up ramp rate, and power-down ramp
rate.
2) Slave BW. The slave DC-DC supplies control loop
bandwidths.
3) Tracking BW. The DS4426 tracking circuit band-
width.
To ensure stable operation and minimize peaking, the
bandwidths should follow the following rule:
Master BW and Slave BW < (Tracking BW/10)
Quad-Channel, I
2
C-Margining IDACs with
Three Channels of Power-Supply Tracking
8 _______________________________________________________________________________________
V
DS4426 TRACKING DISABLED
GAIN
ERROR
TRACKING RANGE: DS4426 OVERRIDES SLAVE'S FEEDBACK LOOP
V MASTER
V
SLAVE
V THRESHOLD
t
Figure 4. Enabling Power-Supply Tracking Using the THR Input