LTC3633A/LTC3633A-1
APPLICATIONS INFORMATION
this tracking mode, V
must be set higher than V
.
with 20Ω output resistance to ground, thus dropping the
PGOODpinvoltage. ThisbehaviorisdescribedinFigure6.
OUT1
OUT2
To implement the ratiometric tracking, the feedback pin of
the master channel should connect to the TRACKSS pin of
the slave channel (as in Figure 5b). By selecting different
resistors, the LTC3633A can achieve different modes of
tracking including the two in Figure 4.
NOMINAL OUTPUT
PGOOD
VOLTAGE
Uponstart-up,theregulatordefaultstoBurstModeopera-
tion until the output exceeds 80% of its final value (V
>
FB
0.48V).Oncetheoutputreachesthisvoltage,theoperating
mode of the regulator switches to the mode selected by
the MODE/SYNC pin as described above. During normal
operation, if the output drops below 10% of its final value
(as it may when tracking down, for instance), the regula-
tor will automatically switch to Burst Mode operation to
prevent inductor saturation and improve TRACKSS pin
accuracy.
–8% –5%
0%
5%
8%
OUTPUT VOLTAGE
3633a F06
Figure 6. PGOOD Pin Behavior
A filter time of 40μs (typical) acts to prevent unwanted
PGOOD output changes during V transient events.
As a result, the output voltage must be within the target
regulation window of 5% for 40μs before the PGOOD pin
pulls high. Conversely, the output voltage must exit the
8% regulation window for 40μs before the PGOOD pin
pulls to ground.
OUT
Output Power Good
The PGOOD output of the LTC3633A is driven by a 20Ω
(typical) open-drain pull-down device. This device will be
turned off once the output voltage is within 5% (typical) of
the target regulation point, allowing the voltage at PGOOD
to rise via an external pull-up resistor. If the output voltage
exits an 8% (typical) regulation window around the target
regulation point, the open-drain output will pull down
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
V
OUT1
V
OUT1
V
OUT2
V
OUT2
R3
R4
R1
R2
R3
R4
R1
R2
R3
R4
TO
TRACKSS2
PIN
TO
TRACKSS2
PIN
TO
FB1
PIN
TO
FB2
PIN
TO
FB2
PIN
TO
V
V
V
V
FB1
PIN
3633a F05
(5a) Coincident Tracking Setup
(5b) Ratiometric Tracking Setup
Figure 5. Setup for Coincident and Ratiometric Tracking
3633a1f
18
Linear [ Linear ]
