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ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - DC/DC voltage regulator Holder TSSOP 28 EP
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LTC3633A-2/LTC3633A-3
18
3633a23f
APPLICATIONS INFORMATION
Upon start-up, the regulator defaults to Burst Mode opera-
tion until the output exceeds 80% of its fi nal value (V
FB
>
0.48V). Once the output reaches this voltage, the operating
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 fi nal 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.
Output Power Good
The PGOOD output of the LTC3633A-2 is driven by a 20
(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
with 20 output resistance to ground, thus dropping the
PGOOD pin voltage. This behavior is described in Figure 7.
Figure 7. PGOOD Pin Behavior
PGOOD
VOLTAGE
OUTPUT VOLTAGE
NOMINAL OUTPUT
0% 8%–5% 5%
3633a23 F07
–8%
R3 R1
R4 R2
R3
V
OUT2
R4
(6a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TRACKSS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3633a23 F06b3633a23 F06a
(6b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TRACKSS2
PIN
TO
V
FB2
PIN
V
OUT1
Figure 6. Setup for Coincident and Ratiometric Tracking
A fi lter time of 40µs (typical) acts to prevent unwanted
PGOOD output changes during V
OUT
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.
TIME
(5a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
3633a23 F05a
V
OUT1
V
OUT2
TIME
3633a23 F05b
(5b) Ratiometric Tracking
OUTPUT VOLTAGE
Figure 5. Two Different Modes of Output Voltage Tracking