LTC3633A-2 Datasheet, PDF (18/28 Pages) Linear Technology – Dual Channel 3A, 20V Monolithic Synchronous Step-Down Regulator

LTC3633A-2/LTC3633A-3
APPLICATIONS INFORMATION
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
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(5a) Coincident Tracking
TIME
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(5b) Ratiometric Tracking
Figure 5. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TRACKSS2
PIN
R3 R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R4 R2
V_OUT2
R3
R4
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(6a) Coincident Tracking Setup
V_OUT1
TO
TRACKSS2
PIN
R1
TO
TO
V_FB1
V_FB2
PIN
PIN
R2
V_OUT2
R3
R4
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(6b) Ratiometric Tracking Setup
Figure 6. Setup for Coincident and Ratiometric Tracking
Upon start-up, the regulator defaults to Burst Mode opera-
tion until the output exceeds 80% of its final 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 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.
PGOOD
VOLTAGE
NOMINAL OUTPUT
–8% –5% 0%
5% 8% OUTPUT VOLTAGE
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Figure 7. PGOOD Pin Behavior
Output Power Good
The PGOOD output of the LTC3633A-2 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
with 20Ω output resistance to ground, thus dropping the
PGOOD pin voltage. This behavior is described in Figure 7.
18
A filter 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.
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