LTC3633 LINER [Linear Technology], LTC3633 Datasheet - Page 18

• Manufacturer Part Number: LTC3633
• Description: Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

LTC3633
APPLICATIONS INFORMATION
The ratio of this divider should be the same as that of the
slave channel’s feedback divider shown in Figure 5a. In
this tracking mode, V
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 LTC3633 can achieve different modes of
tracking including the two in Figure 4.
Upon start-up, the regulator defaults to Burst Mode opera-
tion until the output exceeds 80% of its fi nal value (V
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 LTC3633 is driven by a 15Ω
(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
18
TRACKSS2
V
OUT1
PIN
TO
OUT1
(5a) Coincident Tracking Setup
must be set higher than V
R3
R4
R1
R2
Figure 5. Setup for Coincident and Ratiometric Tracking
TO
V
PIN
FB1
V
PIN
FB2
TO
R3
R4
OUT2
FB
V
OUT2
>
.
15Ω output resistance to ground, thus dropping the PGOOD
pin voltage. This behavior is described in Figure 6.
A fi lter time of 40μs (typical) acts to prevent unwanted
PGOOD output changes during V
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.
Effi ciency Considerations
The percent effi ciency 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
VOLTAGE
TRACKSS2
PGOOD
V
OUT1
PIN
(5b) Ratiometric Tracking Setup
TO
R1
R2
Figure 6. PGOOD Pin Behavior
–8%
TO
V
PIN
FB1
–5%
NOMINAL OUTPUT
V
PIN
FB2
TO
0%
R3
R4
3633 F05
5%
OUT
V
OUT2
8%
transient events.
OUTPUT VOLTAGE
3633 F06
3633f