Datasheet

LTC3613
21
3613fa
APPLICATIONS INFORMATION
TIME
Coincident Tracking
EXTERNAL
SUPPLY
EXTERNAL
SUPPLY
V
OUT
VOLTAGE
V
OUT
TIME
3613 F06
Ratiometric Tracking
VOLTAGE
Figure 6. Two Different Modes of Output Tracking
Figure 7. Setup for Coincident and Ratiometric Tracking
R
FB2
EXT. V
R
FB1
Coincident Tracking Setup
TO
TRACK/SS
R
FB2
V
OUT
TO V
OSNS
+
R
FB1
TO V
OSNS
–
R1
EXT. V
R2
R1+ R2
≥
R2
TO
TRACK/SS
R
FB2
V
OUT
TO V
OSNS
+
R
FB1
TO V
OSNS
–
3613 F07
Ratiometric Tracking Setup
0.6V
EXT. V
Phase and Frequency Synchronization
For applications that require better control of EMI and
switching noise or have special synchronization needs,
the LTC3613 can phase and frequency synchronize the
turn-on of the switching cycle to an external clock signal
applied to the MODE/PLLIN pin. The applied clock signal
needs to be within ±30% of the RT pin programmed free-
running frequency to assure proper frequency and phase
lock. The clock signal levels should generally comply to V
IH
> 2V and V
IL
< 0.5V. The MODE/PLLIN pin has an internal
600k pull-down resistor to ensure pulse-skipping mode
if the pin is left floating.
The LTC3613 uses the voltages on SV
IN
and V
OUT
pins as
well as the RT programmed frequency to determine the
steady-state on-time as follows:
t
ON
≈
V
OUT
V
IN
•f
An internal PLL system adjusts this on-time dynamically
in order to maintain phase and frequency lock with the
external clock. The LTC3613 will maintain phase and fre-
quency lock under steady-state conditions for V
IN
, V
OUT
and load current.
As shown in the previous equation, the on-time is a
function of the switching regulator’s output. This output
is measured by the V
OUT
pin and is used to calculate the
required on-time. Therefore, simply connecting V
OUT
to
the regulator’s local output point is preferable for most
applications. However, there could be applications where
the internally calculated on-time differs significantly from
the real on-time required by the application. For example,
if there are differences between the local output point and
the remotely regulated output point due to line losses, then
the internally calculated on-time will be inaccurate. Lower
efficiencies in the switching regulator can also cause the
real on-time to be significantly different from the internally