LTC3613 Datasheet, PDF (21/36 Pages) Linear Technology – 24V, 15A Monolithic Step Down Regulator

APPLICATIONS INFORMATION
EXTERNAL
SUPPLY
V_OUT
LTC3613
EXTERNAL
SUPPLY
V_OUT
TIME
Coincident Tracking
TIME
3613 F06
Ratiometric Tracking
Figure 6. Two Different Modes of Output Tracking
EXT. V
R_FB2
TO
TRACK/SS
R_FB1
V_OUT
R_FB2
TO V_OSNS+
R_FB1
TO V_OSNS–
Coincident Tracking Setup
EXT. V
TO
TRACK/SS
R1
R2
R1+ R2
≥
0.6V
EXT. V
R2
V_OUT
R_FB2
TO V_OSNS+
R_FB1
TO V_OSNS–
3613 F07
Ratiometric Tracking Setup
Figure 7. Setup for Coincident and Ratiometric Tracking
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
3613fa
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