( )
OUT
IN min
MAX
SW
V
0.75
1
V
CLK _ PW
f
÷
-
÷
è
=
AGND
RT/CLK
RRT
500 kHz to 2 MHz
External Clock
1 k
470 pF
SLUSAN7
– SEPTEMBER 2011
Synchronization (CLK)
An internal phase locked loop (PLL) has been implemented to allow synchronization between 500 kHz and
2 MHz, and to easily switch from RT mode to CLK mode. To implement the synchronization feature, connect a
square wave clock signal to the RT/CLK pin with a minimum pulse width of 75 ns. The maximum clock pulse
width must be calculated using
Equation 2. The clock signal amplitude must transition lower than 0.4 V and
higher than 2.2 V. The start of the switching cycle is synchronized to the falling edge of RT/CLK pin. In
applications where both RT mode and CLK mode are needed, the device can be configured as shown in
Before the external clock is present, the device works in RT mode and the switching frequency is set by RT
resistor (RRT). When the external clock is present, the CLK mode overrides the RT mode. The device switches
from RT mode to CLK mode and the RT/CLK pin becomes high impedance as the PLL starts to lock onto the
frequency of the external clock. The device will lock to the external clock frequency approximately 15
s after a
valid clock signal is present. It is not recommended to switch from CLK mode back to RT mode because the
internal switching frequency drops to a lower frequency before returning to the switching frequency set by the RT
resistor.
(2)
Figure 28. CLK/RT Configuration
The synchronization frequency must be selected based on the output voltages of the devices being
synchronized.
Table 7 shows the allowable frequencies for a given range of output voltages based on a resistive
load. 5V input applications requiring 1.5A or less can synchronize to a wider frequency range. For the most
efficient solution, always synchronize to the lowest allowable frequency. For example, an application requires
synchronizing three TPS84210 devices with output voltages of 1.2V@1.7A, 1.8@1.1AV and 3.3V@ 1.0A, all
powered from VIN = 5V.
Table 7 shows that all three output voltages can be synchronized to any frequency
between 700 kHz to 1 MHz. For best efficiency, choose 700 kHz as the sychronization frequency.
Table 7. Synchronization Frequency vs Output Voltage
VIN = 5 V
VIN = 3.3 V
IOUT ≤ 1.5 A
IOUT > 1.5 A
All IOUT
SYNCHRONIZATION
RRT
FREQUENCY (kHz)
(k
Ω)
VOUT RANGE (V)
MIN
MAX
MIN
MAX
MIN
MAX
500
open
0.8
1.4
0.8
1.1
550
3400
0.8
1.6
0.8
0.9
0.8
1.2
600
1800
0.8
1.9
0.8
1.1
0.8
2.2
650
1200
0.8
2.4
0.8
1.2
0.8
2.2
700
887
0.8
3.6
0.8
1.3
0.8
2.2
750
715
0.9
3.6
0.9
1.5
0.8
2.2
800
590
0.9
3.6
0.9
1.7
0.8
2.2
900
511
1.0
3.6
1.0
2.2
0.8
2.2
1000
348
1.2
3.6
1.2
2.5
0.8
2.2
1250
232
1.4
3.6
1.4
3.3
1.0
2.2
1500
174
1.7
3.6
1.7
3.6
1.1
2.2
1750
137
2.0
3.6
2.0
3.6
1.3
2.2
2000
113
2.3
3.6
2.3
3.6
1.5
2.2
Copyright
2011, Texas Instruments Incorporated
19