LMZ31704_14 Datasheet, PDF (22/34 Pages) Texas Instruments – 4A SIMPLE SWITCHER® Power Module with 2.95V-17V Input and Current Sharing in QFN Package

LMZ31704
SLVSBV8A – JUNE 2013 – REVISED AUGUST 2013
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Synchronization (CLK)
An internal phase locked loop (PLL) has been implemented to allow synchronization between 200 kHz and
1200 kHz, 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 duty cycle between 20% to 80%. The clock signal amplitude
must transition lower than 0.5 V and higher than 2.0 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 Figure 35.
Before the external clock is present, the device works in RT mode and the switching frequency is set by RT
resistor. When the external clock is present, the CLK mode overrides the RT mode. The first time the CLK pin is
pulled above the RT/CLK high threshold (2.0 V), 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. It is not
recommended to switch from CLK mode back to RT mode because the internal switching frequency drops to
100 kHz first before returning to the switching frequency set by the RT resistor (R_RT).
External Clock
200 kHz to 1200 kHz
R_RT
RT/CLK
AGND
Figure 35. RT/CLK Configuration
The synchronization frequency must be selected based on the output voltages of the devices being
synchronized. Table 9 shows the allowable frequencies for a given range of output voltages. For the most
efficient solution, always synchronize to the lowest allowable frequency. For example, an application requires
synchronizing three LMZ31704 devices with output voltages of 1.0 V, 1.2 V and 1.8 V, all powered from
PVIN = 12 V. Table 9 shows that all three output voltages should be synchronized to 300 kHz.
Table 9. Synchronization Frequency vs Output Voltage
SYNCHRONIZATION
FREQUENCY (kHz)
200
300
400
500
600
700
800
900
1000
1100
1200
PVIN = 12 V
V_OUT RANGE (V)
MIN
MAX
0.6
1.3
0.8
2.0
1.1
2.5
1.4
3.4
1.6
5.0
1.9
5.5
2.1
5.5
2.4
5.5
2.7
5.5
2.9
5.5
3.2
5.5
PVIN = 5 V
V_OUT RANGE (V)
MIN
MAX
0.6
1.5
0.6
4.3
0.6
4.3
0.6
4.3
0.7
4.3
0.8
4.3
0.9
4.3
1.0
4.3
1.1
4.3
1.3
4.3
1.4
4.3
22
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