Datasheet

AGND
RT/CLK
R
RT
500 kHz to 2 MHz
External Clock
1 kΩ
470 pF
( )
OUT
IN min
MAX
SW
V
0.75 1
V
CLK _ PW
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LMZ30602
SNVS997 –JULY 2013 www.ti.com
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
Figure 28.
Before the external clock is present, the device works in RT mode and the switching frequency is set by RT
resistor (R
RT
). 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 µs 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. 5-V input applications requiring 1.5 A 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 LMZ30602 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
I
OUT
≤ 1.5 A I
OUT
> 1.5 A All I
OUT
SYNCHRONIZATION R
RT
FREQUENCY (kHz) (kΩ)
V
OUT
RANGE (V) V
OUT
RANGE (V) V
OUT
RANGE (V)
MIN MAX MIN MAX MIN MAX
500 open 0.8 1.4 0.8 0.8 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 2.0
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 2.4
750 715 0.9 3.6 0.9 1.5 0.8 2.5
800 590 0.9 3.6 0.9 1.7 0.8 2.5
900 511 1.0 3.6 1.0 2.2 0.8 2.5
1000 348 1.2 3.6 1.2 2.5 0.8 2.5
1250 232 1.4 3.6 1.4 3.3 1.0 2.5
1500 174 1.7 3.6 1.7 3.6 1.1 2.5
1750 137 2.0 3.6 2.0 3.6 1.3 2.4
2000 113 2.3 3.6 2.3 3.6 1.5 2.3
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