SYNC
LM5005
UP TO 5 TOTAL
DEVICES
LM5005
SYNC
SYNC
AGND
LM5005
SW
CLK
SYNC
SW
500 ns
R
T
=
- 580 x 10
-9
135 x 10
-12
F
1
LM5005
SNVS397D –SEPTEMBER 2005–REVISED MARCH 2013
www.ti.com
Oscillator and Sync Capability
The LM5005 oscillator frequency is set by a single external resistor connected between the RT pin and the
AGND pin. The R
T
resistor should be located very close to the device and connected directly to the pins of the IC
(RT and AGND). To set a desired oscillator frequency (F), the necessary value for the R
T
resistor can be
calculated from the following equation:
(1)
The SYNC pin can be used to synchronize the internal oscillator to an external clock. The external clock must be
of higher frequency than the free-running frequency set by the R
T
resistor. A clock circuit with an open drain
output is the recommended interface from the external clock to the SYNC pin. The clock pulse duration should
be greater than 15 ns.
Figure 11. Sync from External Clock Figure 12. Sync from Multiple Devices
Multiple LM5005 devices can be synchronized together simply by connecting the SYNC pins together. In this
configuration all of the devices will be synchronized to the highest frequency device. The diagram in Figure 13
illustrates the SYNC input/output features of the LM5005. The internal oscillator circuit drives the SYNC pin with
a strong pull-down / weak pull-up inverter. When the SYNC pin is pulled low either by the internal oscillator or an
external clock, the ramp cycle of the oscillator is terminated and a new oscillator cycle begins. Thus, if the SYNC
pins of several LM5005 IC’s are connected together, the IC with the highest internal clock frequency will pull the
connected SYNC pins low first and terminate the oscillator ramp cycles of the other IC’s. The LM5005 with the
highest programmed clock frequency will serve as the master and control the switching frequency of the all the
devices with lower oscillator frequency.
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Product Folder Links: LM5005
