Datasheet

3
SW(nom)
REF
2.5 10
F kHz
RT k
1
V 2.5 V V
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=
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W
+ ´
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3
SW (nom )
2.5 10
F kHz
RT k
1
2.5 V V
æ ö
ç ÷
´
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=
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W
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5 125
R
T
- Resistor - kW
1000
65
F
SW(nom)
- Switching Frequency - kHz
SWITCHING FREQUENCY
vs
RESISTOR RT VALUE
800
200
0
600
400
100
500
15
25
35 55 75 85 95 11545 105
300
700
900
UCC28950
www.ti.com SLUSA16B –MARCH 2010–REVISED OCTOBER 2011
Switching Frequency Setting (RT)
Connecting an external resistor RT between the RT pin and VREF pins sets the fixed frequency operation and
configures the controller as a master providing synchronization output pulses at SYNC pin with 0.5 duty cycle
and frequency equal to the internal oscillator. To set the converter in Slave Mode, connect the external resistor
RT between RT-pin to GND and place an 825-kΩ resistor form SS pin to GND in parallel to the SS_EN
capacitor. This configures the controller as a slave. The slave controller operates with 90° phase shift relatively to
the master converter if their SYNC pins are tied together. The switching frequency of the converter is equal to the
frequency of output pulses. The following Equation 10 defines the nominal switching frequency of the converter
configured as a master (resistor RT between RT-pin and VREF). On the UCC28950 there is an internal clock
oscillator frequency which is twice as that of the controller outputs frequency.
(10)
In this equation the RT is in kΩ, VREF is in volts and F
SW(nom)
is in kHz. This is also empirical approximation and
thus, there is no unit agreement. Assume for example, VREF = 5 V, RT = 65 kΩ. Then the switching frequency
F
SW(nom)
is going to be 92.6 kHz.
The Equation 11 defines the nominal switching frequency of converter if the converter configured as a slave and
the resistor RT is connected between RT pin and GND.
(11)
In this equation the RT is in kΩ, and F
SW(nom)
is in kHz. Notice that for VREF = 5 V, Equation 10 and Equation 11
yield the same results.
The plot in Figure 11 shows how F
SW(nom)
depends on the resistor RT value when the VREF = 5 V. As it is seen
from Equation 10 and Equation 11, the switching frequency F
SW(nom)
is set to the same value for either master, of
slave configuration provided the same resistor value RT is used.
Figure 11. Converter Switching Frequency F
SW(nom)
Over Resistor RT Value
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