Datasheet

() ()
PEAK
ON n p
I
T L Trr n
VBAT
= +
(1)
()
()
PEAK
OFF n
OUTn
I
T N L
V
= ´
(2)
Programming Peak Current
TPS65563A
SLVS858A – JULY 2008 – REVISED AUGUST 2008 ......................................................................................................................................................... www.ti.com
In Figure 5 , ON time is almost the same period in every switch cycle. But the current at SW always starts from
negative value because of the Trr of the diode. Because of this, ON time depends on Trr. ON time is calculated
by Equation 1 .
Where:
T
ON(n)
= ON time at n cycle switching
Lp = Inductance of primary side
I
PEAK
= Peak current at primary side
VBAT = Battery voltage
Trr(n) = Reverse recovery time at n cycle switching
OFF time is dependant on output voltage. As the output voltage gets higher, OFF time gets shorter (see
Equation 2 ).
Where:
T
OFF(n)
= OFF time at n cycle switching
N = Turn ratio of transformer
V
OUT(n)
= Output voltage at n cycle switching
The TPS65563A provides a method to program the peak primary current with a voltage applied to the I_PEAK
pin. Figure 6 shows how to program I
PEAK
.
Figure 6 shows the relationship between I_PEAK pin voltage and a peak current at the primary side. This
function has the analog slope controlled by I_PEAK. The maximum voltage to control peak current at the primary
side is around 1.2 V and it is acceptable to input the voltage up to VCC.
Figure 6. I_PEAK Pin Voltage vs Peak Current at Primary Side
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