Datasheet

T2P-OUT
T2P-MIN T2P
C T2P FWLED
T2P
T2P
a. The approximate forward voltage of the optocoupler diode, V , is 1.1 V from the data sheet.
FWLED
I
0.46mA
I 0.46mA, Select I 1mA
CTR 1.00
b.
V V V
12 V 0.
R
I
= = = =
- -
-
= =
26 V 1.1V
10.6k
1mA
c. Select a 10.7 k resistor.
-
= W
W
OUT T2P OUT
T2P OUT
T2P OUT
V V
5 0.4
I 0.46 mA
R 10000
-
-
-
-
-
= = =
V
C
T2P From
TPS23751/2
R
T2P
R
T2P-OUT
V
OUT
V
T2P-OUT
V
T2P
Low
Indicates
Type 2
I
T2P-OUT
I
T2P
TPS23751
TPS23752
SLVSB97C –JULY 2012–REVISED JANUARY 2014 www.ti.com
T2P Pin Interface
The T2P pin is an active-low, open-drain output which indicates that a high power source is available. An
optocoupler can interface the T2P pin to circuitry on the secondary side of the converter. A high-gain optocoupler
and a high-impedance (for example, CMOS) receiver are recommended. Design of the T2P optocoupler interface
can be accomplished as follows:
Figure 32. T2P Interface
1. As shown in Figure 32, let V
C
= 12 V, V
OUT
= 5 V, R
T2P-OUT
= 10 kΩ, V
T2P
= 260 mV, V
T2P-OUT
= 400 mV.
(15)
2. The optocoupler current transfer ratio, CTR, is not needed to determine R
T2P
. A device with a minimum CTR
of 100% at 1 mA LED bias current, I
T2P
, is selected. Note that in practice, CTR varies with temperature, LED
bias current, and aging. These variations may require some iteration using the CTR-versus- I
DIODE
curve on
the optocoupler data sheet.
(16)
Softstart
Converters require a softstart on the voltage error amplifier to prevent output overshoot on startup. Figure 33
shows a common implementation of a secondary-side softstart that works with the typical TL431 error amplifier.
The softstart components consist of D
SS
, R
SS
, and C
SS
. They serve to control the output rate-of-rise by pulling
V
CTL
down as C
SS
charges through R
OB
, the optocoupler, and D
SS
. This has the added advantage that the TL431
output and C
IZ
are preset to the proper value as the output voltage reaches the regulated value, preventing
voltage overshoot due to the error amplifier recovery. The secondary-side error amplifier does not become active
until there is sufficient voltage on the secondary. The TPS23751 and TPS23752 provide a primary-side softstart
which persists long enough (approximately 3ms) for secondary side voltage-loop softstart to take over. The
primary-side current-loop softstart controls the switching MOSFET peak current by applying a slowly rising ramp
voltage to a second PWM control input. The PWM is controlled by the lower of the softstart ramp or the CTL-
derived current demand. The actual output voltage rise time is usually much shorter than the internal softstart
period. Initially the internal softstart ramp limits the maximum current demand as a function of time. The current
limit, secondary-side softstart, or output regulation assume control of the PWM before the internal softstart period
is over.
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