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TPS23757
SLVS948D – JULY 2009 – REVISED NOVEMBER 2013
www.ti.com
(a)
t RECHARGE =
(C VC1 +C VC2 ) ′ V CUVH
I VC
(10 m F + 0.47 m F) ′ 3.5 V
= = 9.2 ms
4 mA
t DISCHARGE = = = 6.5 ms
t DISCHARGE 6.5 ms
(b)
(c)
(d)
(C VC1 +C VC2 ) ′ V CUVH (10 m F + 0.47 m F) ′ 3.5 V
I TOTAL 5.6 mA
(a) Note that the optocoupler current is 0 mA because the output is in current limit.
(b) Also, it is assumed I APb is 0 mA.
Duty Cycle: D = = = 41%
t DISCHARGE + t RECHARGE 6.5 ms + 9.2 ms
1 1
Hiccup Frequency: F = = = 64 Hz
t DISCHARGE + t RECHARGE 6.5 ms + 9.2 ms
6. The voltage rating of C VC1 and C VC2 should be 16 V minimum.
Switching Transformer Considerations and R VC
Care in design of the transformer and V C bias circuit is required to obtain hiccup overload protection. Leading-
edge voltage overshoot on the bias winding may cause V C to peak-charge, preventing the expected tracking with
output voltage. Some method of controlling this is usually required. This may be as simple as a series resistor, or
an R-C filter in front of D VC1 . Good transformer bias-to-output-winding coupling results in reduced overshoot and
better voltage tracking.
R VC as shown in Figure 27 helps to reduce peak charging from the bias winding. This becomes especially
important when tuning hiccup mode operation during output overload. Typical values for R VC will be between 10
? and 100 ? .
V C
R VC
D VC1
T1
Bias Winding
ARTN
Figure 27. R VC Usage
APb Pin Interface
The APb pin is an active low, open-drain output indicating an adapter power source is available. An optocoupler
is typically used to interface with the APb pin to signal equipment on the secondary side of the converter of APb
status. Optocoupler current-gain is referred to as CTR (current transfer ratio), which is the ratio of transistor
collector current to LED current. To preserve efficiency, a high-gain optocoupler ( 250% ≤ CTR ≤ 500%, or 300%
≤ CTR ≤ 600% ) along with a high-impedance (e.g., CMOS) receiver are recommended. Design of the APb
optocoupler interface can be accomplished as follows:
V C
R APb
V OUT
R APb_OUT
Adapter Present
Indicator
Low = Present
APb From
TPS23757
Figure 28. APb Interface
1. APb ON characteristic: I APb = 2 mA minimum, V APb = 1 V
2. Let V C = 12 V, V OUT = 5 V, R APb-OUT = 10 k ? , V APb-OUT (low) = 400 mV max
28
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