Datasheet
f
D
L
V
ΔI
IN
L
×=
I = I
VALLEY LIM
I
L
f
I
PEAK
Rectifier
Current
I
OUT(DC)
Inductorr
Current
Increased
Load Current
I
IN(DC)
I
IN(DC)
Current Limit
Threshold
I
OUT(CL)
DI
L
DI
L
IN
L
V
D
I =
L f
D g
IN
OUT
V
D 1
V
h
= -
g
- Dg
OUT(CL) VALLEY L
1
I = (1 D) (I + I )
2
TPS61253, TPS61254, TPS61256
TPS61258, TPS61259
SLVSAG8C –SEPTEMBER 2011–REVISED AUGUST 2012
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STANDBY MODE
The TPS6125x device is able to maintain its output biased at the input voltage level. In so called standby mode
(EN = 0, BP = 1), the synchronous rectifier is current limited to ca. 150mA allowing an external load (e.g. audio
amplifier) to be powered with a restricted supply. The output voltage is slightly reduced due to voltage drop
across the rectifier MOSFET and the inductor DC resistance. The device consumes only a standby current of
22µA (typ).
Table 3. Operating Mode Control
OPERATING MODE EN BP
Shutdown, True Load Disconnect (SD) 0 0
Standby Mode, Output Pre-Biased (SM) 0 1
1 0
Boost Operating Mode (BST)
1 1
CURRENT LIMIT OPERATION
The TPS6125x device employs a valley current limit sensing scheme. Current limit detection occurs during the
off-time by sensing of the voltage drop across the synchronous rectifier.
The output voltage is reduced as the power stage of the device operates in a constant current mode. The
maximum continuous output current (I
OUT(CL)
), before entering current limit (CL) operation, can be defined by
Equation 1.
(1)
The duty cycle (D) can be estimated by Equation 2
(2)
and the peak-to-peak current ripple (ΔI
L
) is calculated by Equation 3
(3)
The output current, I
OUT(DC)
, is the average of the rectifier ripple current waveform. When the load current is
increased such that the lower peak is above the current limit threshold, the off-time is increased to allow the
current to decrease to this threshold before the next on-time begins (so called frequency fold-back mechanism).
When the current limit is reached the output voltage decreases during further load increase.
Figure 36 illustrates the inductor and rectifier current waveforms during current limit operation.
Figure 36. Inductor/Rectifier Currents in Current Limit Operation
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