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Current Sensing and Balancing (U1, U9 and U18)

Hiccup Mode

PowerGood

Soft-Start

SS

SS

6

0.6

C

t

5

´

»

´

(1)

Overcurrent Protection

IL IM

P H(m a x)

C S

V

3 .7 5

I

R

=

´

´

(2)

I

I

OUT

2

)

V

V

OUT

2

L

OUT

f

SW

V

IN

(3)

TPS40132

SLUS776A – DECEMBER 2007 – REVISED FEBRUARY 2008

The controller employs peak current mode control scheme, thus naturally provides certain degree of current

balancing. With current mode, the level of current feedback should comply with certain guidelines depending on

duty factor known as “slope compensation” to avoid the sub-harmonic instability. This requirement can prohibit

achieving a higher degree of phase current balance. To avoid the controversy, a separate current loop that

forces phase currents to match is added to the proprietary control scheme. This effectively provides high degree

useable range of the controller bandwidth is also extended.

High bandwidth current amplifiers, U1 and U18 can accept as an input voltage either the voltage drop across

dedicated precise current sense resistors, or inductor’s DCR voltage derived by an RC network. The wide range

of current sense arrangements ease the cost/complexity constraints and provides superior performance

compared to controllers utilizing the low-side MOSFET current sensing. See the Inductor DCR Current Sense

section for more information on selecting component values for the R-C network.

When the soft-start cycle is complete and soft-start voltage exceeds 1 V, the hiccup mode is enabled for output

overcurrent and undervoltage protection. The hiccup mode is invoked when overcurrent or undervoltage faults

are detected. the hiccup mode allows a time for a fault to clear itself, for instance, a momentary short circuit on

the output. The hiccup mode consists of 7 cycles of charging and discharging the soft-start capacitor, and then

attempting a re-start. If the fault has been cleared, the re-start causes the output to come up in regulation. If the

fault has not been cleared, the hiccup mode is initiated again, and another restart occurs after another 7

soft-start cycles.

The PGOOD pin indicates when the inputs and output are within their specified ranges of operation. Also

monitored are the EN/SYNC and SS pins. PGOOD has high impedance when indicating inputs and outputs are

within specified limits and is pulled low to indicate an out-of-limits condition. .

A capacitor connected to the soft start pin (SS) sets the power-up time. When EN is high and POR is cleared,

the calibrated current source starts charging the external soft start capacitor. The PGOOD pin is held low during

the start up. The rising voltage across the capacitor serves as a reference for the error amplifier, U12. When the

soft-start voltage reaches the level of the reference voltage, the converter’s output reaches the regulation point

and further voltage rise of the soft start voltage has no effect on the output. When the soft start voltage reaches

1.0 V, the power good (PGOOD) function is cleared to be reported on the PGOOD pin. Normally the PGOOD pin

goes high at this time. Equation 1 is used to calculate the value of the soft-start capacitor.

The overcurrent function, U19, monitors the output of current sense amplifiers U1 and U18. These currents are

converted to voltages and compared to the voltage on the ILIM pin. The relationship between the maximum

phase current and the current sense resistance is given in the following equation. In case a threshold of

and the voltage to set on the ILIM pin is determined by Equation 2 and Equation 3.

where

• I

• I

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