BUK3F00-50WDXX Datasheet, PDF (18/52 Pages) NXP Semiconductors – Controller for TrenchPLUS FETs

NXP Semiconductors
BUK3F00-50WDxx
Controller for TrenchPLUS FETs
devices have common drain connections, the V_DS of the two devices are equal, and the
correct sense current is being pulled. Current measurement is only possible when the
voltage on pin KELVIN is above the V_{th(on)(bat-KEL)} threshold.
9.4.2 Analog current measurement output
An analog current can be output on pin IMEAS that is proportional to the sense current
measured on a selected FET channel. Any single channel can be multiplexed to this
output at a time.
The accuracy and resolution of analog current measurement is determined by the voltage
across the R_IMEAS resistor with the measurement output current and the measurement
range used. The measurement current is given by I_meas = (I_SENSE / I_{meas(ADC)(fs)}) × 100 µA,
where I_SENSE is the FET sense current and I_{meas(ADC)(fs)} is the set full-scale current for the
measurement range. For reliable current measurement, the voltage on pin IMEAS must
be less than the measurement supply voltage on pin V_CC(MEASC). A resistor value giving
high resolution at low measurement output current (for example, up to I_{meas(ADC)(fs)}) may
not provide the range for high measurement output current (for example, up to
8 × I_{meas(ADC)(fs)}). Conversely, a value giving the range for high measurement current will
give less resolution for low measurement current.
When the selected channel uses PWM, the analog measurement is able to follow the
switched waveform, except when the duty cycle is very low, and high-side FETs are in the
turn-on state. The voltage on pin IMEAS is limited just below the measurement supply
voltage.
Table 11. Analog current channel selection register (address 06h) bit description
Address Register
Bit
Description
06h
CURR_MEAS
7 to 4 not used; must be set to logic 0
3
current measurement setting:
1 = enables current measurement in selected channel
0 = disables current measurement in all channels
2 to 0 selects measurement channel; binary value corresponds
to channel number (0 to 7)
9.4.3 Digital current measurement output
8-bit successive approximation ADCs are used to measure the sense currents of the FET
channels. The measured values are only considered valid when the FET has been on for
the full conversion cycle. Digital measurements are stored and can be read by a controller.
The reading from the ADC may not indicate zero if the channel is requested off. If PWM is
not selected, the values are stored every ADC cycle. For PWM the digital measurement
can be sampled at the start or end of the on time.
The ADC reading, up to the maximum 255 bits, is given by:
reading = 255 × (I_SENSE / I_{meas(ADC)(fs)}) × (50 µA / I_IREFCURR), where I_IREFCURR is the
current through the current reference resistor (R_IREFCURR). At I_IREFCURR = 50 µA this
equation simplifies to give a direct relationship with the analog measurement current.
9.4.4 Low battery supply voltage conditions
The current measurement interface operates at voltages very near the battery voltage. To
permit reasonable headroom in the circuit, the current measurement interface is powered
from the charge pump. The circuit cannot operate correctly when it is close to ground, as
BUK3F00-50WDXX_4
Product data sheet
Rev. 04 — 4 September 2008
© NXP B.V. 2008. All rights reserved.
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