UCD7230
www.ti.com
SLUS741–NOVEMBER 2006
APPLICATION INFORMATION (continued)
Reference / External Bias Supply
The UCD7230 includes a series pass regulator to provide a regulated 3.3 V at the 3V3 pin that can be used to
power other circuits such as the UCD91xx, a microcontroller or an ASIC. 3V3 can source 10 mA of current. For
normal operation, place a 0.22-µF ceramic capacitor between 3V3 and AGND.
Control Inputs
IN and SRE are high impedance digital inputs designed for 3.3-V logic-level signals. They both have 100-kΩ
pull-down resistors. Schmitt Trigger input stage design immunizes the internal circuitry from external noise. IN is
the command input for the upper driver, OUT1, and can function up to 2 MHz. SRE controls the function of the
lower driver, OUT2. When SRE is false (low), OUT2 is held low. When SRE is true, OUT2 is inverted from OUT1
with appropriate delays that preclude cross conduction in the Buck MOSFETs.
Driver Stages
The driver outputs utilize Texas Instruments’ TrueDrive™ architecture, which delivers rated current into the gate
of a MOSFET when it is most needed, during the Miller plateau region of the switching transition. This provides
best switching speeds and reduces switching losses. TrueDrive™ consists of pull-up/ pull-down circuits using
bipolar and MOSFET transistors in parallel. This hybrid output stage also allows relatively constant current
sourcing even at reduced supply voltages.
The low-side high-current output stage of the UCD7230 device is capable of sourcing and sinking 4-A
peak-current pulses and swings from PVDD to PGND. The high-side floating output diver is capable of sourcing
2 A and sinking 4-A peak-current pulses. This ratio of gate currents, common to synchronous buck applications,
minimizes the possibility of parasitic turn on of the low-side power MOSFET due to dv/dt currents during the
rising edge switching transition.
If further limiting of the rise or fall times to the power device is desired, an external resistance can be added
between the output of the driver and the power MOSFET gate. The external resistor also helps remove power
dissipation from the driver.
The driver outputs follows the IN and SRE as previously described provided that VDD and 3V3 are above their
respective under-voltage lockout thresholds. When the supplies are insufficient, the chip holds both OUT1 and
OUT2 low.
It is worth reiterating the need mentioned in the supply section for sound high frequency design techniques in
the circuit board layout and bypass capacitor selection and placement. Some applications may generate
excessive ringing at the switch-inductor node. This ringing can drag SW to negative voltages that might cause
functional irregularities. To prevent this, carefull board layout and appropriate snubbing are essential. In addition,
it may be appropriate to couple SW to the inductor with a 1-Ω resistor, and then bypass SW to PGND with a low
impedance Schottky diode.
Current Sensing and Overload Protection
Since the UCD7230 is physically collocated with the high-current elements of the power converter, it is logical
that current be monitored by the chip. An internal instrumentation amplifier conditions current sense signals so
that they can be used by the control chip generating the pwm signal.
POS and NEG are inputs to an instrumentation amplifier circuit. This amplifier has a nominal gain of 25 and
presents its output at AO. This can be used to monitor a parallel RC around the buck inductor shown in
Figure 3. As long as the RPOS x C time constant is the same as the L/R of the inductor and its parasitic
equivalent series resistance, then the voltage on C is the same as the IR drop on the parasitic inductor
resistance. Signals in this method can be very small, so the amp is necessary to condition the signals to useful
amplitudes. Should more accurate current sensing be required, a sense resistor can be placed between the
buck inductor and output capacitor. Since that resistor represents inefficiency to the converter, it will also be a
very small value of resistance with small signals, and, again, the amp conditions the signal to useful size.
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