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APPLICATION INFORMATION
Introduction
Programming Operating Frequency
R
T +
* 3.98
104
f
SW
2
)
5.14
104
f
SW
* 8.6 (kW)
(1)
Selecting an Inductor Value
L
+
V
IN * VOUT
V
OUT
V
IN
f
SW
DI
(H)
(2)
Selecting the Output Capacitance
SLUS726 – SEPTEMBER 2006
The TPS40101 is a voltage mode synchronous buck controller targeted at applications that require sequencing
and output voltage margining features.Current sensing is true differential and can be done using the inductor DC
resistance (with a R-C filter) or with a separate sense resistor in series with the inductor. The programmable
overcurrent function has user programmable integration to eliminate nuisance tripping and allow the user to tailor
the response to application requirements. The controller provides an integrated method to margin the output
voltage to
±3% and ±5% of its nominal value by simply grounding one of two pins directly or through a
resistance. Powergood and clock synchronization functions are provided on dedicated pins. Users can program
operating frequency and the closed loop soft-start time by means of a resistor and capacitor to ground
respectively. Output sequencing/tracking can be accomplished in one of three ways: sequential (one output
comes up, then a second comes up), ratiometric (one or more outputs reach regulation at the same time – the
voltages all follow a constant ratio while starting) and simultaneous (one or more outputs track together on
startup and reach regulation in order from lowest to highest).
Operating frequency is set by connecting a resistor to GND from the RT pin. The relationship is:
where
f
SW is the switching frequency in kHz
R
T is in k
Figure 25 and Figure 26 show the relationship between the switching frequency and the RT resistor as described in
Equation 1. The scaling is different between them to allow the user a more accurate views at both high and
low frequency.
The inductor value determines the ripple current in the output capacitors and has an effect on the achievable
transient response. A large inductance decreases ripple current and output voltage ripple, but is physically larger
than a smaller inductance at the same current rating and limits output current slew rate more that a smaller
inductance would. A lower inductance increases ripple current and output voltage ripple, but is physically smaller
than a larger inductance at the same current rating. For most applications, a good compromise is selecting an
inductance value that gives a ripple current between 20% and 30% of the full load current of the converter. The
required inductance for a given ripple current can be found from:
where
L is the inductance value (H)
V
IN is the input voltage to the converter (V)
V
OUT is the output voltage of the converter (V)
f
SW is the switching frequency chosen for the converter (Hz)
I is the peak-to-peak ripple current in the inductor (A)
The required value for the output capacitance depends on the output ripple voltage requirements and the ripple
current in the inductor, as well as any load transient specifications that may exist.
The output voltage ripple depends directly on the ripple current and is affected by two parameters from the
output capacitor: total capacitance and the capacitors equivalent series resistance (ESR). The output ripple
voltage (worst case) can be found from:
15