UC1846-SP_15 Datasheet, PDF (17/29 Pages) Texas Instruments – Current-Mode PWM Controller

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UC1846-SP
SLUS871C – JANUARY 2009 – REVISED OCTOBER 2015
Each capacitor type is characterized by its impedance and the frequency range over which it is most effective.
The frequency at which the impedance reaches its minimum is determined by its ESR and ESL. It is known as
the self resonant frequency of the capacitor. The self resonant frequency is considered to be the maximum
usable frequency for a capacitor. Above this frequency the impedance of the capacitor begins to rise as the ESL
of the capacitor begins to dominate. Note that each capacitor type has a specific frequency band over which it is
most effective. Therefore, a capacitor network of multiple capacitor types is more effective in reducing impedance
than just one type.
The current slew rate of a regulator is limited by its output filter inductor. When the amount of current required by
the load changes, the initial current deficit must be supplied by the output capacitors until the regulator can meet
the load demand.
The desired response to a large change in the load current is the first criteria. The output capacitor needs to
supply the load with current when the regulator control loop can not supply the current. This happens when Load
(ie: memory, processor) has a large and fast increase in current, such as a transition from no load to full load.
The regulator typically needs two or more clock cycles for the control loop to see the change in load current,
output voltage and adjust the duty cycle to react to the change. The output capacitor must be properly sized to
supply the extra current to the Load until the control loop responds to the Load change. The output capacitance
must be large enough to supply the difference in current for 2 clock cycles while only allowing a tolerable amount
of droop in the output voltage. Equation 9 shows the minimum output capacitance necessary to accomplish this.
CO > ^{2 ´ DIout}
fSW ´ DVout
(9)
Where ΔIout is the change in output current, f_SW is the regulators switching frequency and ΔVout is the allowable
change in the output voltage. For this example, the transient load response is specified as a 5% change in Vout
for a load step of 1A. For this example, ΔIout = 1.0 A and ΔVout = 0.05 × 3.3 = 0.165 V. Using these numbers
gives a minimum capacitance of 25 μF. This value does not take the ESR of the output capacitor into account in
the output voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation.
8.2.1.2.3 Output Inductor Selection
To calculate the value of the output inductor, use Equation 10. Kind is a coefficient that represents the amount of
inductor ripple current relative to the maximum output current. The inductor ripple current is filtered by the output
capacitor. Therefore, choosing high inductor ripple currents impact the selection of the output capacitor since the
output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. In general,
the inductor ripple value is at the discretion of the designer; however, Kind is normally from 0.1 to 0.3 for the
majority of applications. VinLC refers to the voltage at the input of output LC filter.
L1 = ^{VinLC - Vout} ´ ^Vout
IO ´ Kind VinLC ´ fSW
(10)
The current flowing through the inductor is the inductor ripple current plus the output current. During power up,
faults or transient load conditions, the inductor current can increase above the calculated peak inductor current
level calculated above. In transient conditions, the inductor current can increase up to the switch current limit of
the device. For this reason, the most conservative approach is to specify an inductor with a saturation current
rating equal to or greater than the switch current limit rather than the peak inductor current.
8.2.1.2.4 Switching Frequency
Initial accuracy of UC1846-SP oscillator frequency is 200 kHz ±15% over the temperature range. Switching
frequency selection is a trade-off between the overall design size and efficiency. Operating at lower switching
frequency will result in higher efficiency at the expense of larger solution footprint.
Oscillator frequency can be determined as follows:
R_T = 10 kΩ
(11)
C_T = 1 nF
(12)
2
fT = _{RT ´ CT}
(13)
f_T = 200 kHz
(14)
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