ER2120QI Datasheet, PDF (20/24 Pages) Altera Corporation – 2A Synchronous Buck Regulator with Integrated MOSFETs

Page 20
Output Inductor Selection
The output inductor is selected to meet the output voltage ripple requirements and to minimize the converter’s response time to the
load transient. The inductor value determines the converter’s ripple current, and the ripple voltage is a function of the ripple
current. The ripple voltage and current are approximated by using Equation 8:
VIN - VOUT VOUT
DI =
x
F_SW x L
VIN
DVOUT = DI x ESR
(EQ. 8)
Increasing the value of inductance reduces the ripple current and voltage. However, the large inductance values reduce the
converter response time to a load transient.
One of the parameters limiting converter response to a load transient is the time required to change the inductor current. Given
a sufficiently fast control loop design, the ER2120QI provides either 0% or 100% duty cycle in response to a load transient. The
response time is the time required to slew the inductor current from an initial current value to the transient current level. During
this interval, the difference between the inductor current and the transient current level must be supplied by the output
capacitor. Minimizing the response time can minimize the output capacitance required.
The response time to a transient is different for the application of load and the removal of load. Equation 9 gives the approximate
response time interval for application and removal of a transient load:
t_RISE =
L x I_TRAN
VIN - VOUT
t_FALL =
L x I_TRAN
VOUT
(EQ. 9)
where: I_TRAN is the transient load current step, t_RISE is the response time to the application of load, and t_FALL is the response time to
the removal of load. The worst-case response time can be either at the application or removal of load. Be sure to check both of
these equations at the minimum and maximum output levels for the worst-case response time.
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage overshoot across the MOSFETs. Use small ceramic capacitors for high-
frequency decoupling, and bulk capacitors to supply the current needed each time the upper MOSFET turns on. Place the small
ceramic capacitors physically close to the MOSFETs and between the drain of the upper MOSFET and the source of the lower
MOSFET.
The important parameters for bulk input capacitance are the voltage rating and the RMS current rating. For reliable operation,
select bulk capacitors with voltage and current ratings above the maximum input voltage and largest RMS current required by the
circuit. Their voltage rating should be at least 1.25x greater than the maximum input voltage, while a voltage rating of 1.5x is a
conservative guideline. For most cases, the RMS current rating requirement for the input capacitor of a buck regulator is
approximately one-half the DC load current.
The maximum RMS current through the input capacitors can be closely approximated using Equation 10:
V^-V----_P^-O--_V^-U--_I^--T_N^--

 ^{I O}
U
TMA
2
X


1

–
V^--V--_P^--O-_V^--U--_I^-T_N^--_{}
+
1^--1--2^--




-^V---_L-^I-^N---_--–---_f--^V_O----_S^O---_C-^U---^T-

V^-V----_P^-O--_V^-U--_I^--T_N^--_{}
2_


(EQ. 10)
For a through-hole design, several electrolytic capacitors may be needed. For surface mount designs, solid tantalum capacitors can
be used, but caution must be exercised with regard to the capacitor surge current rating. These capacitors must be capable of
handling the surge current at power-up. Some capacitor series available from reputable manufacturers are surge current tested.
Feedback Compensation
Figure 36 highlights the voltage-mode control loop for a synchronous-rectified buck converter. The output voltage (V_OUT) is
regulated to the reference voltage level. The error amplifier output (V_E/A) is compared with the oscillator (OSC) triangular wave to
provide a pulse-width modulated (PWM) wave with an amplitude of V_PVIN at the SW node. The PWM wave is smoothed by the
output filter (L_O and C_O).
The modulator transfer function is the small-signal transfer function of V_OUT/V_E/A. This function is dominated by a DC gain and
the output filter (L_O and C_O), with a double pole break frequency at F_LC and a zero at F_ESR. The DC gain of the modulator is simply
the input voltage (V_PVIN) divided by the
peak-to-peak oscillator voltage, DV_OSC. The ER2120QI incorporates a feed-forward loop that accounts for changes in the input
voltage. This configuration maintains a constant modulator gain.
ER2120QI 2A Synchronous Buck Regulator with Integrated MOSFETs
09615
March 14, 2014
March 2014 Altera Corporation
Rev A