ER3105DI Datasheet, PDF (20/25 Pages) Altera Corporation – 500mA Wide VIN Synchronous Buck Regulator

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300
200
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500 750 1000 1250 1500 1750 2000
FSW (kHz)
FIGURE 59. R_FSW SELECTION vs F_SW
Synchronization Control
The frequency of operation can be synchronized up to 2MHz by an external signal applied to the SYNC pin. The rising edge on the
SYNC triggers the rising edge of SW. To properly sync, the external source must be at least 10% greater than the programmed free
running IC frequency.
Output Inductor Selection
The inductor value determines the converter’s ripple current. Choosing an inductor current requires a somewhat arbitrary choice of
ripple current, I. A reasonable starting point is 30% of total load current. The inductor value can then be calculated using
Equation 5:
L= ^{VIN - VOUT}
FSW x DI
x ^VOUT
V_IN
(EQ. 5)
Increasing the value of inductance reduces the ripple current and thus, the ripple voltage. However, the larger inductance value
may reduce the converter’s response time to a load transient. The inductor current rating should be such that it will not saturate in
overcurrent conditions. For typical ER3105DI applications, inductor values generally lies in the 10µH to 47µH range. In general,
higher V_OUT will mean higher inductance.
Buck Regulator Output Capacitor Selection
An output capacitor is required to filter the inductor current. The current mode control loop allows the use of low ESR ceramic
capacitors and thus supports very small circuit implementations on the PC board. Electrolytic and polymer capacitors may also be
used.
While ceramic capacitors offer excellent overall performance and reliability, the actual in-circuit capacitance must be considered.
Ceramic capacitors are rated using large peak-to-peak voltage swings and with no DC bias. In the DC/DC converter application,
these conditions do not reflect reality. As a result, the actual capacitance may be considerably lower than the advertised value.
Consult the manufacturers data sheet to determine the actual in-application capacitance. Most manufacturers publish capacitance
vs DC bias so that this effect can be easily accommodated. The effects of AC voltage are not frequently published, but an
assumption of ~20% further reduction will generally suffice. The result of these considerations may mean an effective capacitance
50% lower than nominal and this value should be used in all design calculations. Nonetheless, ceramic capacitors are a very good
choice in many applications due to their reliability and extremely low ESR.
The following equations allow calculation of the required capacitance to meet a desired ripple voltage level. Additional
capacitance may be used.
For the ceramic capacitors (low ESR):
V_OUTripple=
-----------------^----^I-----------------
8^ F_{S W}^ C_{O U T}
(EQ. 6)
ER3105DI 500mA Wide VIN Synchronous Buck Regulator
09616
March 14, 2014
March 2014 Altera Corporation
Rev A