ER3110DI Datasheet, PDF (17/23 Pages) Altera Corporation – Synchronous Buck Regulator

Page 17
Boot Undervoltage Protection
If the Boot capacitor voltage falls below 1.8V, the Boot undervoltage protection circuit will turn on the lower FET for
400ns to recharge the capacitor. This operation may arise during long periods of no switching such as PFM no load
situations. In PWM operation near dropout (V_PVIN near V_OUT), the regulator may hold the upper FET on for multiple
clock cycles. To prevent the boot capacitor from discharging, the lower FET is forced on for approximately 200ns every
10 clock cycles.
Application Guidelines
Simplifying the Design
While the ER3110DI offers user programmed options for most parameters, the easiest implementation with fewest
components involves selecting internal settings for SS, COMP and FSW. Table 1 on page 4 provides component value
selections for a variety of output voltages and will allow the designer to implement solutions with a minimum of
effort.
Operating Frequency
The ER3110DI operates at a default switching frequency of 500kHz if F_SW is tied to V_AVINO. Tie a resistor from F_SW to
GND to program the switching frequency from 300kHz to 2MHz, as shown in Equation 4.
R_FSW[kΩ] = 108.75kΩ∗ (t – 0.2μs ) ⁄ 1μs
(EQ. 4)
Where:
t is the switching period in µs.
300
200
100
0
500
750 1000 1250 1500 1750 2000
FSW (kHz)
FIGURE 46. R_FSW SELECTION vs FSW
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, ΔI. A reasonable starting point is 30% of total load current. The inductor value can
then be calculated using Equation 5:
L= ^{VPVIN - VOUT}
FSW x DI
V_OUT
x
V_PVIN
(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
May 2014 Altera Corporation
10038
Enpirion Power Datasheet ER3110DI Wide PVIN 1A Synchronous Buck Regulator
May 28, 2014
Rev A