ER3110DI Datasheet, PDF (15/23 Pages) Altera Corporation – Synchronous Buck Regulator

Page 15
PWM
DCM
PULSE SKIP
DCM
PWM
CLOCK
I_L
0
8 CYCLES
LOAD CURRENT
V_OUT
V_COMP
V_CSA
DUTY
CYCLE
I_L
V_OUT
FIGURE 43. DCM MODE OPERATION WAVEFORMS
FIGURE 44. PWM OPERATION WAVEFORMS
Light Load Operation
At light loads, converter efficiency may be improved by enabling variable frequency operation (PFM). Connecting the
SYNC pin to GND will allow the controller to choose such operation automatically when the load current is low.
Figure 43 shows the DCM operation. The IC enters the DCM mode of operation when 8 consecutive cycles of inductor
current crossing zero are detected. This corresponds to a load current equal to 1/2 the peak-to-peak inductor ripple
current and set by Equation 2:
I_OUT = ^V----O---2-^U---L^T---F⁽---1_S----_W^–-----D----⁾
(EQ. 2)
where D = duty cycle, F_SW = switching frequency, L = inductor value, I_OUT = output loading current, V_OUT = output
voltage.
While operating in PFM mode, the regulator controls the output voltage with a simple comparator and pulsed FET
current. A comparator signals the point at which FB is equal to the 600mV reference at which time the regulator begins
providing pulses of current until FB is moved above the 600mV reference by 1%. The current pulses are approximately
300mA and are issued at a frequency equal to the converters programmed PWM operating frequency.
Due to the pulsed current nature of PFM mode, the converter can supply limited current to the load. Should load
current rise beyond the limit, V_OUT will begin to decline. A second comparator signals an FB voltage 1% lower than the
600mV reference and forces the converter to return to PWM operation.
Output Voltage Selection
The regulator output voltage is easily programmed using an external resistor divider to scale V_OUT relative to the
internal reference voltage. The scaled voltage is applied to the inverting input of the error amplifier; refer to Figure 45.
May 2014 Altera Corporation
10038
Enpirion Power Datasheet ER3110DI Wide PVIN 1A Synchronous Buck Regulator
May 28, 2014
Rev A