ER3125QI Datasheet, PDF (16/30 Pages) Altera Corporation – MOSFET for Synchronous Buck or Boost Buck Converter

Page 16
AVIN Switch-Over
The ER3125QI has an auxiliary LDO integrated as shown in the “Block Diagram” on page 14. It is used to replace the
internal MAIN LDO function after the IC start-up. “Typical Application Schematic II - AVINO Switch-Over to V_OUT”
on page 4 shows its basic application setup with output voltage connected to AVIN. After IC soft-start is done and the
output voltage is built up to steady state, and once the AVIN pin voltage is over the AUX LDO Switch-over Rising
Threshold, the MAIN LDO is shut off and the AUXILIARY LDO is activated to bias AVINO. Since the AVIN pin
voltage is lower than the input voltage V_PVIN, the internal LDO dropout voltage and the consequent power loss is
reduced. This feature brings substantial efficiency improvements in light load range, especially at high input voltage
applications.
When the voltage at AVIN falls below the AUX LDO Switch-over Falling Threshold, the AUXILIARY LDO is shut off
and the MAIN LDO is re-activated to bias AVINO. At the OV/UV fault events, the IC also switches back over from
AUXILIARY LDO to MAIN LDO.
The AVIN switchover function is offered in buck configuration. It is not offered in boost configuration when the AVIN
pin is used to monitor the boost output voltage for OVP.
Input Voltage
With the part switching, the operating ER3125QI input voltage must be under 36V. This recommendation allows for
short voltage ringing spikes (within a couple of ns time range) due to part switching while not exceeding the 44V, as
stated in the Absolute Maximum Ratings.
The lowest IC operating input voltage (PVIN pin) depends on AVINO voltage and the Rising and Falling V_AVINO POR
Threshold in the Electrical Specifications table on page 6. At IC start-up when AVINO is just over rising POR
threshold, there is no switching before the soft-start starts. Therefore, the IC minimum start-up voltage on the PVIN
pin is 3.05V (MAX of Rising V_AVINO POR). When the soft-start is initiated, the regulator is switching and the dropout
voltage across the internal LDO increases due to driving current. Thus, the IC PVIN pin shutdown voltage is related to
driving current and AVINO POR falling threshold. The internal upper side MOSFET has typical 10nC gate drive. For a
typical example of synchronous buck with 4nC lower MOSFET gate drive and 500kHz switching frequency, the
driving current is 7mA total causing 70mV drop across internal LDO under 3V V_PVIN. Then the IC shutdown voltage
on the PVIN pin is 2.87V (2.8V + 0.07V). In practical design, extra room should be taken into account with concern to
voltage spikes at PVIN.
With boost buck configuration, the input voltage range can be expanded further down to 2.5V or lower depending on
the boost stage voltage drop upon maximum duty cycle. Since the boost output voltage is connected to the PVIN pin
as the buck inputs, after the IC starts up, the IC will keep operating and switching as long as the boost output voltage
can keep the AVINO voltage higher than falling threshold. Refer to “2-Stage Boost Buck Converter Operation” on page 17
for more details.
Output Voltage
The output voltage can be programmed down to 0.8V by a resistor divider from V_OUT to FB. For Buck, the maximum
achievable voltage is (V_PVIN*D_MAX - V_DROP), where V_DROP is the voltage drop in the power path including mainly the
MOSFET R_DS(ON) and inductor DCR. The maximum duty cycle D_MAX is decided by (1 - f_SW * t_MIN(OFF)).
Output Current
With the high-side MOSFET integrated, the maximum output current, which the ER3125QI can support is decided by
the package and many operating conditions including input voltage, output voltage, duty cycle, switching frequency
and temperature, etc. From the thermal perspective, the die temperature shouldn’t exceed +125°C with the power loss
dissipated inside of the IC. Figures 10 and 11 show the thermal performance of this part operating at different
conditions.
Figures 10 and 11 show 2A and 2.5A buck applications under +25°C still air conditions over V_PVIN range. The
temperature rise data in these figures can be used to estimate the die temperature at different ambient temperatures
under various operating conditions. Note that more temperature rise is expected at higher ambient temperature due to
more conduction loss caused by R_DS(ON) increase.
Generally, the part can output 2.5A in typical buck application conditions (V_PVIN 8~30V, V_O 5V, 500kHz, still air and
+85°C ambient conditions). For any other operating conditions, refer to the previous mentioned thermal curves to
estimate the maximum output current. The output current should be derated under any conditions causing the die
temperature to exceed +125°C.
Enpirion Power Datasheet ER3125QI 2.5A Regulator with Integrated High-Side MOSFET for
Synchronous Buck or Boost Buck Converter
10040
May 28, 2014
May 2014 Altera Corporation
Rev A