ER3125QI Datasheet, PDF (19/30 Pages) Altera Corporation – MOSFET for Synchronous Buck or Boost Buck Converter

Page 19
Non-Inverting Single Inductor Buck Boost Converter Operation
In “Typical Application Schematic III - Boost Buck Converter” on page 4, schematic (b) shows non-inverting single
inductor buck boost configuration. The recommended setting is to use resistor divider 1MΩ and 130kΩ (as shown in
TYP Schematic III b) connecting from AVINO to both BOOST and AVIN pins (BOOST and AVIN pin are directly
connected). In this way, the BOOST pin voltage is a fixed voltage 0.52V that is higher than the boost mode detection
threshold 0.2V to set IC in boost mode and lower than the boost switching threshold 800mV to have boost being
constantly switching (during and after soft-start).
As the same in 2-stage boost buck mode, LGATE is switching ON with the same phase of upper FETs switching ON,
meaning both upper and lower side FETs are ON and OFF at the same time with the same duty cycle. When both FETs
ON, input voltage charges inductor current ramping up for duration of DT; when both FETs OFF, inductor current is
free wheeling through the 2 power diodes to output, and output voltage discharge the inductor current ramping down
for (1-D)T (in CCM mode). The steady state DC transfer function is:
V_OUT = _{1-----D–}-----D-- ⋅ V_PVIN
(EQ. 9)
where V_PVIN is the input voltage, V_OUT is the buck boost output voltage, D is duty cycle.
Another useful equation is to calculate the inductor DC current as below:
IL_DC = _1-----–^1-----D-- ⋅ I_OUT
(EQ. 10)
where IL_DC is the inductor DC current and I_OUT is the output DC current.
Equation 10 shows the inductor current is charging output only during (1-D)T, which means inductor current has
larger DC current than output load current. Thus, for this part with high-side FET integrated, the non-inverting buck
boost configuration has less load current capability compared with buck and 2-stage boost buck configurations. Its
load current capability depends mainly on the duty cycle and inductor current.
Inductor ripple current can be calculated below:
IL_RIPPLE = -V---^O----^U---^T---⁽--L-1----^–-----D----⁾--T--
(EQ. 11)
The inductor peak current is,
IL_PEAK = IL_DC + ^12-- ⋅ IL_RIPPLE
(EQ. 12)
In power stage DC calculations, use Equation 9 to calculate D, then use Equation 10 to calculate IL_DC. D and IL_DC are
useful information to estimate the high-side FET’s power losses and check if the part can meet the load current
requirements.
Oscillator and Synchronization
The oscillator has a default frequency of 500kHz with the f_SW pin connected to AVINO, or ground, or floating. The
frequency can be programmed to any frequency between 200kHz and 2.2MHz with a resistor from f_SW pin to GND.
R_FSW[kΩ] = ^{1---4---5---0----0---0---f-–}-_S---_W-1---6-_[---k^â‹---H---f-z-^S--_]-^W-----^[--k---H-----z---^]
(EQ. 13)
May 2014 Altera Corporation
10040
Enpirion Power Datasheet ER3125QI 2.5A Regulator with Integrated High-Side MOSFET for
Synchronous Buck or Boost Buck Converter
May 28, 2014
Rev A