ER3125QI Datasheet, PDF (23/30 Pages) Altera Corporation – MOSFET for Synchronous Buck or Boost Buck Converter

Page 23
Boost Inductor (2-Stage Boost Buck)
Besides the need to sustain the current ripple to be within a certain range (30% to 50%), the boost inductor current at its
soft-start is a more important perspective to be considered in selection of the boost inductor. Each time the boost starts
up, there is a fixed 500µs soft-start time when the duty cycle increases linearly from t_MIN(ON)*f_SW to ~50%. Before and
after boost start-up, the boost output voltage will jump from V_{PVIN_BOOST} to voltage (V_{PVIN_BOOST} + V_{OUT_BUCK}). The
design target in boost soft-start is to ensure the boost input current is sustained to minimum but capable to charge the
boost output voltage to have a voltage step equaling to V_{OUT_BUCK}. A big inductor will block the inductor current to
increase and not high enough to be able to charge the output capacitor to the final steady state value (V_{PVIN_BOOST} +
V_{OUT_BUCK}) within 500µs. A 6.8µH inductor is a good starting point for its selection in design. The boost inductor
current at start-up must be checked by oscilloscope to ensure it is under acceptable range.
Boost Output Capacitor (2-Stage Boost Buck)
Based on the same theory in boost start-up previously described in the boost inductor selection, a large capacitor at
boost output will cause high in-rush current at boost PWM start-up. 22µF is a good choice for applications with a buck
output voltage less than 10V. Also some minimum amount of capacitance has to be used in boost output to keep the
system stable.
Loop Compensation Design - Buck
The ER3125QI uses constant frequency peak current mode control architecture to achieve fast loop transient response.
An accurate current sensing pilot device in parallel with the upper MOSFET is used for peak current control signal and
overcurrent protection. The inductor is not considered as a state variable since its peak current is constant, and the
system becomes single order system. It is much easier to design the compensator to stabilize the loop compared with
voltage mode control. Peak current mode control has inherent input voltage feed-forward function to achieve good
line regulation. Figure 30 shows the small signal model of a buck regulator.
^iin
V^PVIN
+
^iL _LP
R_LP
ILd^ 1:D ^VPVIN^{d^}
RT
vo^
Rc
Ro
Co
d^
Fm
T_i(S)
(S)
+
He(S)
Tv (S)
v^comp
-Av(S)
FIGURE 30. SMALL SIGNAL MODEL OF BUCK REGULATOR
PWM Comparator Gain F_m
The PWM comparator gain F_m for peak current mode control is given by Equation 20:
F_m
=
-------dˆ--------
vˆ _comp
=
-₍--S----_e---₊----1-S----_n---₎--T----_s
(EQ. 20)
Where, S_e is the slew rate of the slope compensation and S_n is given by Equation 21:
S_n = R_t^V----P---^V----^I-L-^N---_P⁽---^–---V----^o---⁾
(EQ. 21)
where, R_t is the gain of the current amplifier.
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