ER3125QI Datasheet, PDF (22/30 Pages) Altera Corporation – MOSFET for Synchronous Buck or Boost Buck Converter

Page 22
where ΔI is the inductor’s peak to peak ripple current, f_SW is the switching frequency and C_OUT is the output capacitor.
If using electrolytic capacitors then:
V_OUTripple= ΔI*ESR
(EQ. 16)
Regarding transient response needs, a good starting point is to determine the allowable overshoot in V_OUT if the load is
suddenly removed. In this case, energy stored in the inductor will be transferred to C_OUT causing its voltage to rise.
After calculating capacitance required for both ripple and transient needs, choose the larger of the calculated values.
The Equation 17 determines the required output capacitor value in order to achieve a desired overshoot relative to the
regulated voltage.
C_OUT
=
---------------------------------I-^O----^U----^T---2---*----L---------------------------------
V_OUT^2*(V_OUTMAX ⁄ V_OUT)² – 1 )
(EQ. 17)
where V_OUTMAX/V_OUT is the relative maximum overshoot allowed during the removal of the load.
Input Capacitors - Buck
Depending on the system input power rail conditions, the aluminum electrolytic type capacitor is normally needed to
provide the stable input voltage. Thus, restrict the switching frequency pulse current in a small area over the input
traces for better EMC performance. The input capacitor should be able to handle the RMS current from the switching
power devices.
Ceramic capacitors must be used at PVIN pin of the IC and multiple capacitors including 1µF and 0.1µF are
recommended. Place these capacitors as closely as possible to the IC.
Output Inductor - Buck
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% to 40% of total load current. The inductor
value is calculated using Equation 18:
L = ^V----P--f-^V-_S---^I-_W-^N----^–-_×----V-_Δ--^O--I--^U----^T- × _V-V---P-^O--_V--^U--_I-^T-_N-
(EQ. 18)
Increasing the value of inductance reduces the ripple current and thus ripple voltage. However, the larger inductance
value may reduce the converter’s response time to a load transient. The inductor current rating should be as such that
it will not saturate in overcurrent conditions.
Low-Side Power MOSFET
In synchronous buck application, a power N MOSFET is needed as the synchronous low-side MOSFET and a good
one should have low Qgd, low R_DS(ON) and small Rg (Rg_typ < 1.5Ω recommended). The Vgth_min is recommended to
be or higher than 1.2V. A good example is SQS462EN.
A 5.1k or smaller value resistor has to be added to connect LGATE to ground to avoid falsely turn-on of LGATE caused
by coupling noise.
Output Voltage Feedback Resistor Divider
The output voltage can be programmed down to 0.8V by a resistor divider from V_OUT to FB according to Equation 19.
V_OUT
=
0.8 ⋅
⎛
⎜
⎝
1
+
R---R-_L^---UO---P-_W^---_{⎠⎟⎞}
(EQ. 19)
In an application requiring least input quiescent current, large resistors should be used for the divider. Generally, a
resistor value of 10k to 300k can be used for the upper resistor.
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