PM6675AS Datasheet, PDF (39/48 Pages) STMicroelectronics – High efficiency step-down controller with embedded 2 A LDO regulator

PM6675AS
Application information
8.1.4
In Table 13 some tested polymer capacitors are listed.
Table 13. Evaluated output capacitors
Manufacturer
Series
Capacitance Rated voltage
(µF)
(V)
SANYO
4TPE220MF
220
4V
4TPE150MI
150
4V
4TPC220M
220
4V
HITACHI
TNCB OE227MTRYF
220
2.5 V
ESR max
@100kHz (mΩ)
15 to 25
18
40
25
MOSFETs selection
In SMPS converters, power management efficiency is a high level requirement, so the
power dissipation on the power switches becomes an important factor in switches selection.
Losses of high-side and low-side MOSFETs depend on their working condition.
Considering the high-side MOSFET, the power dissipation is calculated as:
Equation 40
P_DHighSide = P_conduction + P_switching
Maximum conduction losses are approximately given by:
Equation 41
P_conduction
= R_DSon
⋅
V_OUT
V_IN.min
⋅ I_LOAD,MAX ²
where R_DSon is the MOSFET drain-source on-resistance.
Switching losses are approximately given by:
Equation 42
P_switching
=
V_IN ⋅ (I_LOAD (max) −
2
∆I_L
2
)
⋅
t _on
⋅
f_sw
+
V_IN ⋅ (I_LOAD (max) +
2
∆I_L
2
)
⋅
t _off
⋅ f_sw
where t_ON and t_OFF are the turn-on and turn-off times of the MOSFET and depend on the
gate-driver current capability and the gate charge Q_gate. A greater efficiency is achieved with
low R_DSon. Unfortunately low R_DSon MOSFETs have a great gate charge.
As general rule, the R_DSon ^. Q_gate product should be minimized to find out the suitable
MOSFET.
Logic-level MOSFETs are recommended, as long as low-side and high-side gate drivers are
powered by VVCC = +5 V. The breakdown voltage of the MOSFETs (VBRDSS) must be
greater than the maximum input voltage V_INmax.
Below some tested high-side MOSFETs are listed.
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