PM6675 Datasheet, PDF (38/47 Pages) STMicroelectronics – Very fast load transient response using constant on-time control loop

Application information
PM6675
where R_DS(on) is the drain-source on-resistance of the control MOSFET.
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_DS(on) x Q_gate product should be minimized to find the suitable
MOSFET.
Logic-level MOSFETs are recommended, as long as low-side and high-side gate drivers are
powered by V_VCC = +5V. The breakdown voltage of the MOSFETs (V_BRDSS) must be
greater than the maximum input voltage V_INmax.
) _{Below some tested high-side MOSFETs are listed.}
ct(s _{Table 12. Evaluated high-side MOSFETs}
rodu ^Manufacturer
Type
R_DS(on)
(mΩ)
P ST
STS12NH3LL
10.5
te ^IR
IRF7811
9
Gate charge
(nC)
12
18
Rated reverse
voltage (V)
30
30
le _{In buck converters the power dissipation of the synchronous MOSFET is mainly due to}
so conduction losses:
) ^{- Ob Equation 43}
P_DLowSide ≅ P_conduction
t(s _{Maximum conduction losses occur at the maximum input voltage:}
Produc ^{Equation 44}
P_conduction
= R_DSon
⋅ _{⎜⎜⎝⎛}1−
V_OUT
V_IN,MAX
⎟⎟⎠⎞ ^{⋅ ILOAD,MAX2}
leteThe synchronous rectifier should have the lowest R_DS(on) as possible. When the high-side
o _{MOSFET turns on, high dV/dt of the phase node can bring up even the low-side gate}
bs ^{through its gate-drain capacitance CRRS, causing a cross-conduction problem. Once again,}
O ^{the choice of the low-side MOSFET is a trade-off between on resistance and gate charge; a}
good selection should minimizes the ratio C_RSS / C_GS where
Equation 45
C_GS = C_ISS − C_RSS
Below some tested low-side MOSFETs are listed.
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