ISL6398 Datasheet, PDF (50/57 Pages) Intersil Corporation – Programmable soft-start rate and DVID rate

ISL6398
LOWER MOSFET POWER CALCULATION
The calculation for heat dissipated in the lower MOSFET is
simple, since virtually all of the heat loss in the lower MOSFET is
due to current conducted through the channel resistance
(r_DS(ON)). In Equation 33, I_M is the maximum continuous output
current; I_P-P is the peak-to-peak inductor current (see Equation 1
on page 15); d is the duty cycle (V_OUT/V_IN); and L is the
per-channel inductance.
P_{LOW 1}
=
r_{DSON}



I-N-_{M---}
2
+
-I-^p_1---^--2-^p--²
 1 – d
(EQ. 33)
An additional term can be added to the lower MOSFET loss
equation to account for additional loss accrued during the dead
time when inductor current is flowing through the lower MOSFET
body diode. This term is dependent on the diode forward voltage
at I_M, V_D(ON); the switching frequency, F_SW; and the length of
dead times, t_d1 and t_d2, at the beginning and the end of the
lower MOSFET conduction interval respectively.
P_{LOW 2}
=
V_{DON} F_SW


-IN-^M---
+
I--_{p--2----p- }
t_d1
+



I-N-_M---
–
-I-^p--2-^---^p- _{}
t_d2
(EQ. 34
Finally, the power loss of output capacitance of the lower
MOSFET is approximated in Equation 35:
P_LOW,3  ²³--  V_I1N.5  C_{OSS_LOW}  V_{DS_LOW}  F_SW
(EQ. 35)
where C_{OSS_LOW} is the output capacitance of lower MOSFET at the
test voltage of V_{DS_LOW}. Depending on the amount of ringing, the
actual power dissipation will be slightly higher than this.
Thus the total maximum power dissipated in each lower MOSFET is
approximated by the summation of P_LOW,1, P_LOW,2 and P_LOW,3.
UPPER MOSFET POWER CALCULATION
In addition to r_DS(ON) losses, a large portion of the upper MOSFET
losses are due to currents conducted across the input voltage (V_IN)
during switching. Since a substantially higher portion of the upper
MOSFET losses are dependent on switching frequency, the power
calculation is more complex. Upper MOSFET losses can be divided
into separate components involving the upper-MOSFET switching
times; the lower MOSFET body-diode reverse-recovery charge, Q_rr;
and the upper MOSFET r_DS(ON) conduction loss.
When the upper MOSFET turns off, the lower MOSFET does not
conduct any portion of the inductor current until the voltage at
the phase node falls below ground. Once the lower MOSFET
begins conducting, the current in the upper MOSFET falls to zero
as the current in the lower MOSFET ramps up to assume the full
inductor current. In Equation 36, the required time for this
commutation is t₁ and the approximated associated power loss
is P_UP,1.
P _{U P,1}

V_IN


I-N-_M---
+
-I-^p--2-^---^p- _



-t2-¹--



F_SW
(EQ. 36)
At turn on, the upper MOSFET begins to conduct and this
transition occurs over a time t₂. In Equation 37, the approximate
power loss is P_UP,2.
P _{U P,}
2

V_IN



I-N-_M---
–
-I-^p--2-^---^p- _{}



t-2-_2--



F_SW
(EQ. 37)
A third component involves the lower MOSFET’s reverse-recovery
charge, Q_rr. Since the inductor current has fully commutated to the
upper MOSFET before the lower MOSFET’s body diode can draw all
of Q_rr, it is conducted through the upper MOSFET across V_IN. The
power dissipated as a result is P_UP,3 and is approximated in
Equation 38:
P_UP,3 = V_IN Q_rrF_SW
(EQ. 38)
The resistive part of the upper MOSFET’s is given in Equation 33
as P_UP,4.
P_UP,4  r_{DSON}



-IN-^M---_{}
2
+
-I-^p_1---^--2-^p--²
d
(EQ. 39)
Equation 40 accounts for some power loss due to the drain-
source parasitic inductance (L_DS, including PCB parasitic
inductance) of the upper MOSFETs, although it is not the exact:
P_UP,5  L_{DS-IN-}^M_--- + ^I--p--2-^---^p-_{} ²
(EQ. 40)
Finally, the power loss of output capacitance of the upper
MOSFET is approximated in Equation 41:
P_UP,6  ²³--  V_I1N.5  C_{OSS_UP}  V_{DS_UP}  F_SW
(EQ. 41)
where C_{OSS_UP} is the output capacitance of lower MOSFET at
test voltage of V_{DS_UP}. Depending on the amount of ringing, the
actual power dissipation will be slightly higher than this.
The total power dissipated by the upper MOSFET at full load can
now be approximated as the summation of the results from
Equations 36 to 41. Since the power equations depend on
MOSFET parameters, choosing the correct MOSFETs can be an
iterative process involving repetitive solutions to the loss
equations for different MOSFETs and different switching
frequencies.
Current Sensing Resistor
The resistors connected to the ISEN+ pins determine the gains in
the load-line regulation loop and the channel-current balance
loop as well as setting the overcurrent trip point. Select values for
these resistors by using Equation 42:
R_ISEN = -1---0---0---R--_---^X_{1---0----–}---₆- ^I--O---N--^C----^P--
(EQ. 42)
where R_ISEN is the sense resistor connected to the ISEN+ pin, N
is the active channel number, R_X is the resistance of the current
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August 13, 2015