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參數資料
| 型號： | ISL6561CR |
| 廠商： | INTERSIL CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Multi-Phase PWM Controller with Precision Rds(on) or DCR Differential Current Sensing for VR10.X Application |
| 中文描述： | SWITCHING CONTROLLER, 1500 kHz SWITCHING FREQ-MAX, PQCC40 |
| 封裝： | 6 X 6 MM, PLASTIC, MO-220VJJD-2, MLFP, QFN-40 |
| 文件頁數： | 20/26頁 |
| 文件大小： | 782K |
| 代理商： | ISL6561CR |
20
FN9098.5
May 12, 2005
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 15, I
M
is the maximum
continuous output current; I
PP
is the peak-to-peak inductor
current (see Equation 1); d is the duty cycle (V
OUT
/V
IN
); and
L is the per-channel inductance.
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
S
; and the length of dead times, t
d1
and t
d2
, at the
beginning and the end of the lower-MOSFET conduction
interval respectively.
Thus the total maximum power dissipated in each lower
MOSFET is approximated by the summation of P
LOW,1
and
P
LOW,2
.
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 17,
the required time for this commutation is t
1
and the
approximated associated power loss is P
UP,1
.
At turn on, the upper MOSFET begins to conduct and this
transition occurs over a time t
2
. In Equation 18, the
approximate power loss is P
UP,2
.
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 VIN. The power
dissipated as a result is P
UP,3
and is approximately
Finally, the resistive part of the upper MOSFET’s is given in
Equation 19 as P
UP,4
.
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 17, 18, 19 and 20. 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 between these pins and the
respective phase nodes 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 based on the room temperature r
DS(ON)
of the
lower MOSFETs, DCR of inductor or additional resistor; the
full-load operating current, I
FL
; and the number of phases, N
using Equation 21.
In certain circumstances, it may be necessary to adjust the
value of one or more ISEN resistor. When the components of
one or more channels are inhibited from effectively dissipating
their heat so that the affected channels run hotter than
desired, chose new, smaller values of R
ISEN
for the affected
phases (see the section entitled
Channel-Current Balance
).
Choose R
ISEN,2
in proportion to the desired decrease in
temperature rise in order to cause proportionally less current
to flow in the hotter phase.
P
LOW 1
,
r
DS ON
)
I
M
N
-----
2
1
d
–
(
)
I
,
-------------12
2
1
d
–
(
)
+
=
(EQ. 15)
P
LOW 2
V
D ON
)
f
S
I
M
N
-----
I
2
--------
+
t
d1
I
M
N
-----
I
---2
–
t
d2
+
=
(EQ. 16)
P
UP 1
V
IN
I
M
N
-----
I
---2
+
t
1
2
----
f
S
≈
(EQ. 17)
P
UP 2
,
V
IN
I
M
N
-----
I
---2
–
t
2
2
----
f
S
≈
(EQ. 18)
P
UP 3
V
IN
Q
rr
f
S
=
(EQ. 19)
P
UP 4
r
DS ON
)
I
M
N
-----
2
d
I
2
--12
+
≈
(EQ. 20)
R
ISEN
R
×
70
10
6
–
-----------------------
I
N
-------
=
(EQ. 21)
R
ISEN 2
R
ISEN
T
2
T
1
----------
=
(EQ. 22)
ISL6561
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