ISL6563 Intersil Corporation, ISL6563 Datasheet - Page 14

• Manufacturer Part Number: ISL6563
• Description: Two-Phase Multi-Phase Buck PWM Controller with Integrated MOSFET Drivers
• Manufacturer: Intersil Corporation

lower-MOSFET body diode. This term is dependent on the
diode forward voltage at I
frequency, f
the beginning and the end of the lower-MOSFET conduction
interval, respectively.
The above equation assumes the current through the lower
MOSFET is always positive; if so, the total power dissipated
in each lower MOSFET is approximated by the summation of
P
UPPER MOSFET POWER CALCULATION
In addition to r
MOSFET losses are switching losses, due to currents
conducted through the device while the input voltage is
present as V
separate components, separating the upper-MOSFET
switching losses, the lower-MOSFET body diode reverse
recovery charge loss, and the upper MOSFET r
conduction loss.
In most typical circuits, when the upper MOSFET turns off, it
continues to conduct the inductor current until the voltage at
the phase node falls below ground. Once the lower
MOSFET begins conducting (via its body diode or
enhancement channel), the current in the upper MOSFET
falls to zero. In the following equation, the required time for
this commutation is t
P
Similarly, the upper MOSFET begins conducting as soon as
it begins turning on. Assuming the inductor current is in the
positive domain, the upper MOSFET sees approximately the
input voltage applied across its drain and source terminals,
while it turns on and starts conducting the inductor current.
This transition occurs over a time t
the power loss is P
A third component involves the lower MOSFET’s reverse-
recovery charge, Q
diode conducts the full inductor current before it has fully
switched to the upper MOSFET, the upper MOSFET has to
provide the charge required to turn off the lower MOSFET’s
body diode. This charge is conducted through the upper
MOSFET across VIN, the power dissipated as a result,
P
P
P
P
P
LMOS1
UMOS,1
UMOS,3
LMOS 2
UMOS 1 ,
UMOS 2 ,
UMOS 3 ,
and P
.
=
≈
≈
can be approximated as:
=
V
V
V
S
V
IN
IN
DS
D ON
; and the length of dead times, t
IN
(
DS(ON)






LMOS2
. Upper MOSFET losses can be divided into
I
-------------
Q
I
-------------
OUT
OUT
N
N
rr
)
f
UMOS,2
f
RR
S
S
1
+
–
and the associated power loss is
.



losses, a large portion of the upper-
. Since the lower MOSFET’s body
I
------------ -
I
------------ -
I
------------ -
L PP
L PP
OUT
,
2
,
2
2
M
.
, V
 t


 t


+






14
----
----
2
D(ON)
I
-------- -
2
2
1
PP
2






f
f



S
S
2
t
d1
, and the approximate
; the switching
+



I
------------ -
OUT
2
d1
–
DS(ON)
I
-------- -
and t
PP
2



t
d2
d2
, at
ISL6563
ISL6563
Lastly, the conduction loss part of the upper MOSFET’s
power dissipation, P
following equation:
In this case, of course, r
upper MOSFET.
The total power dissipated by the upper MOSFET at full load
can be approximated as the summation of these results.
Since the power equations depend on MOSFET parameters,
choosing the correct MOSFETs can be an iterative process
that involves repetitively solving the loss equations for
different MOSFETs and different switching frequencies until
converging upon the best solution.
Current Sensing
The resistor connected between the ISEN and VCC pins
determines the gain in the load-line regulation and the
channel-current balance loop. Select the value for this
resistor based on the room temperature r
MOSFETs and the full-load total output current, I
Load Line Regulation Resistor
The load-line regulation resistor is labeled, R1 in Figure 1,
depends on the desired full-load droop voltage. At full load,
the current determined by R
creates the output voltage droop across R1. Thus, the load
line regulation resistor can be computed using the following
equation:
Frequency Compensation
The load-line regulated converter behaves in a similar
manner to a peak-current mode controller because the two
poles at the output filter LC resonant frequency split with the
introduction of current information into the control loop. The
final location of these poles is determined by the system
function, the gain of the current signal, and the value of the
compensation components, R
The solution to the system equations can be fairly
complicated. Fortunately, there is a simple approximation
that comes very close to an optimal solution. Treating the
system as though it were a voltage mode regulator by
compensating the LC poles and the ESR zero of the voltage
P
R
R
UMOS 4 ,
ISEN
1
=
V
------------------------------------------------------ -
=
DROOP
r
---------------------- -
50 10
r
=
DS ON
DS ON
r
×
(
DS ON
(
⋅
(
–
2 R
)
6
)
⋅
⋅
⋅
I
)
I
------- -
FL
FL
2
ISEN
UMOS,4,



I
-------------
OUT
N
DS(ON)



2
d
ISEN
can be calculated using the
2
+
I
--------- -
and C
is the ON resistance of the
PP
12
is fed into the FB pin and
2
2
.
DS(ON)
of the lower
FL
.