25
FN6849.3
December 16, 2011
Next, calculate the switching time using:
gdr
g
sw
I
Q
t =
(20)
where Q
g
is the gate charge of the selected QH and
I
gdr
is the peak gate drive current available from the
ZL2005P.
Although the ZL2005P has a typica
l gate drive current
of 3 A, use the minimum guaranteed current of 2 A for
a conservative design. Using the calculated switching
time, calculate the switching power loss in QH using
(21)
swOUTswINMswtop
fItVP ×××=
The total power dissipated by QH is given by the fol-
lowing equation:
swtopQHQHtot
PPP +=
(22)
MOSFET Thermal Check
Once the power dissipations for QH and QL have been
calculated, the M
OSFET’s junction temperature can
be estimated. Using the junction-to-case thermal resis-
tance (R
th
) given in the MOSFET manufacturer’s data-
sheet and the expected maximum printed circuit board
tempera
ture, calculate the junction temperature as fol-
lows:
(23)
thQpcbj RPTT ×+=max
Current Sensing Components
Once the current sense method has
been selected
(Refer to Section 5.9, “Current Limit Threshold Selec-
tion,” ), the procedure to select the
component is the
following:
When using the inductor DCR sensing method, the
us
er must also select an R/C network comprised of R1
and CL (see Figure 17).
Figure 17. DCR Current Sensing
These components should be sele
cted according to the
following equation:
τ
RC
= L / DCR-------------------------- (24)
R1 should be in the range of 500 Ω
to 5 kΩ in order to
minimize the power dissipation through it. The user
should make sure the resistor package size is appropri-
ate for the power dissipated. Once
R1 has been calcu-
lated, the value of R2 should be
selected based on the
following equation:
R2 = 5 x R1 ------
-----------------------(25)
If R
DS(ON)
is being used the external low side MOS-
FET will act as the sensing element as indicated in
Figure 18.
5.9 Current Limit Threshold Selection
It is recommended that the user include a current limit-
ing mechanism in their design to protect the
power
supply from damage and prevent excessive current
from being drawn from the input supply in the event
that the output is shorted to ground or an overload con-
dition is imposed on the output. Current limiting is
acco
mplished by sensing the current flowing through
the circuit during a portion of the duty cycle.
Output current sensing can be accomplished by mea-
suring the voltage across a serie
s resistive sensing ele-
ment according to equation 26.
V
LIM
= I
LIM
x R
SENSE
---------- -------(26)
Where:
I
LIM
is the desired maximum current that should
flow in the circuit
R
SENSE
is the resistance of the sensing element
V
LIM
is the voltage across the sensing element at
the point the circuit should start limiting the out-
put current.
GH
GL
ISENA
ZL
ISENB
SW
V
IN
CL
V
OUT
R1
R2
ZL2005P
