zl2005 Intersil Corporation, zl2005 Datasheet - Page 23

• Manufacturer Part Number: zl2005
• Description: Digital-dc? Integrated Power Management And Conversion Ic
• Manufacturer: Intersil Corporation

Next, calculate the switching time using
where Q
I
ZL2005.
Although the ZL2005 has a typical 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
The total power dissipated by QH is given by the fol-
lowing equation:
MOSFET Thermal Check
Once the power dissipations for QH and QL have been
calculated, the MOSFETs junction temperature can be
estimated. Using the junction-to-case thermal resis-
tance (R
datasheet and the expected maximum printed circuit
board temperature, calculate the junction temperature
as follows:
For further details of thermal analysis and design see
Application Note AN10.
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
user must also select an R/C network comprised of R1
and CL (see Figure 13).
gdr
t
sw
P
is the peak gate drive current available from the
=
P
T
swtop
j
QHtot
I
Q
max
gdr
g
g
th
=
=
is the gate charge of the selected QH and
) given in the MOSFET manufacturer’s
V
T
INM
=
pcb
+
P
×
QH
P
t
sw
Q
×
×
+
I
R
OUT
th
23
P
swtop
×
f
sw
(23)
(21)
(20)
(22)
ZL2005
These components should be selected according to the
following equation:
τ
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
FET will act as the sensing element as indicated in
Figure 14.
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
accomplished 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 series resistive sensing ele-
ment according to equation 26.
V
Where:
RC
LIM
DS(ON)
= L /
I
flow in the circuit
R
V
the point the circuit should start limiting the out-
put current.
= I
LIM
SENSE
LIM
Figure 13. DCR Current Sensing
ZL2005
LIM
DCR--------------------------
is the desired maximum current that should
is being used the external low side MOS-
is the voltage across the sensing element at
x R
is the resistance of the sensing element
SENSE
ISENA
ISENB
GH
SW
GL
----------
V
IN
R2
-------(26)
R1
(24)
CL
February 18, 2009
V
OUT
FN6848.0