HIP6028EVAL1 INTERSIL [Intersil Corporation], HIP6028EVAL1 Datasheet - Page 13

• Manufacturer Part Number: HIP6028EVAL1
• Description: Advanced PWM and Dual Linear Power Control with Integrated ACPI Support Interface
• Manufacturer: INTERSIL [Intersil Corporation]

ripple current and the ripple voltage is a function of the ripple
current. The ripple voltage and current are approximated by
the following equations:
Increasing the value of inductance reduces the ripple current
and voltage. However, the large inductance values reduce
the converter’s response time to a load transient.
One of the parameters limiting the converter’s response to a
load transient is the time required to change the inductor
current. Given a sufficiently fast control loop design, the
HIP6028 will provide either 0% or 100% duty cycle in
response to a load transient. The response time is the time
interval required to slew the inductor current from an initial
current value to the post-transient current level. During this
interval the difference between the inductor current and the
transient current level must be supplied by the output
capacitors. Minimizing the response time can minimize the
output capacitance required.
The response time to a transient is different for the
application of load and the removal of load. The following
equations give the approximate response time interval for
application and removal of a transient load:
where: I
response time to the application of load, and t
response time to the removal of load. With a +5V input
source, the worst case response time can be either at the
application or removal of load, and dependent upon the
output voltage setting. Be sure to check both of these
equations at the minimum and maximum output levels for the
worst case response time.
ESR ( )
t
RISE
I
=
V
------------------------------- -
=
IN
F
0.7
0.6
0.5
0.4
0.3
0.2
0.1
TRAN
S
------------------------------- -
V
L
–
10
O
IN
FIGURE 14. C
V
L
OUT
–
O
I
TRAN
V
is the transient load current step, t
OUT
V
--------------- -
V
OUT
IN
OUT2
2-323
CAPACITANCE ( F)
t
FALL
OUTPUT CAPACITOR
V
100
=
OUT
L
------------------------------ -
O
V
=
OUT
I
TRAN
I ESR
FALL
RISE
is the
is the
1000
HIP6028
Input Capacitor Selection
The important parameters for the bulk input capacitor are the
voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and current
ratings above the maximum input voltage and largest RMS
current required by the circuit. The capacitor voltage rating
should be at least 1.25 times greater than the maximum
input voltage and a voltage rating of 1.5 times is a
conservative guideline.
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use ceramic capacitance
for the high frequency decoupling and bulk capacitors to
supply the RMS current. Small ceramic capacitors should be
placed very close to the upper MOSFET to suppress the
voltage induced in the parasitic circuit impedances.
For a through hole design, several electrolytic capacitors
(Panasonic HFQ series or Nichicon PL series or Sanyo MV-
GX or equivalent) may be needed. For surface mount
designs, solid tantalum capacitors can be used, but caution
must be exercised with regard to the capacitor surge current
rating. These capacitors must be capable of handling the
surge-current at power-up. The TPS series available from
AVX, and the 593D series from Sprague are both surge
current tested.
MOSFET Selection/Considerations
The HIP6028 requires 3 power transistors. Two N-channel
MOSFETs are used in the synchronous-rectified buck
topology of the PWM converter. The linear controller drives a
MOSFET or a bipolar NPN as a pass transistor. These
components should be selected based upon r
supply requirements, and thermal management
requirements.
PWM MOSFET Selection and Considerations
In high-current PWM applications, the MOSFET power
dissipation, package selection and heatsink are the
dominant design factors. The power dissipation includes two
loss components; conduction loss and switching loss. These
losses are distributed between the upper and lower
MOSFETs according to duty factor (see the equations
below). The conduction loss is the only component of power
dissipation for the lower MOSFET. Only the upper MOSFET
has switching losses, since the lower device turns on into
near zero voltage.
The equations below assume linear voltage-current
transitions and do not model power loss due to the reverse-
recovery of the lower MOSFET’s body diode. The gate-
charge losses are proportional to the switching frequency
(F
contributing to the MOSFETs’ temperature rise. However,
large gate charge increases the switching interval, t
which increases the upper MOSFET switching losses.
Ensure that both MOSFETs are within their maximum
junction temperature at high ambient temperature by
S
) and are dissipated by the HIP6028, thus not
DS(ON)
SW
, gate