HIP6005 INTERSIL [Intersil Corporation], HIP6005 Datasheet - Page 9

• Manufacturer Part Number: HIP6005
• Description: Buck Pulse-Width Modulator (PWM) Controller and Output Voltage Monitor
• Manufacturer: INTERSIL [Intersil Corporation]

careful not to add inductance in the circuit board wiring that
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load on
specific decoupling requirements. For example, Intel
recommends that the high frequency decoupling for the
Pentium Pro be composed of at least forty (40) 1 F ceramic
capacitors in the 1206 surface-mount package.
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors. The
bulk capacitor’s ESR will determine the output ripple voltage
and the initial voltage drop after a high slew-rate transient.
An aluminum electrolytic capacitor's ESR value is related to
the case size with lower ESR available in larger case sizes.
However, the equivalent series inductance (ESL) of these
capacitors increases with case size and can reduce the
usefulness of the capacitor to high slew-rate transient
loading. Unfortunately, ESL is not a specified parameter.
Work with your capacitor supplier and measure the
capacitor’s impedance with frequency to select a suitable
component. In most cases, multiple electrolytic capacitors of
small case size perform better than a single large case
capacitor.
Output Inductor Selection
The output inductor is selected to meet the output voltage
ripple requirements and minimize the converter’s response
time to the load transient. The inductor value determines the
converter’s 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
HIP6005 will provide either 0% or 100% duty cycle in
response to a load transient. The response time is the time
required to slew the inductor current from an initial current
value to the transient current level. During this interval the
difference between the inductor current and the transient
current level must be supplied by the output capacitor.
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:
t
RISE
I
=
V
------------------------------- -
=
IN
F
------------------------------- -
V
–
S
L I
IN
V
OUT
L
–
TRAN
V
OUT
V
--------------- -
V
OUT
IN
t
FALL
9
V
=
OUT
L I
------------------------- -
V
OUT
TRAN
=
I ESR
HIP6005
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
DACOUT setting. Be sure to check both of these equations
at the minimum and maximum output levels for the worst
case response time. With a +12V input, and output voltage
level equal to DACOUT, t
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic
capacitors for high frequency decoupling and bulk
capacitors to supply the current needed each time Q1 turns
on. Place the small ceramic capacitors physically close to
the MOSFETs and between the drain of Q1 and the anode
of Schottky diode D2 .
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. The RMS current rating
requirement for the input capacitor of a buck regulator is
approximately 1/2 the DC load current.
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 HIP6005 requires an N-Channel power MOSFET. It
should be selected based upon r
requirements, and thermal management requirements.
In high-current 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.
The conduction losses are the largest component of power
dissipation for the MOSFET. Switching losses also
contribute to the overall MOSFET power loss (see the
equations below). These equations assume linear voltage-
current transitions and are approximations. The gate-
charge losses are dissipated by the HIP6005 and do not
heat the MOSFET. However, large gate-charge increases
the switching interval, t
MOSFET switching losses. Ensure that the MOSFET is
within its maximum junction temperature at high ambient
TRAN
is the transient load current step, t
SW
FALL
, which increases the upper
is the longest response time.
DS(ON)
, gate supply
FALL
RISE
is the
is the