HIP6020A Datasheet, PDF (13/16 Pages) Intersil Corporation – Advanced Dual PWM and Dual Linear Power Controller

HIP6020A
Linear Output Capacitors
The output capacitors for the linear regulators provide
dynamic load current. Thus capacitors C_OUT3 and C_OUT4
should be selected for transient load regulation.
PWM Output Inductor Selection
Each PWM converter requires an output inductor. The
output inductor is selected to meet the output voltage ripple
requirements and sets the converter’s response time to a
load transient. Additionally, PWM2 output inductor has to
meet the minimum value criteria for loop stability as
described in paragraph ‘PWM2 Controller Feedback
Compensation’. 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:
∆I = ^V-----^I--^N-_F----–_S----^V-_×----^O-_L---^U----^T-- × -^V---_V-^O---_I-^U-_N---^T--
∆V_OUT = ∆I × ESR
Increasing the value of inductance reduces the ripple current
and voltage. However, the large inductance values increase
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
HIP6020A 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
capacitor(s). 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
=
-^L---^O------^×-----^I--^T---^R----^A----^N---
V_IN – V_OUT
t_FALL = ^L----^O----_V--^×--_O--^I--^T-_U---^R-_T---^A----^N--
where: I_TRAN is the transient load current step, t_RISE is the
response time to the application of load, and t_FALL is the
response time to the removal of load. Be sure to check both
of these equations at the minimum and maximum output
levels for the worst case response time.
Input Capacitor Selection
The important parameters for the bulk input capacitor are the
voltage rating and the RMS current rating. For reliable
operation, select bulk input capacitors 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. The RMS current rating requirement
for the input capacitors of a buck regulator is approximately
1/2 of the summation of the DC output load current.
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 can 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 HIP6020A requires 5 external transistors. Three
N-Channel MOSFETs are employed by the PWM
converters. The GTL and memory linear controllers can
each drive a MOSFET or a NPN bipolar as a pass transistor.
All these transistors should be selected based upon
r_DS(ON), current gain, saturation voltages, gate supply
requirements, and thermal management considerations.
PWM1 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 the duty factor. The conduction losses are the
main component of power dissipation for the lower MOSFETs.
Only the upper MOSFET has significant switching losses, since
the lower device turns on and off into near zero voltage.
The equations presented 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 dissipated by the HIP6020A and don't
heat the MOSFETs. However, large gate-charge increases
the switching time, t_SW, which increases the upper MOSFET
switching losses. Ensure that both MOSFETs are within their
maximum junction temperature at high ambient temperature
by calculating the temperature rise according to package
thermal resistance specifications. A separate heatsink may
be necessary depending upon MOSFET power, package
type, ambient temperature and air flow.
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