HIP6018BCB-T Datasheet, PDF (10/15 Pages) Intersil Corporation – Advanced PWM and Dual Linear Power Control

HIP6018B
Application Guidelines
Soft-Start Interval
Initially, the soft-start function clamps the error amplifier’s output
of the PWM converter. After the output voltage increases to
approximately 80% of the set value, the reference input of the
error amplifier is clamped to a voltage proportional to the SS pin
voltage. Both linear outputs follow a similar start-up sequence.
The resulting output voltage sequence is shown in Figure 6.
The soft-start function controls the output voltage rate of rise
to limit the current surge at start-up. The soft-start interval is
programmed by the soft-start capacitor, C_SS. Programming
a faster soft-start interval increases the peak surge current.
The peak surge current occurs during the initial output
voltage rise to 80% of the set value.
Shutdown
The PWM output does not switch until the soft-start voltage
(V_SS) exceeds the oscillator’s valley voltage. Additionally,
the reference on each linear’s amplifier is clamped to the
soft-start voltage. Holding the SS pin low with an open drain
or collector signal turns off all three regulators.
The VID codes resulting in an INHIBIT as shown in Table 1
also shuts down the IC.
Layout Considerations
MOSFETs switch very fast and efficiently. The speed with which
the current transitions from one device to another causes voltage
spikes across the interconnecting impedances and parasitic
circuit elements. The voltage spikes can degrade efficiency,
radiate noise into the circuit, and lead to device over-voltage
stress. Careful component layout and printed circuit design
minimizes the voltage spikes in the converter. Consider, as an
example, the turn-off transition of the upper PWM MOSFET.
Prior to turn-off, the upper MOSFET was carrying the full load
current. During the turn-off, current stops flowing in the upper
MOSFET and is picked up by the lower MOSFET (and/or parallel
Schottky diode). Any inductance in the switched current path
generates a large voltage spike during the switching interval.
Careful component selection, tight layout of the critical
components, and short, wide circuit traces minimize the
magnitude of voltage spikes. Contact Intersil for evaluation board
drawings of the component placement and printed circuit board.
There are two sets of critical components in a DC-DC converter
using a HIP6018B controller. The power components are the
most critical because they switch large amounts of energy. The
critical small signal components connect to sensitive nodes or
supply critical by-passing current.
The power components should be placed first. Locate the
input capacitors close to the power switches. Minimize the
length of the connections between the input capacitors and
the power switches. Locate the output inductor and output
capacitors between the MOSFETs and the load. Locate the
PWM controller close to the MOSFETs.
The critical small signal components include the by-pass
capacitor for VCC and the soft-start capacitor, C_SS. Locate
these components close to their connecting pins on the
control IC. Minimize any leakage current paths from SS
node because the internal current source is only 11μA.
+5V_IN
+3.3V_IN
C_IN
Q3
V_OUT3
V_OUT2
+12V
C_VCC
C_OCSET1
VCC GND
VIN2 _OCSET1
R_OCSET1
Q1
GATE3UGATE1
PHASE1
Q2
HIP6018B
VOUT2 _LGATE1
L_{OUT1 VOUT1}
C_OUT1
CR1
SS PGND
C_SS
C_OUT2
KEY
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT PLANE LAYER
VIA CONNECTION TO GROUND PLANE
FIGURE 10. PRINTED CIRCUIT BOARD POWER PLANES AND
ISLANDS
A multi-layer printed circuit board is recommended. Figure 10
shows the connections of the critical components in the
converter. Note that capacitors C_IN and C_OUT could each
represent numerous physical capacitors. Dedicate one solid
layer for a ground plane and make all critical component
ground connections with vias to this layer. Dedicate another
solid layer as a power plane and break this plane into
smaller islands of common voltage levels. The power plane
should support the input power and output power nodes.
Use copper filled polygons on the top and bottom circuit
layers for the phase nodes. Use the remaining printed circuit
layers for small signal wiring. The wiring traces from the
control IC to the MOSFET gate and source should be sized
to carry 1A currents. The traces for OUT2 need only be
sized for 0.2A. Locate C_OUT2 close to the HIP6018B IC.
PWM Controller Feedback Compensation
Both PWM controllers use voltage-mode control for output
regulation. This section highlights the design consideration
for a voltage-mode controller. Apply the methods and
considerations to both PWM controllers.
Figure 11 highlights the voltage-mode control loop for a
synchronous-rectified buck converter. The output voltage is
regulated to the reference voltage level. The reference
voltage level is the DAC output voltage for the PWM
controller. The error amplifier output (V_E/A) is compared with
the oscillator (OSC) triangular wave to provide a pulse-width
modulated wave with an amplitude of V_IN at the PHASE node.
The PWM wave is smoothed by the output filter (L_O and C_O).
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