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ISL6553 Datasheet(PDF) 12 Page - Intersil Corporation

Part No. ISL6553
Description  Microprocessor CORE Voltage Regulator Multi-Phase Buck PWM Controller
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Manufacturer  INTERSIL [Intersil Corporation]
Direct Link  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL6553 Datasheet(HTML) 12 Page - Intersil Corporation

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12
As discussed previously, the voltage drop across each Q2
transistor at the point in time when current is sampled is
rDSON (Q2) x ISAMPLE. The voltage at Q2’s drain, the
PHASE node, is applied through the RISEN resistor to the
ISL6553 ISEN pin. This pin is held at virtual ground, so the
current into ISEN is:
ISENSE = ISAMPLE x rDS(ON) (Q2) / RISEN.
RIsen
= ISAMPLE x rDS(ON) (Q2) / 50µA
Example: From the previous conditions,
where ILT
= 50A,
ISAMPLE
= 25.49A,
rDS(ON) (Q2)
= 4m
Then: RISEN
= 2.04K and
ICURRENT TRIP = 165%
Short circuit ILT
= 82.5A.
Channel Frequency Oscillator
The channel oscillator frequency is set by placing a resistor,
RT, to ground from the FS/DIS pin. Figure 10 is a curve
showing the relationship between frequency, FSW, and
resistor RT. To avoid pickup by the FS/DIS pin, it is important
to place this resistor next to the pin. If this pin is also used to
disable the converter, it is also important to locate the pull-
down device next to this pin.
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. These 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 turnoff
transition of the upper PWM MOSFET. Prior to turnoff, the
upper MOSFET was carrying channel current. During the
turnoff, current stops flowing in the upper MOSFET and is
picked up by the lower MOSFET. 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 ISL6553 controller and a HIP6601 gate
driver. The power components are the most critical because
they switch large amounts of energy. Next are small signal
components that connect to sensitive nodes or supply
critical bypassing current and signal coupling.
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, CIN,
and the power switches. Locate the output inductors and
output capacitors between the MOSFETs and the load.
Locate the gate driver close to the MOSFETs.
The critical small components include the bypass capacitors
for VCC and PVCC on the gate driver ICs. Locate the
bypass capacitor, CBP, for the ISL6553 controller close to
the device. It is especially important to locate the resistors
associated with the input to the amplifiers close to their
respective pins, since they represent the input to feedback
amplifiers. Resistor RT, that sets the oscillator frequency
should also be located next to the associated pin. It is
especially important to place the RSEN resistors at the
respective terminals of the ISL6553.
A multi-layer printed circuit board is recommended. Figure 11
shows the connections of the critical components for one
output channel of the converter. Note that capacitors CIN and
COUT could each represent numerous physical capacitors.
Dedicate one solid layer, usually the middle layer of the PC
board, 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. Keep the metal runs from
the PHASE terminal to output inductor short. 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 driver IC to the
MOSFET gate and source should be sized to carry at least 1A
of current.
50
100
10
20
200
500 1,000
5,000 10,000
2,000
1
2
5
10
20
50
100
200
500
1,000
CHANNEL OSCILLATOR FREQUENCY, FSW (kHz)
FIGURE 10. RESISTANCE RT vs FREQUENCY
ISL6553


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