ISL6552_04 Datasheet, PDF (14/18 Pages) Intersil Corporation – Microprocessor CORE Voltage Regulator Multi-Phase Buck PWM Controller

ISL6552
Example: From the previous conditions,
where:
Then:
I_LT
I_SAMPLE
r_DS(ON) (Q2)
R_ISEN
I_{CURRENT TRIP}
Short circuit I_LT
= 100A,
= 25.49A,
= 4mΩ
= 2.04K and
= 165%
= 165A.
Channel Frequency Oscillator
The channel oscillator frequency is set by placing a resistor,
R_T, to ground from the FS/DIS pin. Figure 10 is a curve
showing the relationship between frequency, F_SW, and
resistor R_T. To avoid pickup by the FS/DIS pin, it is important
to place this resistor next to the 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 ISL6552 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, C_IN,
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, C_BP, for the ISL6552 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 R_T, that sets the oscillator frequency
should also be located next to the associated pin. It is
especially important to place the R_SEN resistors at the
respective terminals of the ISL6552.
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 C_IN and
C_OUT 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
one ampere of current.
1,000
500
200
100
50
20
10
5
2
1
10 20
50 100 200 500 1,000 2,000 5,000 10,000
CHANNEL OSCILLATOR FREQUENCY, F_SW (kHz)
FIGURE 10. RESISTANCE R_T vs FREQUENCY
Component Selection Guidelines
Output Capacitor Selection
The output capacitor is selected to meet both the dynamic
load requirements and the voltage ripple requirements. The
load transient for the microprocessor CORE is characterized
by high slew rate (di/dt) current demands. In general,
multiple high quality capacitors of different size and dielectric
are paralleled to meet the design constraints.
Modern microprocessors produce severe transient load rates.
High frequency capacitors supply the initially transient current
and slow the load rate-of-change seen by the bulk capacitors.
The bulk filter capacitor values are generally determined by
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