ISL6521

FN9148 Rev 2.00

Page 9 of 14

Feb 8, 2005

off of the pull-down device, the switching regulator undergoes

a soft-start cycle.

To disable a particular linear controller, pull and hold the

respective FB pin above a typical threshold of 1.25V. One way

to achieve this task is by using a logic gate coupled through a

small-signal diode. The diode should be placed as close to the

FB pin as possible to minimize stray capacitance to this pin.

Upon turn-off of the pull-up device, the respective output

undergoes a soft-start cycle, bringing the output within

regulation limits. On regulators implementing this feature, the

parallel combination of the feedback resistors has to be

sufficiently high to allow ease of driving from the external

device. Considering the other restriction applying to the upper

range of this resistor combination (see ‘Output Voltage

Selection’ paragraph), it is recommended the values of the

feedback resistors on the linear regulator output meet the

following constraint:

Important Note When Using External Pass Devices

If the collector voltage to a linear regulator pass transistor (Q3,

Q4, or Q5 shown in Figure 7) is lost, the respective regulator

has to be shut down by pulling high its FB pin. This measure is

necessary in order to avoid possible damage to the ISL6521 as

a result of overheating. Overheating can occur in such

situations due to sheer power dissipation inside the chip’s

linear drivers.

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 overvoltage 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 or 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.

There are two sets of critical components in a DC-DC

converter using an ISL6521 controller. The switching power

components are the most critical because they switch large

amounts of energy, and as such, they tend to generate equally

large amounts of noise. The critical small signal components

are those connected to sensitive nodes or those supplying

critical bypass current.

The power components and the controller IC should be placed

first. Locate the input capacitors, especially the high-frequency

ceramic decoupling capacitors, close to 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 bypass

capacitor for VCC and the feedback resistors. Locate these

components close to their connecting pins on the control IC.

A multi-layer printed circuit board is recommended. Figure 7

shows the connections of the critical components in the

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, but do not unnecessarily oversize these

particular islands. Since the PHASE nodes are subjected to

very high dv/dt voltages, the stray capacitor formed between

these islands and the surrounding circuitry will tend to couple

switching noise. 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 2A

peak currents.

2k





+

---------------------- 5k



