ISL6545, ISL6545A

FN6305 Rev 7.00

Page 10 of 17

October 7, 2015

sense resistor), the regular boot refresh circuit will still be

active.

Current Sinking

The ISL6545x incorporates a MOSFET shoot-through

protection method which allows a converter to sink current as

well as source current. Care should be exercised when

designing a converter with the ISL6545x when it is known that

the converter may sink current.

When the converter is sinking current, it is behaving as a boost

converter that is regulating its input voltage. This means that

the converter is boosting current into the VCC rail, which

supplies the bias voltage to the ISL6545x. If there is nowhere

for this current to go, such as to other distributed loads on the

VCC rail, through a voltage limiting protection device, or other

methods, the capacitance on the VCC bus will absorb the

current. This situation will allow voltage level of the VCC rail to

increase. If the voltage level of the rail is boosted to a level that

exceeds the maximum voltage rating of the ISL6545x, then the

IC will experience an irreversible failure and the converter will

no longer be operational. Ensuring that there is a path for the

current to follow other than the capacitance on the rail will

prevent this failure mode.

Application Guidelines

Layout Considerations

As in any high frequency switching converter, layout is very

important. Switching current from one power device to another

can generate voltage transients across the impedances of the

interconnecting bond wires and circuit traces. These

interconnecting impedances should be minimized by using wide,

short printed circuit traces. The critical components should be

located as close together as possible, using ground plane

construction or single point grounding.

Figure 7 shows the critical power components of the converter. To

minimize the voltage overshoot, the interconnecting wires

indicated by heavy lines should be part of a ground or power

plane in a printed circuit board. The components shown should be

located as close together as possible. Please note that the

capacitors CIN and CO may each represent numerous physical

capacitors. For best results, locate the ISL6545x within 1 inch of

the MOSFETs, Q1 and Q2. The circuit traces for the MOSFET

gate and source connections from the ISL6545x must be sized to

handle up to 1A peak current.

Figure 8 shows the circuit traces that require additional layout

consideration. Use single point and ground plane construction

for the circuits shown. Minimize any leakage current paths on

the COMP/SD pin and locate the resistor, ROSCET close to the

COMP/SD pin because the internal current source is only

20µA. Provide local VCC decoupling between VCC and GND

pins. Locate the capacitor, CBOOT as close as practical to the

BOOT and PHASE pins. All components used for feedback

compensation (not shown) should be located as close to the IC

as practical.

Feedback Compensation

This section highlights the design consideration for a

voltage-mode controller requiring external compensation. To

address a broad range of applications, a type-3 feedback

network is recommended, as shown in the top part of Figure 9.

Figure 9 also highlights the voltage-mode control loop for a

synchronous-rectified buck converter, applicable to the ISL6545x

circuit. The output voltage (VOUT) is regulated to the reference

voltage, VREF. The error amplifier output (COMP pin voltage) is

compared with the oscillator (OSC) modified sawtooth wave to

provide a pulse-width modulated wave with an amplitude of VIN at

the PHASE node. The PWM wave is smoothed by the output filter

(L and C). The output filter capacitor bank’s equivalent series

resistance is represented by the series resistor E.

LO

CO

LGATE/OCSET

UGATE

PHASE

Q1

Q2

VIN

VOUT

RETURN

ISL6545,

CIN

FIGURE 7. PRINTED CIRCUIT BOARD POWER AND

GROUND PLANES OR ISLANDS

ISL6545A

FIGURE 8. PRINTED CIRCUIT BOARD SMALL SIGNAL

LAYOUT GUIDELINES

+VCC

ISL6545,

LGATE/OCSET

GND

VCC

BOOT

LO

CO

VOUT

Q1

Q2

PHASE

+VIN

CBOOT

CVCC

ISL6545A

ISL6545, ISL6545A