ISL6545 Datasheet, PDF (10/16 Pages) Intersil Corporation – 5V or 12V Single Synchronous Buck Pulse-Width Modulation (PWM) Controller

ISL6545, ISL6545A
help refresh the boot. But if OCP is disabled (no current
sense resistor), the regular boot refresh circuit will still be
active.
Current Sinking
The ISL6545 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 ISL6545 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 V_CC
rail, which supplies the bias voltage to the ISL6545. If there
is nowhere for this current to go, such as to other distributed
loads on the V_CC rail, through a voltage limiting protection
device, or other methods, the capacitance on the V_CC bus
will absorb the current. This situation will allow voltage level
of the V_CC rail to increase. If the voltage level of the rail is
boosted to a level that exceeds the maximum voltage rating
of the ISL6545, 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.
V_IN
ISL6545
UGATE
Q₁
PHASE
Q₂
LGATE/OCSET
L_O
V_OUT
C_IN
C_O
RETURN
FIGURE 7. PRINTED CIRCUIT BOARD POWER AND
GROUND PLANES OR ISLANDS
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 C_IN and C_O may each represent numerous physical
capacitors. For best results, locate the ISL6545 within 1 inch of
the MOSFETs, Q₁ and Q₂. The circuit traces for the MOSFET
gate and source connections from the ISL6545 must be sized
to handle up to 1A peak current.
BOOT
C_BOOT
ISL6545
LGATE/OCSET
PHASE
VCC _+VCC
GND
C_VCC
+V_IN
Q₁ L_O
Q₂
C_O
V_OUT
FIGURE 8. PRINTED CIRCUIT BOARD SMALL SIGNAL
LAYOUT GUIDELINES
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,
R_OSCET close to the COMP/SD pin because the internal
current source is only 20µA. Provide local V_CC decoupling
between V_CC and GND pins. Locate the capacitor, C_BOOT
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
ISL6545 circuit. The output voltage (V_OUT) is regulated to the
reference voltage, V_REF. 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 V_IN 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.
10
FN6305.5
April 29, 2010