ISL6567 Datasheet, PDF (16/26 Pages) Intersil Corporation – Multipurpose Two-Phase Buck PWM Controller with Integrated MOSFET Drivers

ISL6567
OVERVOLTAGE PROTECTION
Although the normal feedback loop operation naturally
counters overvoltage (OV) events the ISL6567 benefits from
a secondary, fixed threshold overvoltage protection. Should
the output voltage exceed 120% of the reference, the lower
MOSFETs are turned on. Once turned on, the lower
MOSFETs are only turned off when the sensed output
voltage drops below the 110% falling threshold of the OC
comparator. The OVP behavior repeats for as long as the
ISL6567 is biased, should the sensed output voltage rise
back above the designated threshold. The occurrence of an
OVP event does not latch the controller; should the
phenomenon be transitory, the controller resumes normal
operation following such an event.
When operating in external-reference mode, the OVP
monitoring is enabled when the MON pin exceeds its rising
threshold. For as long as the ISL6567 is biased, OVP has
the highest priority, bypassing all other control mechanisms
and acting directly onto the lower MOSFETs, as described.
Disabling the IC via the EN pin does not turn off OVP
protection.
START-UP INTO A PRE-CHARGED OUTPUT
The ISL6567 also has the ability to start up into a
pre-charged output, without causing any unnecessary
disturbance. The FB pin is monitored during soft-start, and
should it be higher than the equivalent internal ramping
reference voltage, the output drives hold both MOSFETs off.
Once the internal ramping reference exceeds the FB pin
potential, the output drives are enabled, allowing the output
to ramp from the pre-charged level to the final level dictated
by the circuit setting.
OUTPUT PRE-CHARGED:
ABOVE INTERNAL REFERENCE
ABOVE EXTERNAL REFERENCE
BELOW REFERENCE
V_OUT (1.0V/DIV)
GND>
OUTPUT INITIALLY
DISCHARGED
SS (1V/DIV)
GND>
EN (5V/DIV)
FIGURE 15. SOFT-START WAVEFORMS INTO A PRE-
CHARGED OUTPUT CAPACITOR BANK
As shown in Figure 15, while operating in internal reference
mode, should the output be pre-charged to a level exceeding
the circuit’s output voltage setting, the output drives are
enabled at the conclusion of the internal reference ramp,
leading to an abrupt correction in the output voltage down to
the set level.
When operating in external reference mode, should the
output voltage be pre-charged above the regulation level
driven by the external reference, the output drives are fully
enabled when the SS pin levels out at the top of its range.
CONTROL OF ISL6567 OPERATION
The internal power-on reset circuit (POR) prevents the
ISL6567 from starting before the bias voltage at VCC and
PVCC reach the rising POR thresholds, as defined in the
“Electrical Specifications” table starting on page 5. The POR
levels are sufficiently high to guarantee that all parts of the
ISL6567 can perform their functions properly once bias is
applied to the part. While bias is below the rising POR
thresholds, the controlled MOSFETs are kept in an off state.
ISL6567
EXTERNAL CIRCUIT
+5V
POR
CIRCUIT
V_REF
(0.61V)
VCC
V_IN
R_S
+
EN ^-
COMP
I_H
(20µA)
VCC
EN
R_P
Q1
OFF
ON
FIGURE 16. START-UP COORDINATION USING THE EN PIN
A secondary disablement feature is available via the
threshold-sensitive enable input, the EN. This optional
feature prevents the ISL6567 from operating until a certain
chosen voltage rail is available and above some selectable
threshold. One example would be the input voltage: it may
be desirable the ISL6567-based converter does not start up
until the input voltage is sufficiently high. The schematic in
Figure 16 demonstrates coordination of the ISL6567 start-up
with such a rail. The internal current source, I_H, provides the
means to a user-adjustable hysteresis. The resistor value
selection process follows the same equations as those
presented in “External Reference Operation” on page 14.
Additionally, an open-drain or open-collector device (Q1) can
be used to wire-AND a second (or multiple) control signal. To
defeat the threshold-sensitive enable, connect EN to VCC
directly or via a pull-up resistor.
In summary, for the ISL6567 to operate, VCC and PVCC
must be greater than their respective POR thresholds and
the voltage at EN must be greater than the comparator’s
reference (see typical threshold in the ”Electrical
16
FN9243.3
May 28, 2009