ISL6540A Intersil Corporation, ISL6540A Datasheet - Page 12

• Manufacturer Part Number: ISL6540A
• Description: Single-Phase Buck PWM Controller
• Manufacturer: Intersil Corporation

An OV event is detected asynchronously and causes the
high side MOSFET to turn off, the low side MOSFET to turn
on (effectively a 0% duty cycle), and PGOOD to pull low. The
regulator stays in this state and overrides sourcing and
sinking OCP protections until the OV event is cleared.
An UV event is detected asynchronously and results in the
PGOOD pulling low.
Overcurrent Protection
The ISL6540A monitors both the high side MOSFET and low
side MOSFET for overcurrent events. Dual sensing allows the
ISL6540A to detect overcurrent faults at the very low and very
high duty cycles that can result from the ISL6540A’s wide input
range. The OCP function is enabled with the drivers at startup
and detects the peak current during each sensing period. A
resistor and a capacitor between the LSOC pin and GND set
the low side source and sinking current limits. A 100
source develops a voltage across the resistor which is then
compared with the voltage developed across the low side
MOSFET at conduction mode. The measurement comparator
uses offset correcting circuitry to provide precise current
measurements with roughly ±2mV of offset error. An ~120ns
blanking period, implemented on the upper and lower MOSFET
current sensing circuitries, is used to reduce the current
sampling error due to the leading-edge switching noise. An
additional 120ns low pass filter is used to further reduce
measurement error due to noise. In sourcing current
applications, the LSOC voltage is inverted and compared with
the voltage across the MOSFET while on. When this voltage
exceeds the LSOC set voltage, a sourcing OCP fault is
triggered. A 1000pF or greater filter capacitor should be used in
parallel with R
impacting the accuracy of the OCP measurement.
The ISL6540A’s sinking current limit is set to the same
voltage as its sourcing limit. In sinking applications, when the
voltage across the MOSFET is greater than the voltage
developed across the resistor (R
is triggered. To avoid non-synchronous operation at light
load, the peak to peak output inductor ripple current should
not be greater than twice of the sinking current limit.
The high side sourcing current limit is set by connecting the
HSOC pin with a resistor (R
of the high side MOSEFT. A 100
voltage across the resistor which is then compared with the
voltage developed across the high side MOSFET while on.
When the voltage drop across the MOSFET exceeds the
voltage drop across the resistor, a sourcing OCP event
occurs. A 1000pF or greater filter capacitor should be used in
parallel with R
impacting the accuracy of the OCP measurement and to
LSOC
HSOC
to prevent on chip parasitics from
to prevent on chip parasitics from
HSOC
12
μ
LSOC
) and a capacitor to the drain
A current source develops a
) a sinking OCP event
μ
A current
ISL6540A
smooth the voltage across R
switching noise on the input bus.
Sourcing OCP faults cause the regulator to disable (Ugate and
Lgate drives pulled low, PGOOD pulled low, soft-start capacitor
discharged) itself for a fixed period of time after which a normal
soft-start sequence is initiated. The period of time the regulator
waits before attempting a soft-start sequence is set by three
charge and discharge cycles of the soft-start capacitor.
Sinking OCP faults cause the low side MOSFET drive to be
disabled, effectively operating the ISL6540A in a
non-synchronous manner. The fault is maintained for three
clock cycles at which point it is cleared and normal operation
is restored. OVP fault implementation overrides sourcing
and sinking OCP events, immediately turning on the low side
MOSFET and turning off the high side MOSFET. The OC trip
point varies mainly due to the MOSFETs r
and system noise. To avoid overcurrent tripping in the
normal operating load range, find the R
resistor from the previous detailed equations with:
Simple High Side OCP Equation
R
Detailed High Side OCP Equation
R
N
Simple Low Side OCP Equation
R
Detailed Low Side OCP Equations
R
ΔI =
I
N
1. Maximum r
2. Minimum I
3. Determine the overcurrent trip point greater than the
OC_SINK
HSOC
HSOC
U
LSOC
LSOC
L
maximum output continuous current at maximum
inductor ripple current.
=
=
V
------------------------------- -
Number of low side MOSFETs
Number of high side MOSFETs
IN
=
=
=
=
F
- V
S
=
I
-------------------------------------------------------------------------------------- -
⎛
⎝
------------------------------------------------------------------------------------- -
I
---------------------------------------------------------------------------------------- -
⎛
⎝
-------------------------------------------------------------------------------------- -
OC_SOURCE
OC_SOURCE
I
I
L
OC_SOURCE
OUT
OC_SOURCE
I
------------------------------------------------------- -
LSOC
LSOC
DS(ON)
•
r
DS ON
V
--------------- -
•
V
OUT
I
I
N
LSOC
HSOC
and/or I
IN
(
L
100μA
100μA
at the highest junction temperature.
r •
•
r •
+
+
),L
R
DS ON
DS ON
I Δ
---- -
I Δ
---- -
2
2
•
LSOC
•
⎞
⎠
⎞
⎠
N
HSOC
(
(
N
HSOC
r •
r •
L
U
DS ON
DS ON
)LowSide
)HighSide
–
I Δ
---- -
(
2
(
in the presence of
from specification table.
),L
),U
HSOC
DS(ON)
and/or R
March 12, 2007
variations
FN6288.2
LSOC