ISL78268 Datasheet, PDF (23/33 Pages) Intersil Corporation – Integrated 2A sourcing

ISL78268
TO BOOT
REFRESH
CONTROL
-
+
3.3V
PVCC
PWM
SIGNAL
DEAD TIME
CONTROL
LEVEL
SHIFT
PVCC
BOOT
DBT
VIN
UG
PH
LG
RC
CBOOT
L
RSENSE
VOUT
TO CURRENT
SENSE AMP
PGND
FIGURE 43. OUTPUT BOOT CONTROL
MINIMUM OFF-TIME CONSIDERATION
To ensure the charging of the boot capacitor, the device has
internally fixed minimum off time (t_minoff) for the high-side
MOSFET. Just after the high-side MOSFET turns off, the PH node
goes down to GND level and boot capacitor will be charged from
PVCC via an external diode (Schottky diode is recommended).
However, when an NMOS with large Q_g is selected to support
heavy load application, the internally fixed t_minoff may not be
enough to charge the boot capacitor sufficiently. For this case, it is
recommended to adjust the switching frequency or input voltage
as the system has sufficient off time of high-side transistor.
PWM Operation
The switching cycle is defined as the time between UG pulse
initiation signals. The cycle time of the pulse initiation signal is
the inversion of the switching frequency set by the resistor
between the FSYNC pin and ground.
The ISL78268 uses peak current mode control. The PWM
operation is initialized by the clock from the oscillator. The
high-side MOSFET is turned on (UG) by the clock at the beginning
of a PWM cycle and the inductor current flows in the high-side
MOSFET and ramps up. When the sum of the current sense
signal (through I_SEN1 current sense amplifier) and the slope
compensation signal reaches the error amplifier output voltage,
the PWM comparator is triggered and UG is turned off to shut
down the high-side MOSFET. The high-side MOSFET stays off until
the next clock signal comes for the next cycle.
After the high-side MOSFET is turned off, the low-side MOSFET
turns on with the fixed dead-time. The off timing of low-side
MOSFET is determined by either the next high-side on timing at
next PWM cycle or when the inductor current become zero if the
Diode Emulation mode is selected.
To prevent undesired shoot-through current at external high-side
and low-side MOSFETs, the device has adaptive dead-time
control and internally fixed dead-time. The internally fixed
dead-time is typically 55ns, for both high-side to low side and
low-side to high-side switching transition.
The output voltage is sensed by a resistor divider from V_OUT to the
FB pin. The difference between the FB voltage and 1.6V (typ)
reference is amplified and compensated to generate the error
voltage signal at the COMP pin that is used for PWM generation
circuits.
Current Sensing
The ISL78268 has two current sense amplifiers: one for high-side
MOSFET peak current sensing for PWM control and overcurrent
protections, and the other for output inductor current sensing for
average current control and diode emulation timing control.
CURRENT SENSE AMPLIFIER 1 (CSA1)
The current-sense amplifier (CSA1) is used to sense the inductor
current in the current-sense resistor placed in series with the
high-side MOSFET. The sensed current information (I_SEN1) is
used for peak current mode control and overcurrent protection.
Peak current mode control is implemented using CSA1 in the
PWM control loop as described in “PWM Operation”.
The cycle-by-cycle peak current limit (OC1) is implemented by
comparing I_SEN1 with an 70µA threshold. At the peak current
limit comparator threshold, the PWM pulse is terminated.
During an overload condition when I_SEN1 reaches 93µA (OC2
threshold), the IC enters into latch-off or hiccup mode, which is
defined by the HIC/LATCH pin configuration. If latch-off mode is
selected, the device stops switching when OC2 is tripped and will
not restart until the EN or VIN is toggled. If Hiccup mode is
selected, the PWM is disabled for 500ms (typ) before beginning
a soft-start cycle. Three consecutive OC2 faults are required to
enter hiccup or latch-off. OC2 hiccup or latch-off is enabled
during soft-start and normal operating modes.
CURRENT SENSE AMPLIFIER 2 (CSA2)
The current-sense amplifier (CSA2) is used to sense the
continuous (not pulsing as in R_SEN1) inductor current either by
DCR sensing method or using a sense resistor in series with the
inductor for more accurate sensing. The sensed current signal is
used for three functions:
• Average constant current control
• Diode emulation
• Average OC protection
The ISEN2P voltage is also used to monitor the minimum output
voltage. Under the overload condition (OC1) or under the average
constant current control, if the voltage become lower than about
1.2V (typ), the device stops switching and enters Latch-off/
Hiccup mode.
If these three functions are not required in the application, CSA2
should be connected to VCC (or VIN).
SENSE RESISTOR CURRENT SENSING
A sense resistor can be placed in series with the inductor. As
shown in Figure 44, the ISL78268 senses the voltage across the
sense resistor. CSA1 is used to sense the high-side MOSFET’s
current. The sense resistor is placed between the input
capacitors and the high-side MOSFET.
CSA2 is used to sense the inductor current. A sense resistor is
placed between the inductor and the output capacitors.
The voltage on the ISEN(n)P and ISEN(n)N of the current sense
amplifier are forced to be equal. The voltage across R_SET(n) is
equivalent to the voltage drop across the R_SEN(n) resistor. The
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FN8657.3
December 12, 2014