ISL8014A
FN7525 Rev 2.00 Page 13 of 16
December 16, 2014
Synchronization Control
The frequency of operation can be synchronized up to 4MHz by an
external signal applied to the SYNCH pin. The falling edge on the
SYNCH triggers the rising edge of the LX pulse. Make sure that the
minimum on time of the LX node is greater than 140ns.
Overcurrent Protection
The overcurrent protection is realized by monitoring the CSA output
with the OCP comparator, as shown in the “Block Diagram”. The
current sensing circuit has a gain of 200mV/A, from the P-MOSFET
current to the CSA output. When the CSA output reaches 1.4V,
which is equivalent to 5.7A for the switch current, the OCP
comparator is tripped to turn off the P-MOSFET immediately. The
overcurrent function protects the switching converter from a shorted
output by monitoring the current flowing through the upper MOSFET.
Upon detection of overcurrent condition, the upper MOSFET will
be immediately turned off and will not be turned on again until
the next switching cycle. Upon detection of the initial overcurrent
condition, the overcurrent fault counter is set to 1. If, on the
subsequent cycle, another overcurrent condition is detected, the
OC fault counter will be incremented. If there are 17 sequential
OC fault detections, the regulator will be shut down under an
overcurrent fault condition. An overcurrent fault condition will
result in the regulator attempting to restart in a hiccup mode
within the delay of four soft-start periods. At the end of the fourth
soft-start wait period, the fault counters are reset and soft-start is
attempted again. If the overcurrent condition goes away during
the delay of four soft-start periods, the output will resume back
into regulation point after hiccup mode expires.
Short-Circuit Protection
The short-circuit protection SCP comparator monitors the VFB pin
voltage for output short-circuit protection. When the VFB is lower
than 0.2V, the SCP comparator forces the PWM oscillator
frequency to drop to 1/3 of the normal operation value. This
comparator is effective during start-up or an output short-circuit
event.
PG
During power-up, the open-drain power-good output holds low for
about 1ms after VOUT reaches the regulation voltage. The PG
output also serves as a 1ms delayed the power-good signal when
the pull-up resistor R1 is installed.
UVLO
When the input voltage is below the undervoltage lockout (UVLO)
threshold, the regulator is disabled. To adjust the voltage level of
power on and UVLO, use a resistive divider across EN. The input
voltage programming resistor R4 will depend on the bottom
resistor R5, as referred to in Figure 36. The value of R5 is typically
between 10kΩ and 100kΩ.
Soft Start-Up
The soft start-up reduces the inrush current during the start-up.
The soft-start block outputs a ramp reference to the input of the
error amplifier. This voltage ramp limits the inductor current as
well as the output voltage speed so that the output voltage rises
in a controlled fashion. When VFB is less than 0.2V at the
beginning of the soft-start, the switching frequency is reduced to
1/3 of the nominal value so that the output can start up
smoothly at light load condition. During soft-start, the IC operates
in the SKIP mode to support pre-biased output condition.
Enable
The enable (EN) input allows the user to control the turning on or
off the regulator for purposes such as power-up sequencing.
When the regulator is enabled, there is typically a 600µs delay
for waking up the bandgap reference and then the soft-start-up
begins. It is recommended that the EN voltage should be kept
logic low (less than 400mV), until VIN reaches 2.5V. Refer to
Figure 37 for suggested circuit implementation with VIN slew
rate.
Let T equal the rise time of VIN. Select the ratio of R5 and R4
such that the voltage is 1.4V (minimum enable logic high
threshold) when VIN is equal to or greater than 2.5V. Set R5
between 10kΩ to 100kΩ, and use Equation 1 to determine R4:
Where VIN is greater than or equal to 2.5V.
Then select C such that the equivalent time constant is at least
2x the rise time, T. This will delay the EN voltage enough so that
the overall EN voltage is less than 400mV by the time VIN
reaches 2.5V. Use Equation 2 to get C:
Where T is the rise time of VIN
R4
R5
1V
EN
+
-
VIN
FIGURE 36. EXTERNAL RESISTOR DIVIDER
C
FIGURE 37. CIRCUIT IMPLEMENTATION WITH VIN SLEW RATE
VIN
EN
<400mV
2.5V
V (VOLTS)
Tt (TIME)
R4
R5VIN 1.4V–
1.4V
---------------------------------------------
=(EQ. 1)
C2T
R4R5
--------------------
(EQ. 2)