isl8033 Intersil Corporation, isl8033 Datasheet - Page 15

• Manufacturer Part Number: isl8033
• Description: Dual 3a Low Quiescent Current High Efficiency Synchronous Buck Regulator
• Manufacturer: Intersil Corporation

Synchronization Control (ISL8033)
The frequency of operation can be synchronized up to 6MHz
by an external signal applied to the SYNC pin. The 1st
falling edge on the SYNC triggered the rising edge of the
PWM ON pulse of Channel 1. The 2nd falling edge of the
SYNC triggers the rising edge of the PWM ON pulse of the
Channel 2. This process alternates indefinitely allowing
180° output phase operation between the two channels.
Overcurrent Protection
CAS1 and CSA2 are used to monitor Output 1 and
Output 2 channels respectively. The overcurrent
protection is realized by monitoring the CSA output with
the OCP threshold logic, as shown in Figure 39. The
current sensing circuit has a gain of 0.20V/A, from the
P-MOSFET current to the CSA output. When the CSA1
output reaches the threshold set by ISET, 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
MOSFETs.
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 and the Overcurrent
Condition Flag is set from LOW to HIGH. 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 shutdown under an Overcurrent Fault Condition.
An Overcurrent Fault Condition will result in the regulator
attempting to restart in a hiccup mode with the delay
between restarts being 4 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 prior to the OC Fault
Counter reaching a count of four, the Overcurrent
Condition Flag will set back to LOW.
If the negative output current reaches -2.5A, the part
enters Negative Overcurrent Protection. At this point, all
switching stops and the part enters tri-state mode while
the pull-down FET is discharging the output until it
reaches normal regulation voltage, then the IC restarts.
PG
There are two independent power-good signals. PG1
monitors the Output Channel 1 and PG2 monitors the
Output Channel 2. When powering up, the open-collector
Power-On Reset output holds low for about 1ms after V
reaches the preset voltage. The PG_ output also serves
as a 1ms delayed Power-Good signal.
UVLO
When the input voltage is below the undervoltage lock
out (UVLO) threshold, the regulator is disabled.
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ISL8033, ISL8033A
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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 will begin.
Soft Start-Up
The soft start-up eliminates the inrush current during the
start-up. The soft-start block outputs a ramp reference to
both the voltage loop and the current loop. The two
ramps limit the inductor current rising 1/2 speed as well
as the output voltage speed so that the output voltage
rises in a controlled fashion. At the very beginning of the
start-up, the output voltage is less than 0.5V; hence the
PWM operating frequency is 1/2 of the normal frequency.
When the IC ramps up at start-up, it can’t sink current
even at PWM mode.
Discharge Mode (Soft-Stop)
When a transition to shutdown mode occurs, or the
output undervoltage fault latch is set, its output
discharges to PGND through an internal 100Ω switch.
Power MOSFETs
The power MOSFETs are optimize for best efficiency. The
ON-resistance for the P-MOSFET is typically 50mΩ and
the ON-resistance for the N-MOSFET is typical 50mΩ.
100% Duty Cycle
The ISL8033 features 100% duty cycle operation to
maximize the battery life. When the battery voltage
drops to a level that the ISL8033, ISL8033A can no
longer maintain the regulation at the output, the
regulator completely turns on the P-MOSFET. The
maximum drop-out voltage under the 100% duty-cycle
operation is the product of the load current and the
ON-resistance of the P-MOSFET.
Thermal Shutdown
The ISL8033, ISL8033A has built-in thermal protection.
When the internal temperature reaches +150°C, the
regulator is completely shutdown. As the temperature
drops to +125°C, the ISL8033, ISL8033A resumes
operation by stepping through a soft start-up.
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operation,
ISL8033 typically uses a 1.5µH output inductor. Higher or
lower inductor value can be used to optimize the total
converter system performance. For example, for a higher
output voltage 3.3V application, in order to decrease the
inductor current ripple and output voltage ripple, the
output inductor value can be increased. The inductor
ripple current can be expressed in Equation 1:
ΔI
=
V
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1
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V
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October 21, 2010
(EQ. 1)
FN6854.1