isl8501 Intersil Corporation, isl8501 Datasheet - Page 13

• Manufacturer Part Number: isl8501
• Description: Triple Output Controller With 1a Standard Buck Pwm And Dual Ldos
• Manufacturer: Intersil Corporation

Detailed Description
The ISL8501 combines a standard buck PWM controller with
an integrated switching MOSFET and two low dropout (LDO)
linear regulators with internal pass devices. The buck
controller drives an internal N-Channel MOSFET and
requires an external diode to deliver load current up to 1A.
A Schottky diode is recommended for improved efficiency
and performance over a standard diode. The standard buck
regulator can operate from either an unregulated DC source,
such as a battery, with a voltage ranging from +6.0V to +25V,
or from a regulated system rail of +5V. When operating from
+6V or greater, the controller is biased from an internal +5V
LDO voltage regulator. The converter output is regulated
down to 0.6V from either input source. Each LDO linear
regulator can source up to 500mA continuous output current
with +2V or greater input supply and +1.0V or higher output
voltage. These features make the ISL8501 ideally suited for
FPGA and wireless chipset power applications.
The PWM control loop uses a single output voltage loop with
input voltage feed forward which simplifies feedback loop
compensation and rejects input voltage variation. External
feedback loop compensation allows flexibility in output filter
component selection. The regulator switches at a fixed
500kHz.
The buck regulator and LDOs are provided with independent
current limits. The current limit in the buck regulator is
achieved by monitoring the drain-to-source voltage drop of
the internal switching power MOSFET. The current limit
threshold is internally set at 2A. The part also features
undervoltage protection by latching the switching MOSFET
driver to the OFF state during an overcurrent, when the
output voltage is lower than 70% of the regulated output.
This helps minimize power dissipation during a short-circuit
condition. Due to only the switching power MOSFET
integration, there is no overvoltage protection feature for this
part.
The ISL8501 monitors and controls the pass transistor’s
gate voltage to limit the output current. The current limits for
both LDOs is 800mA typical. Neither LDO has overvoltage or
undervoltage protection. When the current limit in either
output is reached, the output no longer regulates the voltage,
but regulates the current to the value of the current limit.
+5V Internal Bias Supply (VCC)
Voltage applied to the VIN pin with respect to GND is
regulated to +5V DC by an internal LDO regulator. The
output of the LDO, VCC, is the bias voltage used by all the
internal control and protection circuitry. The VCC pin
requires a ceramic capacitor connected to GND. The
capacitor serves to stabilize the LDO and to decouple load
transients.
The input voltage range for the ISL8501 is specified as +6V
to +25V or +5V ±10%. In the case of an unregulated supply
case, the power supply is connected to VIN only. Once
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ISL8501
enabled, the linear regulator will turn-on and rise to +5V on
VCC. In the +5V supply case, the VCC and VIN pins must be
tied together to bypass the LDO. The external decoupling
capacitor is still required in this mode. Do not short VCC to
GND.
Operation Initialization
The power-on reset circuitry and enable inputs prevent false
startup of the PWM regulator and LDO outputs. Once all the
input criteria are met, the controller soft-starts the output
voltage to the programmed level.
Power-On Reset and Undervoltage Lockout
The PWM portion of the ISL8501 automatically initializes
upon receipt of input power. The power-on reset (POR)
function continually monitors the VCC and PVCC voltages.
While both are below their POR thresholds, the controller
inhibits switching of the internal power MOSFET. Once
exceeded, the controller initializes the internal soft-start
circuitry. If either input supply drops below their falling POR
threshold during soft-start or operation, the buck regulator
latches off.
Independent LDO supply inputs (VIN_LDO1 and VIN_LDO2)
allow flexibility in partitioning linear regulator power. Power
supplies connected to either LDO supply input must exceed
the undervoltage lockout (UVLO) threshold before that LDO
is initialized. If the input supply drops below the falling UVLO
threshold during operation, the low dropout voltage regulator
latches off.
Enable and Disable
All internal power devices are held in a high-impedance
state, which ensures they remain off while in shutdown
mode. Typically the enable input for a specific output is
toggled high after the input supply to that regulator is active
and the internal LDO has exceeded its POR threshold.
The EN pin enables the buck controller portion of the
ISL8501. When the voltage on the EN pin exceeds the POR
rising threshold, the controller initiates the soft-start function
for the PWM regulator. If the voltage on the EN pin drops
below the POR falling threshold, the buck regulator shuts
down.
Individual LDO enable inputs (EN_LDO1 and EN_LDO2)
allow independent control of each regulator. Make sure EN
is on, when the voltage on either pin exceeds the POR rising
threshold, linear regulator operation is initiated for that
controller. If the voltage then drops below the hysteresis
level for the enable pin, the LDO shuts down.
Pulling the EN low simultaneously puts all outputs into
shutdown mode, and supply current drops to 10μA typical.
Soft-Start
Once the input supply latch and enable threshold are met,
the soft-start function is initialized. The soft-start circuitry
begins sourcing 30μA, from an internal current source, which
July 12, 2007
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