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10/41
XC9509
Series
●
DC/DC Converter (Continued)
<Current Limit>
The current limiter circuit of the XC9509 series monitors the current flowing through the P-channel MOS driver transistor
connected to the Lx pin, and features a combination of the constant-current type current limit mode and the operation
suspension mode..
①
When the driver current is greater than a specific level, the constant-current type current limit function operates to
turn off the pulses from the Lx pin at any given timing.
②
When the driver transistor is turned off, the limiter circuit is then released from the current limit detection state.
③
At the next pulse, the driver transistor is turned on. However, the transistor is immediately turned off in the case of
an over current state.
④
When the over current state is eliminated, the IC resumes its normal operation.
The IC waits for the over current state to end by repeating the steps
①
through
③
. If an over current state continues
for 8msec* and the above three steps are repeatedly performed, the IC performs the function of latching the OFF state
of the driver transistor, and goes into operation suspension mode. Once the IC is in suspension mode, operations can
be resumed by either turning the IC off via the CE pin, or by restoring power to the V
IN
pin. The suspension mode does
not mean a complete shutdown, but a state in which pulse output is suspended; therefore, the internal circuitry remains
in operation. The constant-current type current limit of the XC9509 series can be set at 1.1A for MSOP-10 package
and 0.7A for USP-10 package.
*
<U.V.L.O. Circuit>
When the V
IN
pin voltage becomes 1.4 V or lower, the P-channel output driver transistor is forced OFF to prevent false
pulse output caused by unstable operation of the internal circuitry. When the VI
N
pin voltage becomes 1.8 V or higher,
switching operation takes place. By releasing the U.V.L.O. function, the IC performs the soft start function to initiate
output startup operation. The soft start function operates even when the V
IN
pin voltage falls momentarily below the
U.V.L.O. operating voltage. The U.V.L.O. circuit does not cause a complete shutdown of the IC, but causes pulse
output to be suspended; therefore, the internal circuitry remains in operation.
●
High Speed LDO Voltage Regulator
The voltage regulator block of the XC9509 series consists of a reference voltage source, error amplifier, and current
limiter circuit. The voltage divided by split resistors is compared with the internal reference voltage by the error
amplifier. The P-channel MOSFET, which is connected to the V
R
OUT
pin, is then driven by the subsequent output
signal. The output voltage at the VR
OUT
pin is controlled and stabilized by a system of negative feedback. A stable
output voltage is achievable even if used with low ESR capacitors as a phase compensation circuit is built-in.
<Reference Voltage Source>
The reference voltage source provides the reference voltage to ensure stable output voltage of the regulator.
<Error Amplifier>
The error amplifier compares the reference voltage with the signal from VR
OUT
, and the amplifier controls the output of
the Pch driver transistor.
<Current Limit Circuit>
The voltage regulator block includes a combination of a constant current limiter circuit and a foldback circuit. The
voltage regulator senses output current of the built-in P channel output driver transistor inside. When the load current
reaches the current limit level, the current limiter circuit operates and the output voltage of the voltage regulator block
drops. As a result of this drop in output voltage, the foldback circuit operates, output voltage drops further and the load
current decreases. When the V
ROUT
and GND pin are shorted, the load current of about 30mA flows.
■
OPERATIONAL EXPLANATION (Continued)
*