XCL210 datasheet(13/24 Pages) TOREX

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XCL210

Series

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OPERATIONAL EXPLANATION (Continued)

By continuously adjusting the interval of the linked operation of (1), (2) and (3) above in response to the load current, the output voltage is stabilized

with high efficiency from light loads to heavy loads.

The PFM switching current monitors the current that flows through the Pch driver Tr, and is a value that limits the Pch driver Tr current.

The Pch driver Tr remains on until the coil current reaches the PFM switching current (IPFM). An approximate value for this on-time tON can be

calculated using the following equation:

tON = L × IPFM / (VIN – VOUT)

To avoid excessive ripple voltage in the event that the coil current does not reach the PFM switching current within a certain interval even though

the Pch driver Tr has turned on and the FB voltage is above VREF, the Pch driver Tr can be turned off at any timing using the maximum on-time

function of the PFM control circuit. If the Pch driver Tr turns off by the maximum on-time function instead of the current sense circuit, the Nch

synchronous rectification switch Tr will not turn on and the coil current will flow to the VOUT pin by means of the parasite diode of the Nch

synchronous rectification switch Tr.

When the VIN voltage is lower than the output voltage, through mode automatically activates and the Pch driver Tr stays on continuously.

(1) In through mode, when the load current is increased and the current that flows through the Pch driver Tr reaches a load current that is several tens

of mA lower than the set PFM switching current (IPFM), the current sense circuit sends a signal through the PFM control circuit to the buffer driver. This

signal turns off the Pch driver Tr and turns on the Nch synchronous rectification switch Tr.

(2) After the on-time (off-time) of the Nch synchronous rectification switch Tr, the Pch driver Tr turns on until the current reaches the set PFM

switching current (IPFM) again.

If the load current is large as described above, operations (1) and (2) above are repeated. If the load current is several tens of mA lower than the

PFM switching current (IPFM), the Pch driver Tr stays on continuously.

When the VIN voltage rises, VIN start mode stops the short-circuit protection function during the interval until the FB voltage approaches VREF. After

the VIN voltage rises and the FB voltage approaches VREF by step-down operation, VIN start mode is released. In order to prevent an excessive

rush current while VIN start mode is activated, the coil current flows to the VOUT pin by means of the parasitic diode of the Nch synchronous

rectification Tr. In VIN start mode as well, the coil current is limited by the PFM switching current.

<Short-circuit protection function>

The short-circuit protection function monitors the VOUT voltage. In the event that the VOUT pin is accidentally shorted to GND or an excessive load

current causes the VOUT voltage to drop below the set short-circuit protection voltage, the short-circuit protection function activates, and turns off

and latches the Pch driver Tr at any selected timing. Once in the latched state, the IC is turned off and then restarted from the CE pin, or operation

is started by re-applying the VIN voltage.

When the VIN pin voltage drops below the UVLO detection voltage, the IC stops switching operation at any selected timing, turns off the Pch driver

Tr and Nch synchronous rectification switch Tr (UVLO mode). When the VIN pin voltage recovers and rises above the UVLO release voltage, the

IC restarts operation.

On the XCL210 series, a CL discharge function is available as an option (XCL210C/XCL210D types). This function enables quick discharging of

the CL load capacitance when “L” voltage is input into the CE pin by the Nch Tr connected between the VOUT-GND pins, or in UVLO mode. This

prevents malfunctioning of the application in the event that a charge remains on CL when the IC is stopped. The discharge time is determined by

CL and the CL discharge resistance RDCHG, including the Nch Tr (refer to the diagram below). Using this time constant τ= CL×RDCHG, the discharge

time of the output voltage is calculated by means of the equation below.

V = VOUT × e - t /

τ, or in terms of t, t = τIn(VOUT / V)

V: Output voltage after discharge

VOUT : Set output voltage

ｔ

: Discharge time

CL: Value of load capacitance (CL)

RDCHG : Value of CL discharge resistance Varies by power supply voltage.

: CL × RDCHG

The CL discharge function is not available on the XCL210A/XCL210B types.

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