TCV7108FN datasheet(7/16 Pages) TOSHIBA

7 / 16 page

TCV7108FN

2011-10-03

Output Filter Capacitor Selection

Use a low-ESR electrolytic or ceramic capacitor as the output filter capacitor. Since a capacitor is generally

sensitive to temperature, choose one with excellent temperature characteristics. When the output voltage

exceeds 1.2 V, the capacitance should be 40

F or greater for applications. Meanwhile 60 F or greater

capacitance is desirable when the output voltage is less than 1.2 V. The capacitance should be set to an optimal

value that meets the system’s ripple voltage requirement and transient load response characteristics. The phase

margin tends to decrease as the output voltage is getting low. Enlarge a capacitance for output flatness when

phase margin is insufficient, or the transient load response characteristics cannot be satisfied. Since the ceramic

capacitor has a very low ESR value, it helps reduce the output ripple voltage; however, because the ceramic

capacitor provides less phase margin, it should be thoroughly evaluated.

Soft-Start Feature

The TCV7108FN has a soft-start feature.

If the SS pin is left open, the soft-start time, tSS, for VOUT defaults to 0.45 ms (typ.) internally.

The soft-start time can be extended by adding an external capacitor (CSS) between the SS and SGND pins. The

soft-start time can be calculated as follows:

SS2





···························· (3)

tSS2: Soft-start time (in seconds) when an external capacitor is

connected between SS and SGND.

CSS: Capacitor value (F)

The soft-start feature is activated when the TCV7108FN exits the undervoltage lockout (UVLO) state after

power-up and when the voltage at the EN pin has changed from logic low to logic high.

Overcurrent Protection（OCP）

The TCV7108FN has maximum current limiting. The TCV7108FN limits the ON time of high side switching

transistor and decreases output voltage when the peak value of the Lx terminal current exceeds switching

terminal peak current limitation ILIM1=3.3A(typ.)@ VIN = 4.3V / ILIM2=2.9A(typ.)@ VIN = 3.3V. When VIN≧4.3V,

The TCV7108FN can operate at IOUT = 2.5A(max). Meanwhile, use it at IOUT = 2A(max) when VIN＜4.3V.

Undervoltage Lockout (UVLO)

The TCV7108FN has undervoltage lockout (UVLO) protection circuitry. The TCV7108FN does not provide

output voltage (VOUT) until the input voltage (VIN2) has reached VUVR (2.55 V typ.). UVLO has hysteresis of 0.1

V (typ.). After the switch turns on, if VIN2 drops below VUV (2.45 V typ.), UVLO shuts off the switch at VOUT.

Figure 4 Undervoltage Lockout Operation

Soft start

VIN2

Hysteresis:

VUV

Undervoltage lockout

detection voltage VUV

Switching operation

stops

GND

VOUT

GND

Undervoltage lockout

recovery voltage VUVR

Switching operation starts

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