MCP1790-3302ESLASHEB datasheet(15/34 Pages) MICROCHIP

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DS22075A-page 15

MCP1790/MCP1791

4.5

Shutdown (SHDN)

The MCP1791 has a Shutdown (SHDN) input signal

that enables or disables the regulator output voltage.

When the SHDN input signal is greater than 2.40V, the

regulator output voltage is enabled. Note that the

regulator output may still be disabled by the undervolt-

age lockout incorporated within the VIN circuitry.

The value of the SHDN signal to put the regulator into

Shutdown mode is ≤ 0.8V. The SHDN pin is pulled low

by an internal resistor. If the SHDN pin is left floating,

the internal pull-down resistor will put the regulator into

shutdown mode.

When the SHDN input signal is pulled to a logic-low, the

PWRGD output signal will also go low and the regulator

will enter a low quiescent current state where the typi-

cal quiescent current is 10 µA. There is a short time

delay (approximately 400 ns) when the SHDN input

signal transitions from high to low to prevent signal

noise from disabling the regulator. The SHDN pin will

ignore low-going pulses that are up to 400 ns in pulse

width. If the SHDN input is pulled low for more than

400 ns, the regulator will enter Shutdown mode. This

small bit of filtering helps to reject any system noise

spikes on the SHDN input signal.

On the rising edge of the SHDN input, the shutdown

circuitry will have a 100 µs delay before allowing the

regulator output to turn on. This delay helps to reject

any false turn-on signals or noise on the SHDN input

signal. After the 100 µs delay, the regulator will start

charging the output capacitor as the regulator output

voltage rises from 0V to its final regulated value. The

charging current will be limited by the short circuit

current value of the device. If the SHDN input signal is

pulled low during the 100 µs delay period, the timer will

be reset and the delay time will start over again on the

next rising edge of the SHDN input. The total time from

the SHDN input going high (turn-on) to the regulator

output being in regulation shall typically be 200 µs

(100 µs + 100 µs) for a CLOAD = 1.0 µF.

FIGURE 4-2:

Shutdown Input Timing

Diagram.

4.6

Low Voltage Shutdown

The MCP1790/MCP1791 incorporates a Low Voltage

Shutdown circuit that turns off the output of the regula-

tor whenever the input voltage, VIN, is below the

specified turn off voltage, VOFF. When the input voltage

(VB) drops below the differential needed to provide

stable regulation, the output voltage (VREG) shall track

the input down to approximately +4.00V. The regulator

will turn off the output at this point.

The output will turn on when VIN rises above the VON

value specified in the data sheet. This feature is

independent of the Shutdown input signal (SHDN) that

is provided for external regulator control. If the SHDN

input signal is active (LOW), then the output of the

regulator shall be disabled regardless of input voltage.

TABLE 4-1:

SHUTDOWN LOGIC

4.7

Output Capacitor

The MCP1790/MCP1791 requires a minimum output

capacitance of 1 µF tantalum or electrolytic capaci-

tance. The minimum value for ceramic capacitors is

4.7 µF. The regulator is stable for all three types of

capacitors from 4.7 µF to 1000 µF (see Figure 4-3).

The MCP1790/MCP1791 regulator may be used with a

1 µF ceramic output capacitor if a 0.300

Ω resistor is

placed in series with the capacitor. The low ESR and

corresponding pole of the ceramic capacitor causes the

instability below 4.7 µF.

The Equivalent Series Resistance (ESR) of the output

capacitor must be no greater than 3 ohms. The output

capacitor should be located as close to the regulator

output as is practical. Ceramic materials X7R and X5R

have

low

temperature

coefficients

and

are

recommended because of their size, cost, and

environmental robustness qualities.

SHDN

VOUT

100 µs

TOR

400 ns (typ)

CLOAD = 1.0 µF

CLOAD CHARGING TIME

VIN

SHDN

VOUT

< VOFF

LOFF

< VOFF

HOFF

> VON

LOFF

> VON

HON

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