MCP16501 datasheet(33/42 Pages) MICROCHIP

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DS20006388E-page 33

MCP16501

5.7

nSTRT Capacitor for Automatic

Turn-On on VIN Ramping

As shown in Figure 4-1, it is possible to configure the

MCP16501 for automatic start-up upon input voltage

(VIN) ramping/power cycling by connecting a capacitor

on nSTRT pin. The purpose of the capacitor (C in

Figure 4-1) is to delay the rise of the nSTRT pin when

VIN ramps up, such that after the VIN pin voltage

(connected to VIN) has stabilized, the logic level of the

nSTRT pin will still be low for a long time which is

enough to be interpreted as a valid start-up event (i.e.,

longer than t1; see Section 4.4.1 “Typical Power-up

Sequence and Timing”).

Under the assumption that the VIN ramping is much

shorter than the rise time of the nSTRT pin voltage, the

recommended amount of capacitance needed for

generation of a valid start-up event is given by the

formula in Equation 5-3.

After the selection of capacitor C, the resulting C

RPU_nSTRT time constant must be compared to the

actual VIN ramping time to verify that indeed the nSTRT

rise time is much slower.

It is recommended that the time constant C RPU_nSTRT

be at least one order of magnitude larger than the

slowest VIN ramping time expected in the application. If

this is not true, the capacitor value C must be increased

further. Using a 1 µF capacitor for C, the C RPU_nSTRT

time constant will be 40 ms, which is adequate for VIN

ramping times in the ms range.

In atypical application in which the input voltage is

unreliable or the ramp-up time is unknown or too long,

an alternative to the startup using the nSTRT capacitor

is to use a voltage supervisor circuit whose output will

drive the PWRHLD directly.

EQUATION 5-3:

t1 t

RPU_nSTRT

VIN

VIN VIL_nSRT

–

------------------------------------





log



--------------------------------------------------------------------------------------



t1 t

RPU_nSTRT

10.36

–

-------------------





log



--------------------------------------------------------------------

2.24

t1 t

RPU_nSTRT

---------------------------



Where:

RPU_nSTRT = nSTRT Pin Pull-up Resistance (40 kΩ typical value)

VIL_nSTRT = nSTRT Logic Low Input Voltage (0.36 VIN, maximum value)

VIN = Input Voltage (VIN pin is connected to the voltage VIN)

t1 = Delay from nSTRT Falling to First Output VOUT1 Starting (≈ 0.5 ms); see Section 4.4.1

“Typical Power-up Sequence and Timing”

Δt = Additional Delay from VIN Voltage established to the Detection of a Logic Low Level on

nSTRT (due to internal logic wake-up time): ≈ 100 µs

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