ADP7185ACPZN3.3-R7 datasheet(14/19 Pages) AD

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ADP7185

Data Sheet

Rev. 0 | Page 14 of 19

START-UP TIME

When the output is enabled, the ADP7185 uses an internal soft

start to limit the inrush current. The start-up time for a −1.2 V

output is approximately 12 ms from the time the EN active

threshold is crossed to the time when the output reaches 90%

of its final value (see Figure 44). As shown in Figure 44 and

Figure 45, the start-up time is dependent upon the output

voltage option and the value of the CAFB capacitor.

TIME (ms)

VOUT = –4.5V

VOUT = –3.3V

VOUT = –2.5V

VOUT = –1.2V

–6

–5

–4

–3

–2

–1

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Figure 44. Start-Up Time at Various Output Voltages, CAFB = 10 nF, CA = 1 μF

The total start-up time depends mostly on the CA and CAFB

values expressed by the τ1 and τ2 equations (see Equation 2 and

Equation 3). During startup, an internal circuit, GM_START, turns

on and helps charge CA up to 90% of the final value. Estimate the

first time constant, τ1, due to CA by

τ1 ≈ CA × ((R1 + R2)//ZOUT)

(2)

During this time, keep ZOUT low to approximately 1 kΩ to allow

quick start-up times, keeping τ1 in the order of 1 ms.

TIME (ms)

–4.5

–4.0

–3.5

–3.0

–2.5

–2.0

–1.5

–1.0

–0.5

0.5

110 130 150 170 190 210 230

CAFB = 1nF

CAFB = 10nF

CAFB = 100nF

CAFB = 1µF

Figure 45. Start-Up Time at Various CAFB Capacitor Values, CA = 1 μF

A second time constant, τ2, is dependent mainly on CAFB. Figure 45

shows how the CAFB value affects the start-up time. Estimate τ2 by

τ2 ≈ CAFB × R1

(3)

The R1 value scales vs. the VOUT option. Table 6 shows the R1

value depending on the fixed output voltage option, whereas R2

is constant at 500 kΩ. For example, at a fixed VOUT = −3.3 V, R1

equals 2.8 MΩ. To keep τ2 at a minimum, it is recommended that

CAFB be in the approximately nanofarad range. A typical setup for

the ADP7185 is CAFB = 10 nF; therefore, τ2 = 28 ms. The total

time constant, τTOTAL, is the sum of τ1 and τ2. At 2.2 × τTOTAL, VA is

equal to ~90% of the final value. Therefore, for a fixed VOUT =

−3.3 V, the output voltage is ~90% of the final value after 63.8 ms.

Table 6. R1 and R2 Values for the Fixed Output Options

Output Voltage (V)

R1 (Ω)

R2 (kΩ)

−1.2

700 k

500

−2.5

2 M

500

−3.3

2.8 M

500

−4.5

4 M

500

Note that τ1 and τ2 are estimates only and do not take into account

that GM and ZOUT dynamically change. It is an accurate estimate

of ~90% of the start-up time for the CAFB < 10 nF recommended

setup, where ~100% of the settling time can easily be achieved.

Note that for setups with CAFB >> 10 nF, the equation may not

hold true anymore. However, it is still a convenient estimate on

the amount of time needed to achieve ~100% of the settling time

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