LTC2913 datasheet(12/22 Pages) AD

12 / 22 page

LTC4368

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

The example of Figure 4 uses standard 1% resistor values.

The following parameters were selected:

VOS(UV) = 3mV

IUV = 10nA

UVTH = 3.5V

OVTH = 18V

The resistor values can then be solved:

1. R1 + R2 =

3mV

10nA

= 300k

2. R3 = 2 •

3mV

10nA

• (3.5V – 0.5V) = 1.8M

The closest 1% value: R3 = 1.82M

3. R1 =

300k + 1.82M

2 • 18V

= 58.9k

The closest 1% value: R1 = 59K

R2 = 300K – 59K = 241K

The closest 1% value: R2 = 243K

Therefore: OV = 17.93V, UV = 3.51V.

Limiting Inrush Current During Turn On

Charging large capacitors on VOUT can lead to excessive

inrush currents when LTC4368 turns on the external

N-channel MOSFET. The maximum slew rate at the GATE

pin can be reduced by adding a capacitor on the GATE pin:

Slew Rate =

IGATE(UP)

CGATE

When both the UV and OV faults are removed, the exter-

nal MOSFETs are not immediately turned on. The input

supply must remain within the user selected power good

window for typically 32ms (tD(ON)) before the load is again

connected to the supply. This recovery timeout period

filters noise (including line noise) at the input supply and

prevents chattering of power at the load.

Procedure for Selecting UV/OV External Resistor Values

The following 3-step procedure helps select the resistor

values for the resistive divider of Figure 4. This procedure

minimizes UV and OV offset errors caused by leakage

currents at the respective pins.

1. Choose maximum tolerable offset at the UV pin,

VOS(UV). Divide by the worst case leakage current at

the UV pin, IUV (10nA). Set the sum of R1 + R2 equal

to VOS(UV) divided by 10nA. Note that due to the pres-

ence of R3, the actual offset at UV will be slightly

lower.

R1 + R2 =

VOS(UV)

IUV

2. Select the desired VIN UV trip threshold, UVTH. Find

the value of R3:

R3 =

VOS(UV)

IUV

•

UV TH – 0.5V

0.5V

⎛

⎝

⎜

⎞

⎠

⎟

3. Select the desired VIN OV trip threshold, OVTH. Find

the values of R1 and R2:

R1 =

VOS(UV)

IUV

⎛

⎝

⎜

⎞

⎠

⎟

⎟ + R3

OV TH

• 0.5V

R2 =

VOS(UV)

IUV

– R1

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