LP4950C-5V, LP4951C

SNVS208C – SEPTEMBER 2002 – REVISED APRIL 2013

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The error comparator has an open-collector output which requires an external pullup resistor. This resistor may

be returned to the output or some other supply voltage depending on system requirements. In determining a

value for this resistor, note that while the output is rated to sink 400

μA, this sink current adds to battery drain in a

low battery condition. Suggested values range from 100k to 1 M

Ω. The resistor is not required if this output is

unused.

*When VIN ≤ 1.3V, the error flag pin becomes a high impedance, and the error flag voltage rises to its pull-up voltage.

Using VOUT as the pull-up voltage (see Figure 37), rather than an external 5V source, will keep the error flag voltage

under 1.2V (typ.) in this condition. The user may wish to divide down the error flag voltage using equal-value resistors

(10 k

Ω suggested), to ensure a low-level logic signal during any fault condition, while still allowing a valid high logic

level during normal operation.

Figure 36. ERROR Output Timing

PROGRAMMING THE OUTPUT VOLTAGE (LP4951C)

The LP4951C may be pin-strapped for 5V using its internal voltage divider by tying the pin 1 (output) to pin 2

(sense) pins together, and also tying the pin 7 (feedback) and pin 6 (VTAP) pins together. Alternatively, it may be

programmed for any output voltage between its 1.235V reference and its 30V maximum rating. As seen in

Figure 37, an external pair of resistors is required.

The complete equation for the output voltage is

(1)

where VREF is the nominal 1.235 reference voltage and IFB is the feedback pin bias current, nominally −20 nA.

The minimum recommended load current of 1

μA forces an upper limit of 1.2 MΩ on the value of R2, if the

regulator must work with no load (a condition often found in CMOS in standby). IFB will produce a 2% typical

error in VOUT which may be eliminated at room temperature by trimming R1. For better accuracy, choosing R2 =

100k reduces this error to 0.17% while increasing the resistor program current to 12

μA. Since the LP4951C

typically draws 60

μA at no load with Pin 2 open-circuited, this is a small price to pay.

12

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