ML4950ES View Datasheet(PDF) - Micro Linear Corporation

Part Name

Description

MFG CO.

ML4950ES

Adjustable Output, Low Current Single Cell Boost Regulator with Detect

Micro Linear Corporation 

ML4950

DESIGN CONSIDERATIONS (Continued)

It is important to note that the accuracy of these resistors

directly affects the accuracy of the output voltage. The

SENSE pin threshold variation is ±3%, and the tolerance of

R1 and R2 will add to this to determine the total output

variation.

Under some circumstances, input ripple cannot be

reduced effectively. This occurs primarily in applications

where inductor currents are high, causing excess output

ripple due to “pulse grouping”, where the charge-

discharge pulses are not evenly spaced in time. In such

cases it may be necessary to add a small 20pF to 100pF

ceramic feedforward capacitor (CFF) from the VIN pin to

the SENSE pin. This is particularly true if the ripple voltage

at VIN is greater than 100mV.

SETTING THE RESET THRESHOLD

To use the RESET comparator as an input voltage monitor,

it is necessary to use an external resistor divider tied to the

DETECT pin as shown in Figure 7. The resistor values RA

and RB can be calculated using the following equation:

1 6 VIN(MIN) = 0.2 ™

RA + RB

RB

(7)

The value of RB should be 100kW or less to minimize bias

current errors. RA is then found by rearranging the

equation:

  RA = RB ™

VIN(MIN) - 1

0.2

(8)

80

LAYOUT

Good PC board layout practices will ensure the proper

operation of the ML4950. Important layout considerations

include:

s Use adequate ground and power traces or planes

s Keep components as close as possible to the ML4950

s Use short trace lengths from the inductor to the VL pin

and from the output capacitor to the VOUT pin

s Use a single point ground for the ML4950 PWR GND

pin and the input and output capacitors, and connect

GND to PWR GND with a separate trace

A typical PC board layout is shown in Figure 8.

VIN

1

RA

DETECT

4

+

COMP

VREF

–

RB

7

RESET

FROM

START-UP

CIRCUITRY

Figure 7. Battery Monitoring Circuit

60

2V OUTPUT

3V OUTPUT

40

20

0

1.0

1.5

2.0

2.5

3.0

VIN (V)

Figure 6. No Load Input Current vs. VIN

For 2V, R1 = 365kW, R2 = 40.2kW

For 3V, R1 = 562kW, R2 = 40.2kW

L = Sumida CD43, 22µH

8

Figure 8. Typical PC Board Layout

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