MAX749 datasheet(8/12 Pages) MAXIM

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counter rolls over and sets the DAC to -VOUT(MIN),

where -VOUT(MIN) = RFB x 6.66µA. In other words, a sin-

gle rising edge of ADJ increments the DAC output by

one, and 63 rising edges of ADJ decrement the DAC

output by one.

Potentiometer Adjustment

It is also possible to adjust the output voltage using a

potentiometer instead of the internal DAC (Figure 8). On

power-up (V+ applied), the internal current source is set

to mid-scale, or 13.33µA. Choose R1 and R2 with the fol-

lowing equations:

R1 = -VOUT(MIN)/13.33µA

R2 = -VOUT(MAX)/13.33µA - R1.

Where the potentiometer can be varied from 0 (producing

VOUT(MIN)) to R2Ω (producing VOUT(MAX)). Notice that ADJ

is connected to ground, allowing the device to be shut

down.

PWM Adjustment

A positive pulse-width modulated (PWM) logic signal

(e.g., from a microcontroller) can control the MAX749’s

output voltage. Use the PWM signal to pull up the FB

pin through a suitable resistor. An RC network on the

PWM output would also be required. In this configura-

tion, the longer the PWM signal remains high, the more

negative the MAX749’s output will be driven.

Current-Sense Resistor

The current-sense resistor limits the peak switch cur-

rent to 140mV/RSENSE, where RSENSE is the value of the

current-sense resistor, and 140mV is the typical cur-

rent-sense comparator threshold (see V+ to CS Voltage

in the

Electrical Characteristics).

To maximize efficiency and reduce the size and cost of

the external components, minimize the peak current.

However, since the output current is a function of the peak

current (Figures 9a-9e), the limit should not be set too low.

No calculations are required to choose the proper cur-

rent-sense resistor; simply follow this two-step procedure:

1. Determine:

- the minimum input voltage, VIN(MIN),

- the maximum output voltage, VOUT(MAX), and

- the maximum output current, IOUT(MAX).

For example, assume that the output voltage must be

adjustable to -24V (VOUT(MAX) = -24V) at up to 30mA

(IOUT(MAX) = 30mA). The supply voltage ranges from

4.75V to 6V (VIN(MIN) = 4.75V).

2. In Figures 9a-9e, locate the graph drawn for the

appropriate output voltage (which is either the

desired output voltage or, if that is not shown, the

graph for the nearest voltage more negative than the

desired output). On this graph find the curve for the

highest RSENSE (the lowest current limit) with an out-

put current that is adequate at the lowest input

voltage.

In this example, select the -24V output graph, Figure 9d.

We then want a curve where IOUT is ≥30mA with a 4.75V

input. The 0.3

Ω R

SENSE graph shows 25mA of output cur-

rent with a 4.75V input, so we look next at the 0.25

Ω

RSENSE graph. It shows IOUT = 30mA for VIN = 4.75V and

VOUT = -24V. Therefore select RSENSE = 0.25Ω. This pro-

vides a current limit in the range 440mA to 720mA.

Alternatively, a 0.2

Ω sense resistor can be used. This

gives a current limit in the range 550mA to 900mA, but

enables over 40mA to be generated at -24V with input

voltages down to 4.5V. A 0.2

Ω resistor may be easier to

obtain than an 0.25

Ω resistor.

The theoretical design curves shown in Figures 9a-9e

assume the minimum (worst-case) value for the current-

limit comparator threshold. Having selected the cur-

rent-sense resistor, the maximum current limit is given

by 180mV/RSENSE. Use the maximum current-limit fig-

ure when choosing the transistor, coil, and diode.

IRC (see Table 2) makes surface-mount resistors with pre-

ferred values including: 0.1

Ω, 0.2Ω, 0.3Ω, 0.5Ω, and 1.0Ω.

Digitally Adjustable LCD Bias Supply

______________________________________________________________________________________

µH

DLOW

VOUT

(NEGATIVE)

+4.5V to +6V

INPUT

µF

0.1

µF

CTRL

DHI

RSENSE

SMD10P05L

1N5819

µF

30V

CCOMP

ADJ

MAX749

V+

GND

VOUT(MIN) = -R1(13.33µA)

VOUT(MAX) = -(R1+R2)(13.33µA)

Figure 8. Using a Potentiometer to Adjust the Output Voltage

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