Triple Output Step-Down Switching Regulator

A4491

9

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

introduce a small voltage offset to the output.

Enable Each regulator channel can be individually enabled via

the corresponding ENBx pin. If any channel is required to start-

up automatically after the VBB voltage is applied, that particu-

lar channel should have the ENB pin tied to the VBB rail via a

pullup resistor.

This resistor should be selected to limit the current to less than

the maximum specified value, 1 mA. This prevents the internal

protection clamps from turning on. It is recommended that a

100 kΩ pull-up resistor be used. This would ensure the current

Soft Start Each regulator channel contains a soft start circuit. A

soft start cycle is initiated when the appropriate regulator enable

ages are above the minimum values; and no thermal shutdown

condition exists. Note that an overload or short circuit will not

cause a soft start cycle, unless a thermal shutdown event occurs.

During a soft start cycle, the reference voltage is ramped from

0 to 0.8 V typical, which in turn forces the current demand signal

to increase in a linear fashion.

Shutdown All converter channels are disabled in the event of

either a thermal shutdown event or an undervoltage on VBB

As soon as the above fault conditions have been removed, and

assuming the ENB inputs are enabled, the appropriate channels

will auto-restart under control of the soft start.

Current Limit The typical peak current limit for each channel is

specified as 2.5 A minimum, with a duty cycle of 0.9. The mini-

mum current limit occurs at maximum duty cycle (0.9), because

the slope compensation has a maximum effect under this condi-

tion. As the duty cycle reduces, the current limit increases. This

means for applications that operate with a narrow duty cycle, it is

possible to operate with a load current greater than 2.0 A.

Figure 3 illustrates the typical peak current limit versus duty

cycle. For example, it is possible to operate with a peak current

limit of 3.75 A with a duty cycle of 0.3.

As well as ensuring the peak current limit is not exceeded, under

worst case load and input voltage conditions, it is also important

to check the implications on the thermal performance. See the

Thermal Considerations section.

Component Selection

Inductor The inductance value, L, determines the ripple current.

It is important to ensure that the minimum current limit is not

It is recommended that gapped ferrite solutions be used as

opposed to powdered iron solutions, the latter of which exhibit

relatively high core losses that can have a large impact on long

term reliability.

Inductors are typically specified at two current levels, rms cur-

rent and saturation current. With regard to the rms current, it is

important to understand how the rms current level is specified,

in terms of ambient temperature. Some manufacturers quote an

ambient only, whilst others quote a temperature that includes a

self-induced temperature rise. For example, if an inductor is rated

for 85°C and includes a self-induced temperature rise of 25°C

at maximum load, then the inductor cannot be safely operated

beyond an ambient temperature of 60°C at full load. The rms cur-

rent can be assumed to be simply the maximum load current, with

perhaps some margin to allow for overloads, and so forth.

The first stage of determining the inductor value is to specify a

peak-to-peak ripple current of typically about 20% to 25% of the

maximum load.

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

0.5

1.0

0.5

0

020

40

60

80

100

Duty Cycle (%)

Figure 3. Current limit versus duty cycle