L4992 Просмотр технического описания (PDF) - STMicroelectronics

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L4992

TRIPLE OUTPUT POWER SUPPLY CONTROLLER

STMicroelectronics 

L4992

DESIGN PROCEDURE (continued)

PCB side and power traces on the other side.

7) Use heavy copper traces: this will reduce their resistance, increasing overall efficiency and will im-

prove their heat-sinking ability.

L4992 EVAL-KIT

The L4992 EVAL KIT is a fully assembled and tested demonstration board that implements a standard

application circuit, configured according to the following specifications:

Input Voltage Range: 6 to 25 V

3.3V Output: Iout3 = 3 A, Vrpp3 ≤ 30 mV

5.1V Output: Iout5 = 3 A, Vrpp5 ≤ 50 mV

12 V Output: Iout12 = 120 mA

Switching frequency: fsw = 300 kHz

The electrical schematic, illustrated in fig. 9, shows that some pull-up/down resistor are added to the

components strictly needed in a real application. Along with a quad dip-switch, they allow to set manually

the logic signals that control the chip operation. These signals are in the present case:

- Switch 1: RUN5 (0= 5.1V OFF, 1= 5.1V ON)

- Switch 2: NOSKIP (0= pulse-skipping ON, 1= pulse-skipping OFF)

- Switch 3: OSC (0= 200 kHz, 1= 300 kHz)

- Switch 4: RUN3 (0= 3.3V OFF, 1= 3.3V ON)

The demonstration board is delivered with the switches configured as illustrated in fig. 8.

Figure 8: Default switches configuration

0

1

1 234

Switches 1 and 4 enable/disable the two PWM sections (switch 4 manages the +12V linear regulator as

well). They must be set on 1 to turn on the regulators.

Please note that as long as each regulator is disabled, the relevant low-side MOSFET is in ON state.

Hence, if the load is capable of sourcing current, it will be short-circuited to ground through the choke

and the low-side MOS.

Although the default switching frequency is 300 kHz (switch 3 set on 1) and the passive components

have been selected for this frequency, the demo board will work satisfactorily at 200 kHz as well. Actu-

ally, at 200 kHz the regulators exhibit the maximum efficiency and the maximum extension of the input

voltage range downwards. On the other hand, the output ripple is greater and the dynamic behaviour

slightly worse.

The demostration board, as it is, does not provide an interface for synchronization. Anyway, it is possible

to synchronize the oscillator (with an appropriate signal: 5V amplitude pulses, spaced out by 400 ns

min.), provided the switch is set on 1, simply by feeding the signal into the middle of the divider R8-R9. In

this way, synchronization can be achieved at a frequency higher than 300 kHz. To synchronize the oscil-

lator to a frequency between 200 and 300 kHz, heavier interventions on the board are needed.

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