Monolithic Step Down Switching Regulator

L4962 is a monolithic step-down  switching regulator. It provides  output voltage of 5.1V to maxi-mum 40V, delivering current up to  1.2A to 1.5A, depending on the type  and package. The theoretical internal  functions are almost the same. The  heart  of  the  device  is  the  regulation  loop  consisting  of  a  saw  tooth  oscillator, error amplifier, comparator and  source-sink output stage.

Circuit diagram :

Fig. 1: Circuit of switching regulator

An error signal is produced by  comparing  the  output  voltage  with  a precise 5.1 volt on chip reference  (which is zener zap trimmed to ±2  per cent). This error signal is then  compared  with  the  saw  tooth  signal  to generate the fixed frequency pulse  width  modulated  pulses  which  drive the output stage.

Fig. 1 shows circuit diagram of  the regulator. The gain and frequency  of the loop can be set by RC network  connected to IC pin 11. When the loop  is  closed  directly  by  connecting  the  supply output to the feedback input  IC pin 10, an output voltage 5.1 volt is  produced. Higher output voltages are  obtained by inserting a voltage divider  in this feedback path. the outputs over current errors generated  at  the  on  switch  are  prevented  by  the  self-start  function.  The  error  amplifier  output is initially  clamped  by  the  external  capacitor ‘Css’ of pin15,  and  is  allowed  to  rise  linearly  as  this  capacitor  is  charged  by  a  constant  current  source.

Output overload protection is pro-vided in the from of a current limiter.  When the load current exceeds a preset  threshold, this comparator sets a flip-flop, which disables the output stage  and discharges the soft start capacitor. Another internal comparator resets  the flip-flop when the voltage across the soft capacitor C3 falls  to 0.4V. The output is thus  re-enabled  and  the  volt-age  rises  under  the  control of soft start network.  If overload condition is  still  present,  the  limiter  will  trigger  again  when  the  threshold  current  is  reached.  The  average  short-circuit is limited to a  safe value by the dead time introduced  in the soft start network. The thermal  overload circuit disables circuit operation when the junction temperature is  about 150°C and has hysteresis to  prevent instability. Frequency is about  100 KHz with parallel RC network connected to this terminal.

 

Fig. 2: Specification of L1

Assemble the circuit on a general-purpose PCB by using two connectors one  for  the  input  and  the  other for the output. You can also use  a  DC-DC  converter  circuit  in  place  of  the  linear  regulator  to  avoid  the  use  of transformer and also to reduce dissipation. Finally, short-circuit protection is provided for all of the auxiliary  outputs by clips, internal current limiter and thermal protection circuit. The  specification of inductor L1 as shown  in Fig. 2. It is a ferrite torroid core T-18  with a small 20 turns of 27 SWG enameled copper wire.

 

Author : S.N. Banerjee - Copyright :EFY