DW8521 2A Peak, 44V Multi-Topology LED Driver

DW8521 General Description

    DW8521 is a current mode monolithic LED driver. The LED current can be controlled with an analog input voltage. With high-side LED current sensing, the DW8521 can be configured as buck, boost or buck-boost topology to serve different LED driver need. The integrated 44V/2A N-Channel MOSFETs ensures high efficiency operation. At light loads, DW8521 operates in low frequency to maintain high efficiency and low output ripple. The overvoltage lockout protection on SW pin protects DW8521 from open LED fault. DW8521 also  has  thermal protection at 140oC. The DW8521 is  available  in  a 6-pin SOT23-6 and  a  8-pin  ESOP8 package,  which  provides  a  compact  solution  with minimal external components.

DW8521 Features

- 4.6V to 44V operating input range

- 2A peak switch current

- Analog dimming and PWM dimming

- Constant off-time operation

- SW over voltage lockout

- Thermal protection

- Available in SOT23-6 and ESOP8 package

DW8521 Applications

  - LED Driver

DW8521 Pin Information

■DW8521 Pin Placement and IC dimension

■DW8521 Pin Description

DW8521 Absolute Maximum Ratings⁽¹⁾

Note (1) Stresses  above  the  max.  Values  listed  here  may  cause  permanent  damage  to  the  device.  Exposure  to absolute maximum rating conditions for extended  periods may  affect device reliability.  Maximum ratings are absolute ratings; exceeding only one of these values may cause irreversible damage to the integrated circuit.

(2) θ_JA is measured in the convection at Ta=30℃  on a high effective thermal conductivity test board(4 Layers, 2S2P) of JEDEC 51-7 thermal measurement standard. PCB dimension is 100x100x1.6mm and 4 layers.  

DW8521 Typical Application Circuit

DW8521 2A Peak, 44V Multi-Topology LED Driver Detailed Descriptions

The DW8521 is a current-mode LED driver that can be configured as boost, buck or buck-boost topology.

■Current-Mode Control

    The  DW8521  operates  in  constant  off-time  in  heavy  load.  At  light  load  DW8521  extends  the  off  time  to minimize power consumption in the IC.

■DW8521 Dimming

     The DW8521 EN/DIM pin serves as analog dimming input. And can also be used for a PWM dimming, where the output crrent is proportional to the duty cycle of the PWM square pulses.  

      The  analog  dimming  range  is  1.2V  ~  2.4V  corresponding  to  the  LED  current  sensing  voltage  of  0~200mV. Refer to the dimming curve.

■Thermal Protection

 DW8521  has  thermal  protection  that  shuts  down  the  part  at  140℃,  and  resumes  operation  after  the temperature drops to below 130℃.LED Current Setting The  output  current  is  determined  by  the  resistor  R_SENSE connected  between  ISP  and  ISN,  and  the  output current can be calculated by:

For example, for I_LED=1A, R_FB=200mΩ.

■DW8521 Analog Dimming

    Apply  a  1.2V~2.4V  DC  voltage  to  the EN/DIM  pin  to  control  the  output  current,  and  the  DIM voltage  is proportional to the output current.

■DW8521 PWM Dimming

     A  PWM  pulse  can also  be  applied  to  the EN/  DIM  pin  to  achieve  dimming.  When  PWM  dimming,  the  LED current is proportional to the duty cycle of the PWM square pulse.

■DW8521 Input Capacitor

     The  input  capacitor  is used to  reduce  the  surge  current  from  the input  supply  and  maintaining  the  DC  input voltage. It  can  be  electrolytic,  tantalum  or  ceramic.  To  minimizing  the  switching  noise,  a  small  X5R  or  X7R ceramic capacitor, i.e. 0.1uF, should be placed as close to the IC as possible when using electrolytic capacitors.A 10uF ceramic capacitor is sufficient for most application.

■DW8521 Output Capacitor

       The output capacitor is required to maintain the DC output voltage, and the capacitance value determines the output  ripple  voltage. It  can  be  low  ESR  electrolytic,  tantalum  or  ceramic,  which  lower  ESR  capacitors  get lower output ripple voltage. The output capacitors also affect the system stability and transient response, and a 22uF ceramic capacitor is recommended in typical application.

■DW8521 Inductor

    The inductor is used to supply constant current to the output load, and the value determines the ripple current which affect the efficiency and the output voltage ripple. The ripple current is typically allowed to be 30% of the maximum switch current limit. DW8521 operates in constant off-time in heavy load, and the inductance can be determined by:

Which toff is the off-time of the MOSFET,  ΔI is the inductor ripple current.

After the inductance is determined, the on-time can be calculated by:

Then the frequency also can be calculated by:

Lower DC resistance of the inductor is preferred for higher efficiency.

■DW8521 Start-up Current limit

    When  using  in  MR16  application,  the  output  capacitance  should  be  very  large  to  maintain  the continuous output current,  which  causes  the  failure  of  the  internal  soft-start. The  following  circuit  is  designed to  limit the start-up current.

The ISN voltage is always following the ISP voltage before the output voltage build up, which causes a low feedback voltage and a large start-up current. The resistor R1, R2 and the capacitor C1 are used to slowing down the ISN voltage rising, and the RC time constant should be larger than the startup time.

 Where t_r is the startup time, and the startup time is about 400us.Because the ISN pin sink current, and to keep the output current sense accuracy, the R1 can bedetermined by :

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  At steady state, R1 and R2 divide the output voltage, and then, R2 should be larger than R1. In most typical application, R2 can be set to 5 times of R1.  

Take all the conditions into consideration, the parameters can be calculated. For example, R1= 300R, R2=2K, and C1=0.1uF.

■DW8521 PCB Layout Precaution

 The PCB layout of DW8521 must be carefully designed. Different PCB layout optimization is required for different topology application of DW8521.  

1.  Place the input decoupling capacitor as close to DW8521 (VIN pin and PGND) as possible to eliminate noise at the input pin. The loop area formed by input capacitor and GND must be minimized.

2.  Put the feedback trace as far away from the inductor and noisy power traces as possible.

3.  The ground plan on the PCB should be as large as possible for better heat dissipation.

DW8521 Typical Application Circuit

■Step-up LED Driver Application DW8521:

■Step-down LED Driver Application  DW8521:

■Step-up/down LED Driver Application DW8521: