PT4205 datasheet(13/18 Pages) POWTECH

13 / 18 page

30V, 1.2A Step-down HB LED Driver

China Resources Powtech (Shanghai) Limited

WWW.CRPOWTECH.COM

Page 13

PT4205_DS Rev EN_1.2

PT4205

VLED is the total LED forward voltage (V)

RSW is the switch resistance (Ω ) {=0.6Ω nominal}

VD is the diode forward voltage at the required load

current (V)

Diode selection

For maximum efficiency and performance, the rectifier

(D1) should be a fast low capacitance Schottky diode

with low reverse leakage at the maximum operating

voltage and temperature.

They also provide better efficiency than silicon diodes,

due to a combination of lower forward voltage and

reduced recovery time.

It is important to select parts with a peak current rating

above the peak coil current and a continuous current

rating higher than the maximum output load current. It

is very important to consider the reverse leakage of the

diode when operating above 85°C. Excess leakage will

increase the power dissipation in the device and if close

to the load may create a thermal runaway condition.

The higher forward voltage and overshoot due to

reverse recovery time in silicon diodes will increase the

peak voltage on the SW output. If a silicon diode is

used, care should be taken to ensure that the total

voltage appearing on the SW pin including supply

ripple, does not exceed the specified maximum value.

The following web sites are useful when finding

alternatives: www.onsemi.com

Reducing output ripple

Peak to peak ripple current in the LED(s) can be

reduced, if required, by shunting a capacitor CLED

across the LED(s) as shown below:

PT4205

VIN

CSN

DIM

GND

68uH

0.28Ω

LED

VIN

A value of 1uF will reduce the supply ripple current by

a factor three (approx.). Proportionally lower ripple can

be achieved with higher capacitor values. Note that the

capacitor will not affect operating frequency or

efficiency, but it will increase start-up delay and reduce

the frequency of dimming, by reducing the rate of rise

of LED voltage.

By adding this capacitor the current waveform through

the LED(s) changes from a triangular ramp to a more

sinusoidal version without altering the mean current

value.

Operation at low supply voltage

The internal regulator disables the drive to the switch

until the supply has risen above the startup threshold

(VUVLO). Above this threshold, the device will start to

operate. However, with the supply voltage below the

specified minimum value, the switch duty cycle will be

high and the device power dissipation will be at a

maximum. Care should be taken to avoid operating the

device under such conditions in the application, in

order to minimize the risk of exceeding the maximum

allowed die temperature. (See next section on thermal

considerations). The drive to the switch is turned off

when the supply voltage falls below the under-voltage

threshold (VUVLO-0.2V).

This prevents the switch working with excessive 'on'

resistance under conditions where the duty cycle is

high.

Thermal considerations

When

operating

the

device

high

ambient

temperatures, or when driving maximum load current,

care must be taken to avoid exceeding the package

power dissipation limits. The graph below gives details

for power derating. This assumes the device to be

mounted on a 25mm2 PCB with 1oz copper standing in

still air.

Max. Power Dissipation

200

400

600

800

1000

1200

1400

1600

-40

-20

100

120

140

Ambient Temperature (Deg C)

Note that the device power dissipation will most often

be a maximum at minimum supply voltage. It will also

increase if the efficiency of the circuit is low. This may

result from the use of unsuitable coils, or excessive

parasitic output capacitance on the switch output. When

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