Rev. C 07/16

3

LYT7503-7504

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Figure 5. Schematic from DER-561 7.5 W, 60 V, 125 mA Dimmable Non-Isolated A19 LED Driver for Wide Input Range: 90 – 300 V VAC using LYT7503D in

Low-Side Buck Configuration.

Applications Design Example

DER-561 a Low-Line Dimmable 7.5 W, Wide Input, High Power factor

LED Bulb Driver.

Circuit Description

The circuit shown in Figure 5 is a LED driver configured as a low-side

buck utilizing the LYT7503D from the LYTSwitch-7 family of ICs. This

is a low component count (20 parts) dimmable LED driver designed to

power a 60 V LED voltage string at 125 mA output current from an

input voltage of 90 VAC to 300 VAC. Dimming performance is

optimized at low-line input (i.e. 120 VAC), while maintaining accurate

regulation for non-dimmable high-line input.

LYTSwitch-7 is a SO-8 package LED driver IC family designed for

non-isolated buck applications. The LYTSwitch-7 family provides high

efficiency, high power factor and accurate LED current regulation. It

incorporates a high-voltage 725 V power MOSFET and a control

engine to switch the MOSFET in critical conduction mode (CM) with

variable on-time and variable frequency which also helps achieve low

EMI, and low THD. The controller also integrates protection features

such as input and output overvoltage protection, thermal fold-back,

over-temperature shutdown, output short-circuit and over-current

protection. The controller also allows natural dimming with only the

addition of a damper resistor and an RC network for damping the

input current ringing when the TRIAC turns on.

Key Design Considerations

Input Stage

The input fusible resistor RF1 provides multiple-purpose function –

safety protection, current limiting against differential surge and acts

as a damping element reducing inrush-current ringing when TRIAC

dimming. Varistor RV1 acts as a voltage clamp that limits the voltage

spike on the primary during line transients and surge events. A 250

VAC rated part was selected with a maximum clamping voltage

specification of 710 VDC – lower than the device Drain voltage (725 V).

The AC input voltage is full wave rectified by BR1 to achieve good

power factor and low THD.

The rectified AC supply is filtered by the input capacitors C1 and C2.

Too much capacitance degrades power factor and THD, so the values

of the input capacitors were set to the minimum necessary to meet

EMI (with suitable margin). Inductor L1, C1 and C2 form a π (pi)

filter, which attenuates conducted differential and common mode EMI

currents. If required a ≥10 kΩ resistor (not shown) can be added

across L1 to damp the Q-factor of the filter inductor to improve the

filtering of high frequency EMI without reducing low frequency

attenuation.

The addition of the RC damper network R2 and C3 makes the driver

compatible with TRAIC (phase-cut) dimmers. The RC damper in the

circuit may be placed before or after the bridge rectifier. In this

design, the RC damper is located after the bridge rectifier for higher

dimming range. Putting the RC damper before the bridge would load

the TRIAC dimmer and maintain full output to a lower conduction

angle but would result in reduced dimming range.

LYTSwitch-7 Controller Stage

The LED driver circuit is a low-side buck configuration operating in

critical conduction mode; the controller allows complete transfer of

the energy stored in the inductor to the load before starting the next

switch cycle. The inductor demagnetization is sensed, detecting

when the voltage across the inductor begins to collapse (towards

zero) as flywheel diode (D1) conduction ceases.

Capacitor C4 provides local decoupling for the BYPASS (BP) pin IC

and provides power to the LYTSwitch-7 controller during the MOSFET

on-time. The IC has an internal regulator that draws power from the

high-voltage DRAIN (D) pin and charges the bypass capacitor C4

during the power switch off-time. The typical BYPASS pin voltage is

5.22 V. To keep the IC operating normally (especially during the dead

time), where V

IN < VOUT, and during dimming at low conduction angles,

resistors R6 and R10 are employed to keep the bypass capacitor

charged. The value of the bypass capacitor should be large enough

to keep the bypass voltage above the V

BP(RESET) reset value of 4.5 V.

The suggested minimum value for the bypass capacitor is 10 µF; an

X7R type is recommended if using a ceramic type capacitor.

Constant output current regulation is achieved through the FEED-

BACK (FB) pin directly sensing the DRAIN pin current during the

MOSFET on-time via external current sense resistors (RFB) R3 and

PI-8023-071416

LYTSwitch-7

U1

LYT7503D

D

S

R8

100 kΩ

R10

51 kΩ

R6

51 kΩ

R2

510 Ω

2 W

L

N

RTN

60 V, 125 mA

+V

D1

US1J-13-F

L1

4.7 mH

T1

EE10

4

6

BR1

B10S-G

1000 V

R3

0.68 Ω

1%

1/3 W

RV1

275 VAC

R1

47 Ω

2 W

R4

51.1 Ω

1%, 1/8 W

R9

402 kΩ

1%

R5

12.4 kΩ

1%

1/8 W

C1

22 nF

450 V

C2

120 nF

450 V

C3

220 nF

450 V

C6

220 µF

80 V

C5

100 pF

1000 V

C4

10 µF

10 V

FB

M

BP

90 - 300

VAC