SP7601_EB datasheet(5/9 Pages) EXAR

5 / 9 page

Sept25-07 Rev D

SP7601 29V High Voltage LED Driver

Page 5

GENERAL OVERVIEW

Modulator Operation and Power Sequencing

The SP7601 has a unique modulator design

which improves the device’s ability to operate

at very high duty cycle.

While seamless in

operation as the duty cycle is increasing (input

voltage falling), when the duty cycle is

decreased (input voltage rising), the user will

observe the switching frequency increasing in

distinct fractions of the switching frequency. If

the device is operating at 100% duty cycle -- a

unique advantage of using a p-channel pass

device -- and then the input voltage is

increased, the frequency will start at 300kHz,

then 400kHz, 600kHz, and then finally 1.2MHz.

The frequency will tend to increase to the next

higher fraction once the duty cycle reaches 75

to 65 percent. This is the normal operation of

the device and should be expected. There is

no impact on the LED current accuracy.

If PWM dimming is being used as the input

voltage is increased, one will see the frequency

increasing when the duty cycle is < 90%. When

power is initially applied, the device will begin

operating as if the input voltage is increasing

and may start operation at one of the fractional

operating frequencies. Many users will prefer

to have the device start operating at the

nominal

operating

frequency,

thus

recommended that VIN be applied after FB is

set at the high state (>1.2V). The regulator is

now in standby and once VIN has reached

steady-state, then FB is transitioned from a

high to a low state. The controller then starts

operating at nominal frequency.

Another benefit of using power sequencing for

power up is that it ensures all internal circuitry

is alive and fully operational before the device

is required to regulate the current through the

LEDs. Depending on the operating conditions,

there may be short duration (<100μs) current

spikes during initial application of VIN. Figure 3

shows an example of the current spikes.

LEDs are capable of handling these short

duration spikes as long as they are not

operating at their thermal limit.

Since the

regulator was “Off” before power was applied, it

is unlikely the LED is under any type of thermal

stress. EXAR does not recommend using the

SP7601 in applications where dimming of the

LED is achieved by PWM’ing the actual input

power as is common in automotive dimming

applications.

Buck Operation Without Output Capacitor

In order to be able to apply the aforementioned

dimming signal to the LED, the output filter

capacitor that is normally used with a buck

converter must be removed from the circuit.

Thus the LED current ripple equals the inductor

current ripple. As a rule of thumb, current ripple

should be limited to 10% of ILED. Allowing for a

higher current ripple, while staying within LED

manufacturer ripple guidelines, will reduce

inductance and possibly inductor size.

Figure 3- current spikes generated at

startup when power sequencing is not

used, ch1:VIN, ch2: IOUT

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