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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 it is
recommended that V IN be applied after FB is
set at the high state (>1.2V). The regulator is
now in standby and once V IN has reached
steady-state, then FB is transitioned from a
high to a low state. The controller then starts
Figure 3- current spikes generated at
startup when power sequencing is not
used, ch1:V IN , ch2: I OUT
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
operating at nominal frequency.
In order to be able to apply the aforementioned
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 (<100 μ s) current
spikes during initial application of V IN . Figure 3
shows an example of the current spikes.
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 I LED . Allowing for a
higher current ripple, while staying within LED
manufacturer ripple guidelines, will reduce
inductance and possibly inductor size.
Sept25-07 Rev D
SP7601 29V High Voltage LED Driver
Page 5
? 2007 Exar Corporation
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