sp7600 Exar Corporation, sp7600 Datasheet - Page 7

• Manufacturer Part Number: sp7600
• Description: 2amp, 29v Buck Converter For Led Driver Applications
• Manufacturer: Exar Corporation

Modulator Operation and Power Sequencing
The SP7600 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 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 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 regulator 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. 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 SP7600 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 has to 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 30% of
I
guidelines, will reduce inductance and possibly inductor size.
LED
Figure 3. LED Duty Cycle Vs (1-D) DIM pin Duty Cycle with 12Vin, 2 Luxeon K2s in parallel at
12/17/07 rev: A
EXAR RESERVES THE RIGHT TO MAKE CHANGES TO THIS DATASHEET. CALL FOR UPDATES: 1-510-668-7000
. Allowing for a higher current ripple, up to 30%, while staying within LED manufacturer ripple
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100Hz, 2A
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(1‐D) PWM Signal (%)
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LED Duty Cycle vs 1‐D
2A total
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©Copyright 2007 Exar Corporation
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SP7600