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DETAILED DESCRIPTION

Operation

Boost Converter

Peak Current Control (Boost Converter)

I

P(typ) +

I

(LIM) )

V

I

L

100 ns

I

P(typ) +

500 mA )

V

I

L

100 ns

Softstart

TPS61042

SLVS441C – DECEMBER 2002 – REVISED MARCH 2007

instead of the output voltage. This gives an accurate regulated LED current independent of the input voltage and

number of LEDs connected. With integrated overvoltage protection (OVP) the TPS61042 is configured as a

current source with overvoltage protection ideally suited to drive LEDs. With the 30V internal switch, the device

can generate output voltages of up to 27.5 V and has an internal 500mA MOSFET switch (Q1). This allows

several LEDs to be connected in series to the output. The internal LED switch (Q2) in series with the LEDs has

a maximum current rating of 60 mA and disconnects the LEDs from ground during shutdown. The LED switch is

driven by a PWM signal applied to the control pin (CTRL), which directly controls the LED brightness. With this

control method the LED brightness depends on the PWM duty cycle only and is independent of the PWM

frequency and amplitude.

The boost converter operates in a pulse frequency modulation (PFM) scheme with constant peak current control.

This control scheme maintains high efficiency over the entire load current range and with a switching frequency

of up to 1 MHz, enables the use of small external components. The converter monitors the sense voltage across

the main switch turns on and the current ramps up. The main switch turns off when the inductor current reaches

the internally set peak current of 500 mA (typ). Refer to the peak current control section for more information.

The second criteria that turns off the main switch is the maximum on-time of 6 µs (typ). This limits the maximum

on-time of the converter in extreme conditions. As the switch is turned off the external Schottky diode is forward

biased, delivering the stored inductor energy to the output. The main switch remains off until the minimum off

time of 400 ns (typ) has passed and the feedback voltage is below the reference voltage again. Using this PFM

peak current control scheme, the converter operates in discontinuous conduction mode (DCM) where the

switching frequency depends on the inductor, input and output voltage, and LED current. Lower LED currents

reduce the switching frequency, which results in high efficiency over the entire LED current range. This

regulation scheme is inherently stable, allowing a wide range for the selection of the inductor and output

capacitor.

Due to the internal current limit delay of 100 ns (typ) the actual current exceeds the DC current limit threshold by

a small amount. The typical peak current limit can be calculated:

The higher the input voltage and the lower the inductor value, the greater the current limit overshoot.

All inductive step-up converters exhibit high in-rush current during start-up if no special precautions are taken.

This can cause voltage drops at the input rail during start-up, which may result in an unwanted or premature

system shutdown.

The TPS61042 limits this in-rush current during start-up by increasing the current limit in two steps starting from

typical start-up behavior.

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