TPS61140_17 Datasheet, PDF (15/26 Pages) Texas Instruments – DUAL OUTPUT BOOST REGULATOR USING SINGLE INDUCTOR

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TPS61140
TPS61141
SLVS624B – JANUARY 2006 – REVISED MARCH 2007
APPLICATION INFORMATION (continued)
To estimate worse case maximum output current, use following conditions
Vin = lowest input voltage
Vf = 1 V
In applications, overcurrent limit I_lim in the PFM mode is typically 60mA higher than the value listed in the
ELECTRICAL CHARACTERISTICS which is measured with DC current. In reality, the inductor current ramps
pass specification value due to the delay of the overcurrent limit comparator.
The LDO has 35 mA maximum output current, regardless of the current output capability of the boost converter.
WLED BRIGHTNESS DIMMING
There are three ways to dynamically change the output current 'on the fly'' for WLED dimming. The first method
parallels an additional resistor with the ISET pin resistor as shown in Figure 18 . The switch, Q1, can change the
ISET pin resistance, and therefore, modify the output current. This method is simple, but can only provide limited
dimming steps.
ISET
R_ISET
R1
Q1
ON/OFF
Logic
Figure 18. Switching In/Out an Additional Resistor to Change Output Current
Alternatively, a PWM dimming signal at the SELI pin will modulate the output current by the duty cycle of the
signal. The logic high of the signal turns on the current sink circuit, while the logic low turns it off. This operation
creates an averaged dc output current proportional to the duty cycle of the PWM signal. The frequency of the
PWM signal must be high enough to avoid flashing of the WLEDs. The soft start of the current sink circuit is
disabled during the PWM dimming to improve linearity.
PWM dimming in the audible frequency range can cause audible noises from the inductor and/or output
capacitor of the boost converter. A voltage ripple in the audible frequency range causes the output capacitor to
vibrate at the same frequency. Because the TPS61140/1 disconnects the WLEDs from the output capacitor
when the SELI pin is low, the output capacitor is not discharged by the WLEDs, which reduces the voltage
ripple, and potential for audible noise from the output capacitor.
Audible noises from both the inductor and output capacitor can be prevented by using a PWM dimming
frequency above or below the audible frequency range. The maximum PWM dimming frequency of the
TPS61140/1 is determined by the current settling time (Tisink) which is the time required for the circuit sink
circuit to reach steady state after the SELI pin transitions from low to high. The maximum dimming frequency
can be calculated by:
F_{PWM_MAX}
+
D_min
T_isink
(8)
D_min = min duty cycle of the PWM dimming required in the application.
For 20% D_min, PWM dimming frequency up to 33 kHz is possible, which is above the audible range.
The third method uses an external dc voltage and resistor as shown in Figure 19 to change the ISET pin current,
and thus control the output current. The dc voltage can be the output of a filtered PWM signal. The equation to
calculate the output current is either
ǒ Ǔ IWLED + KISET
1.229
RISET
)
1.229 *
R1
VDC
for DC voltage input
(9)
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