SN6507 datasheet(13/44 Pages) TI

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VCC

GND

SW1

SW2

LIM

GND

EN/UVLO

CLK

SS/ILIM

VCC

OUT

CLK

LOAD

Figure 8-3. Schematic with duty cycle control

The calculation of DC pin resistor is shown in Equation 1, where both RDC and RCLK are in kΩ.

RDC=0.816×D×VCC× RCLK+1 −1

(1)

For fixed oscillator cases where RCLK is shorted to GND, a value of RCLK = 9.6kΩ should be used in the equation

above to calculate RDC.

The duty cycle control can compensate for input variation up to ±35%, where line regulation within ±5% can be

achieved. To achieve this range, it is recommended that duty cycle at nominal VIN is centered at 25% (D = 0.25).

The transformer turns ratio needs take this duty cycle into calculation to ensure the expected output voltage level

at all VIN voltages, as discussed in Section 9.2.2.5.

The duty cycle control features supports up to a certain duty cycle and VIN range. The minimum duty cycle is

determined by the charge and discharge time of the gate capacitance of Power FETs, while the maximum duty

cycle is limtied by the dead time (70 ns typical). For example, at 1 MHz, the adjustable duty cycle is between

10% and 43%. Exceeding above duty cycle range, the line regulation may saturate and input compensation does

not work anymore. Meanwhile, if the duty cycle is lower than the minimum spec, the part may hit current limit at

heavy loads. The VIN range that duty cycle feature is applicable is from 6 V to 36 V.

To enable the duty cycle control feature, an inductor is required on the output side. The selection of the output

inductor should make sure the inductor current will not go into discountinous conduction mode (DCM), meaning

the inductor current ramp should not drop to zero at any time. The minimum inductance LMIN is therefore

calculated by the conditions that the part stays in continuous conduction mode (CCM) where the load DC current

is smaller than half the current ramp amplitude seen on the inductor. Therefore LMIN is a function of the load

current and switching frequency as shown by below equation where Iload is in A, fSW is in Hz, D is the duty cycle

as a decimal (for 25% duty cycle, 0.25 would be used), and Lmin is in H.

LMIN=VOUT×1−2×D× VIN TYP/VIN MAX

4×ILOAD MIN×fSW

(2)

www.ti.com

SN6507

SLLSFM0A – MARCH 2022 – REVISED JUNE 2022

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