RT2856 datasheet(13/19 Pages) RICHTEK

13 / 19 page

RT2856

DS2856-10

October 2016

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

REF



(2)

where CSS is the external soft-start capacitor, ISS is the

soft-start charge current (2

μA), VREF is the feedback

reference voltage (0.8V).

Once the input voltage falls below UVLO threshold, the

EN pin is pulled low, or the OTP is triggered, the device

stops switching and the SS/TR pin starts to discharge. It

is held such shutdown condition until the event is cleared

and the SS/TR pin has already discharged to ground

ensuring proper soft-start behavior.

During the pre-biased start-up sequence, the output of

device is not discharged by low-side power switch

because the device is designed to prevent low-side

MOSFET sinking. It is allowed to sink when the SS/TR

pin exceeds 2.1V.

Slope Compensation

Slope compensation provides stability in constant

frequency architectures by preventing sub-harmonic

oscillations at duty cycles greater than 50%. It is

accomplished internally by adding a compensating ramp

to the inductor current signal. Normally, the peak inductor

current is remained constant under the whole duty cycle

range when slope compensation is added. For the device,

Figure 2. Enable Timing Control

An external MOSFET can be added to implement digital

control from the EN pin to ground, as shown in Figure 3.

In this case, there is no need to connect a pull-up resistor

between the VIN and EN pins since the EN pin is pulled

up by the internal current source. The device can simply

achieve the digital control only through an external

MOSFET on EN pin.

RT2856

GND

External

Digital Control

VIN

Figure 3. Digital Enable Control

The EN pin can also be applied to adjust its Under-Voltage

Lockout (UVLO) threshold with two external resistors

divider from the both input VIN and PVIN pins used together

or separately, and the application structures can refer to

Figure 4, Figure 5, and Figure 6.

RT2856

GND

VIN

REN

CEN

VIN

separated inductor current signal is used to monitor over-

current condition, so the maximum output current stays

relatively constant regardless of duty cycle. More

discussions about over-current protection are described

in a later section.

Enable Operation

The EN pin is an device enable input. Pulling the EN pin

to logic low that is typically less than the set threshold

voltage 1.17V, the device shuts down and enters to low

quiescent current state about 2

μA. The regulator starts

switching again once the EN pin voltage exceeds the

threshold voltage 1.21V. In additional, the EN pin is

implemented with an internal pull-up current source which

allows to enable the device when the EN pin is floating.

For general external timing control, the EN pin can be

externally pulled high by adding a capacitor and a resistor

from the VIN pin as Figure 2.









OUT

REF

= V

For high efficiency, the divider resistance must adopt larger

values, but too large values may induce noises and voltage

errors by the coupled FB pin input current. It is

recommended to use the values between 10k

Ω and

100k

Ω. The output voltage is set by an external resistive

voltage divider according to the following Equation (1) :

(1)

where VREF is the feedback reference voltage (0.8V typ.).

Soft-Start

The device contains an external soft-start clamp that

gradually raises the output voltage. The soft-start timing

is programmed by the external capacitor between SS/TR

pin and GND. The device provides an internal 2

μA charge

current for the external capacitor. If a 10nF capacitor is

used to set the soft-start, the period can be 4ms. The

calculations for external charge capacitor CSS and soft-

start time TSS are shown in Equation (2) :

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