RT8207LZQW datasheet(18/27 Pages) RICHTEK

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DS8207L/M-08 September 2016

www.richtek.com

RT8207L/M

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT8207L/M PWM controller provides the high

efficiency, excellent transient response, and high DC output

accuracy needed for stepping down high voltage batteries

to generate low voltage chipset RAM supplies in notebook

computers. Richtek's Mach ResponseTM technology is

specifically designed for providing 100ns “instant-on”

response to load steps while maintaining a relatively

constant operating frequency and inductor operating point

over a wide range of input voltages. The topology

circumvents the poor load transient timing problems of

fixed-frequency current mode PWMs, while also avoiding

the problems caused by widely varying switching

frequencies in conventional constant-on-time and constant-

off-time PWM schemes. The DRVTM mode PWM

modulator is specifically designed to have better noise

immunity for such a single output application.

The 1.5A sink/source LDO maintains fast transient

response, only requiring 20

μF of ceramic output

capacitance. In addition, the LDO supply input is available

externally to significantly reduce the total power losses.

The RT8207L/M supports all of the sleep state controls,

placing VTT at high-Z in S3 and discharging VDDQ, VTT

and VTTREF (soft-off) in S4/S5.

PWM Operation

The Mach ResponseTM DRVTM mode controller relies on

the output filter capacitor's Effective Series Resistance

(ESR) to act as a current-sense resistor, so the output

ripple voltage provides the PWM ramp signal. Referring to

the function diagrams of the RT8207L/M, the synchronous

high side MOSFET is turned on at the beginning of each

cycle. After the internal one-shot timer expires, the

MOSFET will be turned off. The pulse width of this one-

shot is determined by the converter's input and output

voltages to keep the frequency fairly constant over the

entire input voltage range. Another one-shot sets a

minimum off-time (400ns typ.).

On-Time Control

The on-time one-shot comparator has two inputs. One

input looks at the output voltage, while the other input

samples the input voltage and converts it to a current.

This input voltage proportional current is used to charge

an internal on-time capacitor. The on-time is the time

required for the voltage on this capacitor to charge from

zero volts to VVDDQ, thereby making the on-time of the

high side switch directly proportional to the output voltage

and inversely proportional to the input voltage. This

implementation results in a nearly constant switching

frequency without the need of a clock generator, as shown

below :



TON

VDDQ

3.85p x R

x V

/ (V

0.5)

And then the switching frequency is :

 VDDQ

/ V

x t

)

where RTON is the resistor connected from VIN to the TON

pin.

Diode-Emulation Mode

In diode-emulation mode, the RT8207L/M automatically

reduces switching frequency at light load conditions to

maintain high efficiency. This reduction of frequency is

achieved smoothly without increasing VDDQ ripples or load

regulation. As the output current decreases from heavy

load condition, the inductor current will also be reduced

and eventually come to the point where its valley touches

zero current, which is the boundary between continuous

conduction and discontinuous conduction modes. By

emulating the behavior of diodes, the low side MOSFET

allows only partial negative current to flow when the

inductor freewheeling current reaches negative. As the load

current is further decreased, it takes longer and longer

time to discharge the output capacitor to the level that

requires the next “ON” cycle. The on-time is kept the

same as that in the heavy load condition. In contrast, when

the output current increases from light load to heavy load,

the switching frequency increases to the preset value as

the inductor current reaches the continuous condition. The

transition load point to the light load operation is shown in

below figure and can be calculated as follows :



 IN

VDDQ

LOAD(SKIP)

x t

where tON is the on-time.

RT8207LZQW Datasheet(PDF) 18 Page - Richtek Technology Corporation

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