RT8205CGQW datasheet(19/28 Pages) RICHTEK

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RT8205A/B/C

DS8205A/B/C-06 July 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT8205A/B/C is a dual, Mach ResponseTM DRVTM

dual ramp valley mode synchronous buck controller. The

controller is designed for low-voltage power supplies for

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

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 dual output application. The RT8205A/

B/C includes 5V (VREG5) and 3.3V (VREG3) linear

regulators. VREG5 linear regulator can step down the

battery voltage to supply both internal circuitry and gate

drivers. The synchronous-switch gate drivers are directly

powered from VREG5. When VOUT1 voltage is above

4.66V, an automatic circuit will switch the power of the

device from VREG5 linear regulator from VOUT1.

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. Refer to the

RT8205A/B/C's function block diagram, the synchronous

high-side MOSFET will be 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 voltage and

the output voltage to keep the frequency fairly constant

over the input voltage range. Another one-shot sets a

minimum off-time (300ns typ.). The on-time one-shot will

be triggered if the error comparator is high, the low-side

switch current is below the current-limit threshold, and

the minimum off-time one-shot has timed out.

PWM Frequency and On-Time Control

The Mach ResponseTM control architecture runs with

pseudo-constant frequency by feed-forwarding the input

and output voltage into the on-time one-shot timer. The

high-side switch on-time is inversely proportional to the

input voltage as measured by the VIN, and proportional to

the output voltage. There are two benefits of a constant

switching frequency. The first is the frequency can be

selected to avoid noise-sensitive regions such as the

455kHz IF band. The second is the inductor ripple-current

operating point remains relatively constant, resulting in

easy design methodology and predictable output voltage

ripple. The frequency for 3V SMPS is set at 1.25 times

higher than the frequency for 5V SMPS. This is done to

prevent audio-frequency “beating” between the two sides,

which switch asynchronously for each side. The

frequencies are set by TONSEL pin connection as Table1.

The on-time is given by :

On-Time = K x (VOUT / VIN)

where “K” is set by the TONSEL pin connection (Table

1). The on-time guaranteed in the Electrical Characteristics

tables are influenced by switching delays in the external

high-side power MOSFET. Two external factors that

influence switching-frequency accuracy are resistive drops

in the two conduction loops (including inductor and PC

board resistance) and the dead-time effect. These effects

are the largest contributors to the change of frequency

with changing load current. The dead-time effect increases

the effective on-time, reducing the switching frequency

as one or both dead times. It occurs only in PWM mode

(SKIPSEL= GND) when the inductor current reverses at

light or negative load currents. With reversed inductor

current, the inductor's EMF causes PHASEx to go high

earlier than normal, extending the on-time by a period

equal to the low-to-high dead time. For loads above the

critical conduction point, the actual switching frequency

is :

f = (VOUT + VDROP1) / (tON x (VIN + VDROP1 -VDROP2) )

where VDROP1 is the sum of the parasitic voltage drops in

the inductor discharge path, including synchronous

rectifier, inductor, and PC board resistances; VDROP2 is

the sum of the resistances in the charging path; and tON

is the on-time calculated by the RT8205A/B/C.

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