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RT8204A

Single Synchronous Buck with LDO Controller

Richtek Technology 

RT8204A

The switching waveforms may appear noisy and

asynchronous when light loading causes diode-emulation

operation, but this is a normal operating condition that

results in high light-load efficiency. Trade-off in DEM noise

vs. light-load efficiency is made by varying the inductor

value. Generally, low inductor values produce a broader

efficiency vs. load curve, while higher values result in higher

full-load efficiency (assuming that the coil resistance

remains fixed) and less output voltage ripple. The

disadvantages for using higher inductor values include

larger physical size and degrades load-transient response

(especially at low input-voltage levels).

Forced-CCM Mode (EN/DEM = floating)

The low noise, forced-CCM mode (EN/DEM = floating)

disables the zero-crossing comparator, which controls the

low-side switch on-time. This causes the low side gate-

drive waveform to become the complement of the high-

side gate-drive waveform. This in turn causes the inductor

current to reverse at light loads as the PWM loop to

maintain a duty ratio VOUT/VIN. The benefit of forced-CCM

mode is to keep the switching frequency fairly constant,

but it comes at a cost: The no-load battery current can be

up to 10mA to 40mA, depending on the external

MOSFETs.

Current Limit Setting (OCP)

The RT8204A has cycle-by-cycle current limiting control.

The current limit circuit employs a unique “valley” current

sensing algorithm. If the magnitude of the current-sense

signal at OC is above the current limit threshold, the PWM

is not allowed to initiate a new cycle (Figure 2).

IL

IL, peak

ILoad

ILIM

0

t

Figure 2. Valley Current-Limit

Current sensing of the RT8204A can be accomplished in

two ways. Users can either use a current sense resistor

or the on-state of the low-side MOSFET (RDS(ON)). For

resistor sensing, a sense resistor is placed between the

source of low-side MOSFET and PGND (Figure 3(a)).

RDS(ON) sensing is more efficient and less expensive (Figure

3(b)). There is a compromise between current limit

accuracy and sense resistor power dissipation.

PHASE

LGATE

OC

RILIM

(a)

PHASE

LGATE

OC

RILIM

(b)

Figure 3. Current-Sense Methods

In both cases, the RILIM resistor between the OC pin and

PHASE pin sets the over current threshold. This resistor

RILIM is connected to a 20μA current source within the

RT8204A which is turned on when the low side MOSFET

turns on. When the voltage drop across the sense resistor

or low-side MOSFET equals the voltage across the RILIM

resistor, positive current limit will be activated. The high

side MOSFET will not be turned on until the voltage drop

across the sense element (resistor or MOSFET) falls

below the voltage across the RILIM resistor.

Choose a current limit resistor by following equation :

RILIM = ILIMIT x RSENSE / 20μA

Carefully observe the PC board layout guidelines to ensure

that noise and DC errors do not corrupt the current-sense

signal seen by OC and PGND. Mount the IC close to the

low-side MOSFET and sense resistor with short, direct

traces, making a Kelvin sense connection to the sense

resistor.

DS8204A-05 April 2011

www.richtek.com

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