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RT8202

Single Synchronous Buck Controller

Richtek Technology 

RT8202/A/B

IL

IL

Slope = (VIN -VOUT) / L

iL, peak

iLoad = iL, peak / 2

IL, peak

ILoad

ILIM

0

tON

t

Figure 3. Boundary Condition of CCM/DEM

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-offs in DEM noise

vs. light-load efficiency are 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)

RT8202/A/B 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 4).

0

t

Figure 4. Valley Current-Limit

Current sensing of the RT8202/A/B 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 5(a)).

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

5(b)). There is a compromise between current-limit

accuracy and sense resistor power dissipation.

PHASE

LGATE

OC

RILIM

PHASE

LGATE

OC

RILIM

(a)

(b)

Figure 5. 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

RT8202/A/B 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 activate.

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

Copyright ©2014 Richtek Technology Corporation. All rights reserved.

www.richtek.com

12

is a registered trademark of Richtek Technology Corporation.

DS8202/A/B-07 April 2014

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