RT8230A datasheet(13/18 Pages) RICHTEK

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RT8237E

DS8237E-02

October 2020

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

the next

“ON” cycle. The on-time is kept the same as

that in heavy load condition. On the contrary, 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. This

is shown in Figure 1. The transition load point to the light

load operation is calculated as follows :





OUT

LOAD





where tON is the on-time.

Figure 1. Boundary Condition of CCM/DCM

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 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 degraded load transient response

(especially at low input voltage levels).

Forced-CCM Mode (FCCM)

The low noise, forced-CCM mode 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 duty

ratio VOUT/VIN. A fairly constant switching frequency is

the benefit of forced-CCM mode, but this comes at a cost.

The no load battery current can be up to 10mA to 40mA,

depending on the external MOSFETs.

Current Limit Setting (CS)

The RT8237E has a cycle-by-cycle current limit control.

The current limit circuit employs a unique

“valley” current

sensing algorithm. If the magnitude of the current sense

signal at PHASE is above the current limit threshold, the

PWM is not allowed to initiate a new cycle (see Figure

2). In order to provide both good accuracy and a cost

effective solution, the RT8237E supports temperature

compensated MOSFET RDS(ON) sensing.

The CS pin of the RT8237E is a multiplexed pin for PWM

enable/disable control and current limit threshold setting.

Connect a setting resistor from this pin to GND via an N-

MOSFET. When the N-MOSFET is turned off, the PWM

is disabled. When the N-MOSFET is turned on, the PWM

is enabled and the current limit threshold is equal to 1/8

of the voltage at this pin.

Choose a current limit resistor by following below equation:

RIPPLE

LOAD_OC

DS(ON)

CS_OC

OC_SET

I8 R



 







Inductor current is monitored by the voltage between the

GND and PHASE pins, so the PHASE pin should be

connected to the Drain terminal of the low-side MOSFET.

ICS has a temperature coefficient to compensate the

temperature dependency of the RDS(ON). GND is used as

the positive current sensing node, so GND should be

connected to the Source terminal of the low-side MOSFET.

As the comparison is being done during the OFF state,

VLIMIT (current limit threshold) sets the valley level of the

inductor current. Thus, the load current at over current

threshold, ILOAD_OC, can be calculated as follows :

CS_OC

RIPPLE

LOAD_OC

DS(ON)

CS_OC

OUT

DS(ON)

) V

2 L f







 

tON

Slope = (VIN -VOUT) / L

IL, PEAK

ILOAD = IL, PEAK / 2

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