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DC/DC Converter

Technical information about Ricoh DC/DC converters.

	PWM Control and VFM Control
Ricoh DC/DC converters may use PWM control or VFM control. PWM is an abbreviation for Pulse Width Modulation and VFM is an abbreviation for Variable Frequency Modulation. For optimal use, some products may be switched between the two types of control, and the switching methods include manual switching by means of a MODE pin and automatic switching.

	Required or Not of Output Transistor
Ricoh DC/DC converters include converters both with and without in-built output transistor. The former have an Lx pin, the latter an EXT pin and they are respectively referred to as DC/DC converters and DC/DC controllers. (If a single product has both types it is listed as a DC/DC converter) When using an external output transistor, please use a power MOS FET. When efficiency during high loads is more important please use a MOS FET with a low ON resistance, but when efficiency during low loads is more important please use a MOS FET with a small gate capacity. Please use a MOS FET that has a maximum drain current higher than the maximum coil current (ILmax). (The R1210N Series shown below are recommended to use the bipolar transistor.)

Products Containing Internal Output Transistor

RN5RK

R1200x

R1201x

R1210Nxx1

R1218x

R1232D

R1240x

R1242S

R1283x

R1285L

R1290K

R5220x

RP400xxx1A

RP400xxx1C

RP500x

RP503x

RP901K

RP902K

Products Requiring External Transistor

RN5RK

R1210Nxx2

R1211x

R1212D

R1215D

R1224N

R1225N

R1280D

R1282D

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Output Voltage Setting

Ricoh DC/DC converters include types in which the output voltage is fixed within the IC and types that allow the output voltage to be set with external resistors. The types in which the output voltage is fixed within the IC have a VOUT pin, while the externally settable products have a VFB pin. When using an externally settable type please select components with attention to a phase compensation and intrusion of noise.

Products with Internally Fixed Output Voltage

Products with Externally Adjustable Output Voltage

Under Voltage Lock Out Function (UVLO)

To provide more stable operation some DC/DC converters contain a UVLO (Under Voltage Lock Out) function. After turning on power, the UVLO function keeps the internal circuit in the standby state until the DC/DC converter input voltage (VIN) reaches the UVLO voltage, so as to reduce supply current and avoid misoperation.

Products Containing Under Voltage Lockout Function

Soft Start Function

Soft Start is a function that operates when the input voltage (VIN) reaches the minimum operating voltage and UVLO voltage after power on, so as to prevent a sudden starting up of the DC/DC converter that could cause a rush current and over/under shoots of output voltage.
The DC/DC converter is made to start up in a gradual manner by provision of a start up time (soft start time). This soft start time is fixed within the IC, but along with the later to be explained oscillator maximum duty cycle setting, in some products it may be set externally.

Products Containing Soft Start Function

Oscillator Maximum Duty Cycle

Some products allow the oscillator maximum duty cycle to externally set by a capacitor and resistors connected to the DTC pin and VREFOUT pin. The soft start function can also be set externally.

Products with Externally Settable Oscillator Maximum Duty Cycle

Protection Circuits (Latch Type/Reset Type/Fold back Type)

Operation of over current protection in each protection circuit type

There are three protection circuit types in Ricoh DC/DC converters, latch type, reset type and fold back type.

		Latch type protection circuits latch the power MOS to the OFF state during high load etc. if the error persists for a specified time (the protection delay time). The latch protective circuit can be released by once putting the IC into the standby mode with the CE pin and then into the active mode, or, by turning the power off and back on. The protection delay time can be set by the capacitance of a capacitor connected to the DLY pin (DELAY pin).
		Reset type protection circuits repeatedly trigger the soft start operation so as to restrict output current during high load etc. if the error persists for a specified time (the protection delay time). The reset protection circuit is automatically released following removal of the high load etc. that is causing the maximum duty to occur. The protection delay time is fixed within the IC.
		The fold back protection circuit reduces the oscillator frequency when the output voltage decreases from set output voltage. Therefore a minimum ON duty decreases, and the inductor ripple current increases. As a result, the output current decreases. The protection is automatically released after the error is removed.

Protection Circuits (Latch Type/Reset Type/Fold back Type)

Products Containing Latch Type Protection Circuit

Products Containing Reset Type Protection Circuit

R1224N

RP901K

RP902K

Products Containing Fold back Type protection circuit

R1240x

R1242S

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External Phase Compensation

The phase compensation of some DC/DC converters can be set with external components. Please set the phase compensation of such products by connecting external capacitor (speed up capacitor) and resistor to the AMPOUT pin.

Products Allowing External Phase Compensation

Frequency Switching Circuit

The R1210Nxx1A PWM controlled DC/DC Step-up Converters contain a frequency switching circuit. When output loads become light, the PWM control oscillator frequency is reduced from 100kHz to 35kHz. This reduces supply current.

Products Containing a Frequency Switching Circuit

R1210Nxx1

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Over Voltage Protection Circuit (OVP)

R1218x contains OVP (Over Voltage Protection). This circuit monitors the VOUT pin voltage and if it reaches the OVP voltage it will stop oscillation. When the VOUT pin voltage decreases it will restart oscillation, but if the cause of the excess VOUT pin voltage is not removed the OVP circuit will operate repeatedly so as to restrict the VOUT pin voltage.

Products Containing Over Voltage Protection (OVP)

Diode Rectifier and Synchronous Rectifier

Ricoh’s step-down DC/DC converters can be classified into diode rectifier type and synchronous rectifier type. The diode rectifier type is used with a schottky diode, and the synchronous rectifier type is used with MOS transistor in the switch on the low side.
In many cases, DC/DC converters have a built-in MOS transistor. Ricoh’s synchronous rectifier type DC/DC converters have MOS both sides, FET inside the IC except for the R1242S.

Diode rectifier type step-down DC/DC converters

R1224N

R1225N

R1240x

Synchronous rectifier type step-down DC/DC converters

Anti-ringing Switch

Ringing occurs, when the inductor finishes discharging energy in the discontinuous mode of the step-up PWM DC/DC converter.
The anti-ringing switch inhibits the ringing.

Products with the anti-ringing switch

RP400xxx1A

RP400xxx1C

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Sequence Control

Start-up and shutdown sequence controls are available in R1283x, R1290K and etc, using the soft start function, output delay function and auto discharge function. Start-up/shutdown sequence for the LCD panel and CCD sensor can be set.

Products with sequence control

Shutdown Function

The step-up DC/DC converter outputs the supply voltage from inductor through diode at standby mode because the circuit is connected. When the OLED is connected to the output, because the voltage is always applied to the OLED, it may have an impact to the life of OLED. In addition, when the input voltage close to the Vf voltage of white LED, the problem may occurs that the white LED doesn't off.	.
The R1200x Series has an internal NPN transistor in place of a diode. At standby mode, the NPN transistor turns off and can separate the output from the input. So These problems are solved.

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