RICOH R1225N Technical data

R1225N SERIES

PWM/VFM step-down DC/DC controller

NO.EA-097-111123

OUTLINE

The R1225N Series are CMOS-based PWM step-down DC/DC Converter controllers with low supply current. Each of these ICs consists of an oscillator, a PWM control circuit, a reference voltage unit, an error amplifier, a soft-start circuit, a latch-type protection circuit, a PWM/VFM alternative circuit, a chip enable circuit, a phase

compensation circuit, and an input voltage detect circuit. Further, protection circuit delay time adjuster circuit, and resistors for voltage detection are included. A low ripple, high efficiency step-down DC/DC converter can be easily composed of this IC with some external components, or a power-transistor, an inductor, a diode and capacitors.

With a PWM/VFM alternative circuit, when the load current is small, the operation is automatically switching into the VFM oscillator from PWM oscillator, therefore the efficiency at small load current is improved. The R1225NxxxC/D/K types, which are without a PWM/VFM alternative circuit, are also available.

If the term of maximum duty cycle keeps on a certain time, the embedded protection circuit works. It is latch-type protection circuit, and it works to latch an external Power MOSFET with keeping it off. To release the condition of protection, after disable this IC with a chip enable circuit, enable it again, or restart this IC with power-on. Delay Time for protection circuit is adjustable with an external capacitor. With a built-in UVLO function, when the input voltage is UVLO threshold or less, this IC keeps standby state, and saves its consumption current and avoids miss-operation. Further, if the set output voltage is equal or more than 2.1V, with a built-in start-up function, at the power-on moment until the input voltage becomes more than the set output voltage, DC/DC operation is halted and avoids miss-operation.

FEATURES

•	Wide Range of Input Voltage ........................................................	2.3V~18.5V
•	Built-in Soft-start Function and Latch-type Protection Function
•	Three options of Oscillator Frequency..........................................	180kHz, 300kHz, 500kHz
•	High Efficiency ..............................................................................	Typ. 90%
•	Output Voltage ..............................................................................	Stepwise Setting with a step of 0.1V
		in the range of 1.2V ~ 6.0V
•	Standby Current ............................................................................	Typ. 0.0µA
•	High Accuracy Output Voltage ......................................................	±2.0%
•	Low Temperature-Drift Coefficient of Output Voltage....................	Typ. ±100ppm/°C

APPLICATIONS

•Power source for hand-held communication equipment, cameras, video instruments such as VCRs, camcorders.

•Power source for battery-powered equipment.

•Power source for household electrical appliances.

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R1225N

BLOCK DIAGRAM

	OSC		Start
VIN				VOUT
			func
-	-
EXT
	Amp
+		Vref
	+
PWM/VFM		Soft Start
CONTROL
	Protection	Chip Enable		CE
DLY	UVLO	+	Vref
		-
GND

SELECTION GUIDE

In the R1225N Series, the output voltage, and the optional function for the ICs can be selected at the user's request.

Product Name

Package

Quantity per Reel

Pb Free

Halogen Free

R1225Nxx2 -TR-FE

SOT-23-6W

3,000 pcs

Yes

Yes

xx The output voltage can be designed in the range from 1.2V(12) to 6.0V(60) in 0.1V steps.

The oscillator frequency and the modulation method are options as follows.

Code

Oscillator

PWM/VFM

frequency

alternative circuit

A

300kHz

Yes

B

500kHz

Yes

C

300kHz

No

D

500kHz

No

J

180kHz

Yes

K

180kHz

No

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R1225N

PIN CONFIGURATION

SOT-23-6W

6 5 4

(mark side)

1

2

3

PIN DESCRIPTION

Pin No.	Symbol	Description
1	EXT	External Transistor Drive Pin (CMOS Output Type)
2	VIN	Power Supply Pin
3	DLY	Pin for Setting External Capacitor for Protection Circuit Delay Time
4	CE	Chip Enable Pin (Active “H”)
5	GND	Ground Pin
6	VOUT	Pin for Monitoring Output Voltage

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R1225N

ABSOLUTE MAXIMUM RATINGS

Symbol	Item	Rating	Unit
VIN	VIN Supply Voltage	20	V
VEXT	EXT Pin Output Voltage	-0.3~VIN+0.3	V
VCE	CE Pin Input Voltage	-0.3~VIN+0.3	V
VOUT	VOUT Pin Input Voltage	-0.3~VIN+0.3	V
VDLY	VDLY Pin Input Voltage	-0.3~+1.0	V
IEXT	EXT Pin Inductor Drive Output Current	±50	mA
IDLY	DLY Pin Output Current	±15	mA
PD	Power Dissipation	430	mW
Topt	Operating Temperature Range	-40~+85	°C
Tstg	Storage Temperature Range	-55~+125	°C

ABSOLUTE MAXIMUM RATINGS

Electronic and mechanical stress momentarily exceeded absolute maximum ratings may cause the permanent damages and may degrade the life time and safety for both device and system using the device in the field.

The functional operation at or over these absolute maximum ratings is not assured.

RECOMMENDED OPERATING CONDITIONS (ELECTRICAL CHARACTERISTICS)

All of electronic equipment should be designed that the mounted semiconductor devices operate within the recommended operating conditions. The semiconductor devices cannot operate normally over the recommended operating conditions, even if when they are used over such conditions by momentary electronic noise or surge. And the semiconductor devices may receive serious damage when they continue to operate over the recommended operating conditions.

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R1225N

ELECTRICAL CHARACTERISTICS

	• R1225Nxx2X (X=A/B/C/D/J/K)					Topt=25°C
	Symbol	Item	Conditions	Min.	Typ.	Max.	Unit
	VIN	Operating Input Voltage		2.3		18.5	V
	VOUT	Step-down Output Voltage	VIN=VCE+VSET+1.5V, IOUT=-100mA	VSET×	VSET	VSET×	V
			When VSET ≤ 1.5V, VIN=VCE=3.0V	0.98		1.02
	VOUT/	Step-down Output Voltage	-40°C ≤ Topt ≤ 85°C		±100		ppm/
	T	Temperature Coefficient					°C
			VIN=VCE=VSET+1.5V, IOUT=-100mA
			When VSET ≤ 1.5V, VIN=VCE=3.0V	144	180	216
	fosc	Oscillator Frequency	J/K version				kHz
			A/C version	240	300	360
			B/D version	400	500	600
	fOSC/ T	Oscillator Frequency	-40°C ≤ Topt ≤ 85°C		±0.2		%/°C
		Temperature Coefficient
			VIN=VCE=VOUT=18.5V		20	50
	IDD1	Supply Current 1	A/B/J/K version				µA
			C version		30	60
			D version		40	80
	Istb	Standby Current	VIN=18.5V, VCE=0V, VOUT=0V		0.0	0.5	µA
	IEXTH	EXT “H” Output Current	VIN=8V, VEXT=7.9V, VOUT=8V,		-17	-10	mA
			VCE=8V
	IEXTL	EXT “L” Output Current	VIN=8V, VEXT=0.1V, VOUT=0V,	20	30		mA
			VCE=8V
	ISW	DLY switch current	VIN=2.3V, VCE=0V, VDLY=0.1V	1.0	2.0		mA
	ICEH	CE “H” Input Current	VIN=VCE=VOUT=18.5V		0.0	0.5	µA
	ICEL	CE “L” Input Current	VIN=VOUT=18.5V, VCE=0V	-0.5	0.0		µA
	VCEH	CE “H” Input Voltage	VIN=8V, VOUT=0V	1.5			V
	VCEL	CE “L” Input Voltage	VIN=8V, VOUT=0V			0.3	V
	Maxdty	Oscillator Maximum Duty Cycle		100			%
	VFMdty	VFM Duty Cycle	A/B/J version		35		%
	VUVLO1	UVLO Voltage	VIN=VCE=2.5V to 1.5V, VOUT=0V	1.8	2.0	2.2	V
	VUVLO2	UVLO Release Voltage	VIN=VCE=1.5V to 2.5V, VOUT=0V		VUVLO1	2.3	V
					+0.1
	Tstart	Delay Time by Soft-Start	VIN=VSET+1.5V, IOUT=-10mA	5	10	20	ms
		function	VCE=0V→VSET+1.5V
	Tprot	Delay Time for protection circuit	VIN=VCE=VSET+1.5V	10	20	35	ms
			VOUT=VSET+1.5V→0V

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R1225N

TYPICAL APPLICATION AND APPLICATION HINTS

L

R1

C1

PMOS

VIN

EXT

VOUT

+

C3

CE

GND

DLY

SD

-

LOAD

C2

C4

CE CONTROL

PMOS: HAT1044M (Hitachi)

L : CR105-270MC (Sumida, 27µH)

SD1

: RB063L-30 (Rohm)

C3 : 47µF (Tantalum Type)

C1

: 10µF (Ceramic Capacitor)

C2 : 0.1µF (Ceramic Capacitor)

C4: 20nF(Ceramic Capacitor)

R1

: 10Ω

When you use these ICs, consider the following issues;

•As shown in the block diagram, a parasitic diode is formed in each terminal, each of these diodes is not formed for load current, therefore do not use it in such a way. When you control the CE pin by another power supply, do not make its “H” level more than the voltage level of VIN pin.

•The operation of Latch-type protection circuit is as follows;

When the maximum duty cycle continues longer than the delay time for protection circuit, (Refer to the Electrical Characteristics) the protection circuit works to shutdown Power MOSFET with latching operation. Therefore when an input/output voltage difference is small, the protection circuit may work with small load current.

To release the protection of latch status, after disable this IC with a chip enable circuit, enable it again, or restart this IC with power-on. However, in the case of restarting this IC with power-on, after the power supply is turned off, if a certain amount of charge remains in CIN, or some voltage is forced to VIN from CIN, this IC might not be restarted even after power-on.

•Set external components as close as possible to the IC and minimize the connection between the components and the IC. In particular, a capacitor should be connected to VOUT pin with the minimum connection. Make grounding sufficient and reinforce supplying. Large switching current flows through the connection of power line, an inductor and the connection of VOUT. If the impedance of the connection of power supply is high, the voltage level of power supply of the IC fluctuates with the switching current. This may cause unstable operation of the IC.

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R1225N

•Use capacitors with a capacity of 22µF or more for VOUT pin, and with good high frequency characteristics such as tantalum capacitors. We recommend to use capacitors with an allowable voltage which is at least twice as much as setting output voltage, in terms of the input capacitors, its voltage rating is twice or more than input voltage. This is because there may be a case where a spike-shaped high voltage is generated by an inductor when an external transistor is on and off.

•Choose an inductor that has sufficiently small D.C. resistance and large allowable current and is hard to reach magnetic saturation. If the value of inductance of an inductor is extremely small, the ILX may exceed the absolute maximum rating at the maximum loading.

Use an inductor with appropriate inductance.

•Use a diode of a Schottky type with high switching speed, and also pay attention to its current capacity.

•Do not use this IC under the condition with VIN voltage at equal or less than minimum operating voltage.

•When the threshold level of an external power MOSFET is rather low and the drive-ability of voltage supplier is small, if the output pin is short circuit, input voltage may be equal or less than UVLO detector threshold. In this case, the devise is reset with UVLO function that is not the latch-protection function.

•With the PWM/VFM alternative circuit, when the on duty cycle of switching is 35% or less, the R1225N alters from PWM mode to VFM mode (Pulse skip mode). The purpose of this circuit is raising the efficiency with a light load by skipping the frequency and suppressing the consumption current. However, the ratio of output voltage against input voltage is 35% or less, (ex. VIN>8.6V and VOUT=3.0V) even if the large current may be loaded, the IC keeps its VFM mode. As a result, frequency might be decreased, and oscillation waveform might be unstable. These phenomena are the typical characteristics of the IC with PWM/VFM alternative circuit.

ÌThe performance of power source circuits using these ICs extremely depends upon the peripheral circuits. Pay attention in the selection of the peripheral circuits. In particular, design the peripheral circuits in a way that the values such as voltage, current, and power of each component, PCB patterns and the IC do not exceed their respected rated values.

7

R1225N

How to set the delay time of protection circuit

The equation describes how to calculate the delay time of protection circuit from the value of an external capacitor C4.

TDLY=C4×106 sec (In this equation, 1000pF ≤ C4 ≤ 1µF

Without the external capacitor, a certain delay time exists, therefore, if the external capacitor is less than 1000pF, the error will increase. Further, if the external capacitor value is beyond 1µF, the time required to discharge the C4 will be long, and this may cause the miss-operation. For example, if the protection circuit may work and released, soon after that the protection may work. In that case, C4 has not discharged completely yet, therefore, the delay time may be shorter than expected.

					L
			PMOS
R1	C1
	VIN	EXT	VOUT
					+	C3
	CE	GND	DLY	SD	-
	C2		C4			LOAD
	CE CONTROL

8

R1225N

OPERATION of step-down DC/DC converter and Output Current

The step-down DC/DC converter charges energy in the inductor when Lx transistor is ON, and discharges the energy from the inductor when Lx transistor is OFF and controls with less energy loss, so that a lower output voltage than the input voltage is obtained. The operation will be explained with reference to the following diagrams:

	<Basic Circuits>		<Current through L>
			IL
			i1	ILmax
			IOUT
VIN	Lx Tr	L	VOUT
		i2	ILmin	topen
	SD		CL
			ton	toff
				T=1/fosc

Step 1: Lx Tr. turns on and current IL (=i1) flows, and energy is charged into CL. At this moment, IL increases from ILmin (=0) to reach ILmax in proportion to the on-time period (ton) of LX Tr.

Step 2: When Lx Tr. turns off, Schottky diode (SD) turns on in order that L maintains IL at ILmax, and current IL (=i2) flows.

Step 3: IL decreases gradually and reaches ILmin after a time period of topen, and SD turns off, provided that in the continuous mode, next cycle starts before IL becomes to 0 because toff time is not enough. In this case, IL value is from this ILmin (>0).

In the case of PWM control system, the output voltage is maintained by controlling the on-time period (ton), with the oscillator frequency (fosc) being maintained constant.

9

R1225N

Discontinuous Conduction Mode and Continuous Conduction Mode

The maximum value (ILmax) and the minimum value (ILmin) current which flow through the inductor is the same as those when Lx Tr. turns on and when it turns off.

The difference between ILmax and ILmin, which is represented by I;

I=ILmax-ILmin=VOUT×topen/L=(VIN-VOUT) ×ton/L ....................... Equation 1

Where, T=1/fosc=ton+toff

duty (%)=ton/T×100=ton×fosc×100 topen ≤ toff

In Equation 1, VOUT×topen/L and (VIN-VOUT)×ton/L are respectively shown the change of the current at ON, and the change of the current at OFF.

When the output current (IOUT) is relatively small, topen < toff as illustrated in the above diagram. In this case, the energy is charged in the inductor during the time period of ton and is discharged in its entirely during the time period of toff, therefore ILmin becomes to zero (ILmin=0). When IOUT is gradually increased, eventually, topen becomes to toff (topen=toff), and when IOUT is further increased, ILmin becomes larger than zero (ILmin>0). The former mode is referred to as the discontinuous mode and the latter mode is referred to as continuous mode.

In the continuous mode, when Equation 1 is solved for ton and assumed that the solution is tonc,

tonc=T×VOUT/VIN...........................................................................

Equation 2

When ton<tonc, the mode is the discontinuous mode, and when ton=tonc, the mode is the continuous mode.

10

R1225N

OUTPUT CURRENT AND SELECTION OF EXTERNAL COMPONENTS

When Lx Tr. is “ON”:

(Wherein, Ripple Current P-P value is described as IRP, ON resistance of LX Tr. is described as Rp the direct current of the inductor is described as RL.)

VIN=VOUT+(Rp+RL) ×IOUT+L×IRP/ton ...............................................	Equation 3
When Lx Tr. is “OFF”:
L×IRP/toff=VF+VOUT+RL×IOUT ..........................................................	Equation 4
Put Equation 4 to Equation 3 and solve for ON duty, ton/(toff+ton)=DON,
DON=(VOUT+VF+RL×IOUT)/(VIN+VF-Rp×IOUT).....................................	Equation 5
Ripple Current is as follows;
IRP=(VIN-VOUT-Rp×IOUT-RL×IOUT)×DON/f/L.........................................	Equation 6
Wherein, peak current that flows through L, Lx Tr., and SD is as follows;
ILmax=IOUT+IRP/2 ..........................................................................	Equation 7

Consider ILmax, condition of input and output and select external components.

ÌThe above explanation is directed to the calculation in an ideal case in continuous mode.

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