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BL8501: Descriptionstep down DC/DC controller; File Size338KB，10 Pages; ManufacturerSHANGHAI

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step down DC/DC controller

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BL8501

Step down DC/DC controller

OUTLINE

The BL8501 is a CMOS-based PWM step-down DC-DC Converter controller. The device can

offer the following advantages: lower supply current and wider operating input-voltage range.

The BL8501 consists of an oscillator, a PWM control circuit, a reference voltage unit, an error

amplifier, a soft-start circuit, a protection circuit, a PWM/VFM alternative circuit, a chip enable

circuit, and detecting input voltage circuit. A low ripple, high efficiency step-down DC-DC

converter can be easily composed of this IC with only several external components, or a

power-transistor, an inductor, a diode and capacitors. Output Voltage can be adjusted with

external resistors.

FEATURES

Range of input Voltage: 3.5V~12V

Built-in Soft-start Function and Protection Function (Reset type protection)

Oscillation Frequency: 500KHz

High efficiency: 90%

High Accuracy Output Voltage: 2.0%

Low Temperature-Drift Coefficient of Output Voltage: Typ:

100ppm/°C

Operating Temperature Range: -20 ~85

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.

PIN CONFIGURATION

SOT-23-5L

PIN DESCRIPTION

Pin No.

Symbol

GND

EXT

Description

Chip Enable Pin (Active with “H”)

Ground Pin

Pin for Feedback Voltage

External Transistor Drive Pin (CMOS Output)

Power Supply Pin

DETAILED DESCRIPTION

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The following description refer to the functional block diagram:

Error Amplifier

The error amplifier is a high-gain differential amplifier used to regulate the converter output

voltage. The amplifier generates an error signal, which is fed to the PWM comparator. The

error signal is generated when a sample of the output voltage is compared to the internal

reference and the difference is amplified. The output sample is obtained from

, which is

obtained from an external resistive divider connected between output voltage and GND.

PWM Comparator

The PWM comparator generates a series of pulses, which is obtained when the error signal is

compared to the oscillator clock. And the series of pulses are used to control the state of the

power switch by turning the driver on or off.

PWM/VFM controller

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.

Oscillator

The oscillator circuit provides a 500kHz clock, to set the converter operating frequency, and a

timing ramp of slope compensation. The clock waveform is a pulse, a few hundred

nanoseconds in duration.

Protection circuit

If the term of maximum duty cycle keeps on a certain time, the embedded protection circuit

works. The protection circuit is Reset-type protection circuit, and it works to restart the

operation with soft-start and repeat this operation until maximum duty cycle condition is

released. When the cause of large load current or something else is removed, the operation

is automatically released and returns to normal operation.

Soft-Start

The Soft-Start circuit causes the output voltage to increase to the regulation point at a

controlled rate of rise. The voltage on the charging soft-start capacitor gradually raises the

reference voltage of the error amplifier to clamp the error signal.

UVLO

UVLO function works when the input voltage is equal or less than UVLO threshold, it makes

this IC be standby and suppresses the consumption current and avoids an unstable

operation.

Enable (CE)

Logic low on CE puts the BL8501 in standby state. In standby mode, the output power switch,

voltage reference, and other functions are shut off, the supply current is reduced to 1 A

maximum.

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frequency characteristics such as tantalum capacitors. We recommend you to use output

capacitors with an allowable voltage at least twice as much as setting output 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. And if the value of inductance of an inductor is

extremely small, the

may exceed the absolute maximum rating at the maximum

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

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 device is reset with UVLO function that is

different from the reset-protection function caused by maximum duty cycle.

With the PWM/VFM alternative circuit, when the on duty cycle of switching is 20% or less,

the IC 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 20% or

less, (ex.

>10V and

OUT

=2V) 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.

The performance of power source circuits using this IC 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 does not exceed their respected rated values.

How to Adjust Output Voltage and about Phase Compensation

As for adjustable Output type, feedback pin (

) voltage is controlled to maintain 1.0V.

Output Voltage,

OUT

is as following equation:

OUT

: (R2+R4)=

: R2

OUT

*(R2+R4)/R2

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