feature

Low Power PFM Boost Regulator

Input voltage range from 1.6V to 4.5V

Output voltage range from 3V to 5V

Maximum load current capability

95% efficient power conversion

2-3 Cell and Single Cell Li-Ion Systems

Variable real-time Pulse Frequency Modulation (PFM)

Internal synchronous rectifier (no external diode required)

Low battery detection

Logic Controlled Shutdown with True Load Disconnect

Low (80µA) Quiescent Current TSSOP-8 Package

application

DSC

PDA system

Mobile phones, smartphones

Portable Instruments

2-3 AA/AAA battery operated unit

Single-cell lithium-ion operating device

General Instructions

FAN4855 is a low-power boost regulator for dual-battery low-voltage DC-DC conversion digital cameras, mobile phones and other power systems and PDAs. The converter starts up at 1.3V and operates from an input voltage as low as 1V after startup. The output voltage can be adjusted from 3.3V to 5V by an external resistor, and the maximum load current is 0.5A. The current in shutdown mode is less than 10 microamps to maximize battery life. The input voltage is varied on time to keep the ripple current constant and provide maximum efficiency over a wide load range while maintaining low peak currents. The combination of built-in power transistors, synchronous rectification, and low supply current make the FAN4855 ideal for portable applications. This FAN4855 is available in 8-lead TSSOP package.

Absolute Maximum Ratings

Absolute Maximum Ratings are those values beyond which the device will be permanently damaged. Absolute maximum ratings are only ratings and functional device operation is not implied.

Function description

The boost regulator FAN4855 is an adjustable boost regulator that combines a variable on and minimum off architecture with synchronous rectification. Unique control circuitry provides light and heavy duty efficient power conversion based on load conditions, applying loads by switching between discontinuous and continuous conduction modes. There's no oscillator there; the constant peak current limit is 0.8A in the inductor to allow inductor current at this peak and some smaller values. The switching frequency is load dependent and the input and output voltage ranges up to 430kHz. The input voltage VIN goes to the VIN pin and passes through the external inductor on the VL pin of the device. Cycle the divider from VOUT through the external resistor voltage to close the feedback pin VFB. This transfer rate divider determines the output voltage. When the VFB voltage is lower than VREF=1.24 volts, the error amplifier A1 sends a signal to the regulator to transfer the charge to the output by triggering the variable once. A shot generates a pulse at the gate of the power NMOS transistor Q1. The time interval TON that this transistor will charge the inductor L1 to generate a peak current is given by:

When a shot times out, the Q1 transistor releases the VL pin, allowing the inductor to fly back and pass transistor Q2. However, as the voltage changes polarity in Q2, its gate will be driven by the synchronous rectifier control circuit (SRC), causing Q2 to short out its body diode. The inductor then transfers the charge to the load that is dropped through Q2. Under light load conditions, the energy delivered in this single pulse satisfies the voltage control loop and the converter no longer needs a pulse of energy until the output falls below the lower voltage threshold again. In medium and heavy load conditions the energy pulse is not sufficient to force the output voltage above its upper limit before the minimum off time expires and a second charge cycle is commanded. Because the inductor current does not reach zero, the peak current is greater than the previous value at the end of the second cycle. The result is a ratcheting effect until the output voltage meets - or the converter reaches the set current limit. After a period of TOFF>1µs, determined by the minimum off-time logic, if VOUT is low (VFB

Synchronous rectifiers significantly improve efficiency - no proficiency in adding external components, so conversion efficiencies can be as high as 94% over a wide load range, as shown in typical operation characteristics. Even at light loads, the efficiency remains the same because the switching losses of the converter are high and minimized by reducing the switching frequency. The Error Detection Comparator (LBI – LBO) provides an additional comparator A3 to detect a low VIN or any other error condition important to the user. The non-inverting input of the comparator is internally connected to the reference threshold voltage Vth and the inverting input is connected to the LBI pin. The output of the low battery comparator is a simple open drain output if the battery voltage falls below the programmed threshold voltage on the LBI. The output requires a pull-up resistor with the recommended value and should only be connected to VOUT. A low battery detection circuit is typically used to monitor the battery voltage and generate an error flag or reset command when the battery voltage falls below a user-set threshold voltage. This feature is only activated when the device is enabled. When the device is disabled, the LBO pin is high impedance. Shutdown When VSHDN is less than about 0.5Vin, the device enters shutdown. Switching is stopped during shutdown of the regulator, all internal control circuits including the low battery comparator are turned off, and the load is disconnected from the input. The output voltage can drop below the input voltage during shutdown. A typical shutdown voltage vs. input voltage and timing process for exiting shutdown is shown in the figure. For normal operation VSHDN should be raised 0.8VIN or connected to VIN.

application information

Choose the output voltage The output voltage VOUT can be adjusted from 3V to 5V, choose the resistors R4 and R5 of the voltage divider in the feedback circuit (see test circuit). It is recommended that the value of R5 be less than 270k. R4 can be calculated using the following formula: R4 = R5 [(VOUT/VREF) – 1] where VREF = 1.24 volts to set the LBI threshold of the low battery detector circuit The typical voltage of 390 when the LBI pin drops below the set threshold millivolts, set by the internal reference voltage. The battery voltage, which senses the circuit switch, can be programmed with a resistor divider to connect to the LBI pin. The resistor divider scale reduces the battery voltage to one-tenth of the voltage level, which is then compared to the LBI threshold voltage. The LBI pin has a built-in hysteresis of 25 mV. Resistor values R1 and R2 can be calculated using the following formula: Minimum VIN = 0.39 x (R1+R2)/R2 The value of R2 should be less than or equal to 270k to minimize the deviation current error. Then, by rearranging the equation, find R1: R1 = R2 x (minimum VIN/0.39–1) If the low battery detection circuit is not used, the LBI pin should be connected to ground (or VIN) and the LBO pin Can be left unconnected or tied to GND. Do not let the LBI pin float. Component Selection Input and Output Capacitor Selection For general use, a 47µF tantalum capacitor is recommended. Ceramic capacitors are recommended only on the input side; if connected on the output side, the voltage ripple will not improve significantly. A more effective way to reduce output ripple at light loads is to connect a small 18 to 100pF capacitor to the VOUT and FB pins.

Inductor selection The inductor parameters that directly affect device performance are saturation current and DC resistance. The FAN4855 operates with a typical inductor The lower the resistance, the higher the efficiency - proficient. The saturation current rating should be higher than 0.8A, which is to turn off the N-channel power FET.

Layout and Grounding Considerations

Careful design of printed circuit boards is recommended because of high frequency switching and high peak currents in DC/DC converter applications. The general rule is to place the converter circuit away from any sensitive analog components. Printed circuit board layout should be based on a few simple rules to minimize EMI and ensure good regulation:

1. Place the IC, inductor, input and output capacitors as close together as possible.

2. Place the output capacitors as close as possible to the fan 4855 trace to the VOUT and GND pins as short as possible. Usually should be within 0.25 inches or 6 inches.

3. Leave traces of power components, usually larger than 50 mils or 1.25 mm.

4. Place the external nets of LBI and FB close to the fan 4855, but away from the power supply components as much as possible to prevent coupling of voltage transients to sensitive nodes.

5. Use component-side copper to ground around the IC on a multilayer board and connect back to the mute ground plane using vias. The ground plane acts as an electromagnetic shield for some RF energy radiated.

6. The ground pin (pin 8) of the integrated circuit and the entire ground system should be directly connected to the bottom of the output filter capacitor. A star radiated from the power supply ground system into the printed circuit board is a recommended practice. Applications

The FAN4855 can be used as a constant current source to drive white LEDs, such as the QTLP670C-IW. As shown in the figure below, the current is wide over the input voltage range.