Micropower Step-Up DC-to-DC Converter

Page Contents:   [ Featured Products | Datasheets | Application Notes | Buy Now ]

The AME5130 is a fixed off-time step-up DC-to-DC converter in a small 5-lead SOT-25 package. It is ideal for LCD panels requiring low current and high efficiency as well as LED applications for cellular phone backlighting, PDAS and other handheld devices. The low 400ns off-time allows the use of tiny external components.

The AME5130 can drive up 8 white LEDs from a single Li-Ion battery DC 2V to 5.5V; can be turned on by putting more than 1V at pin 4(EN). To control LED brightness, the LED current can be pulsed by applying a PWM (pulse width modulated) signal with a frequency range of 100Hz to 50kHz to the EN pin.

The AME5130 features a constant off-time control scheme. When the voltage at the FB pin is less than 1.23V, the Enable Comp in enables the device and the NMOS switch is turned on pulling the SW pin to ground. When the NMOS switch is on, current is supplied by the output capacitor C OUT. Once the current in the inductor reaches the peak current limit, the 400ns One Shot turns off the NMOS switch. The SW voltage will then rise to the output voltage plus a diode drop and the inductor current will begin to decrease. During this time the energy stored in the inductor is transferred to C OUT and the load. After the 400ns off-time the NMOS switch is turned on and energy is stored in the inductor again. This energy transfer from the inductor to the output causes a stepping effect in the output ripple.

This cycle is continued until the voltage at FB reaches 1.23V. When FB reaches this voltage, the enable comparator then disables the device turning off the NMOS switch and reducing the Iq of the device to 64mA. The load current is then supplied solely by C OUT indicated by the gradually decreasing slope at the output. When the FB pin drops slightly below 1.23V, the enable comparator enables the device and begins the cycle described previously. The EN pin can be used to turn off the AME5130 and reduce the Iq to 0.01mA. In shutdown mode the output voltage will be a diode drop lower than the input voltage.

 

Inductor Selection

The appropriate inductor for a given application is calculated using the following equation:

Where VD is the Schottky diode voltage, ICL is the switch current limit found in the Typical Performance Characteristics section, and TOFF is the switch off time. When using this equation be sure to use in minimum input voltage for the application, such as for battery powered applications. Choosing inductors with low ESR decrease power loss and increases efficiency.

Care should be taken when choosing an inductor. For applications that require an input voltage that approaches the output voltage, such as when converting a Li-ion battery voltage to 5V, the 400ns off time may not be enough time to discharge the energy in the inductor and transfer the energy to the output capacitor and load. This can cause a ramping effect in the inductor current waveform and an increased ripple on the output voltage. Using a smaller inductor will cause the IPK to increase and will increase the output voltage ripple further. This can be solved by adding a 4.7pF capacitor across the R1 feedback resistor and slightly increasing the output capacitor. A smaller inductor can then be used to ensure proper discharge in the 400ns off time.

 

Diode Selection

To maintain high efficiency, the average current rating of the Schottky diode should be larger than the peak inductor current, IPK. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency in portable applications. Choose a reverse breakdown of the Schottky diode larger than the output voltage.

 

Diode Selection

Choose low ESR capacitors for the output to minimize output voltage ripple. Multilayer ceramic capacitors are the best choice. For most applications, a 1µF ceramic capacitor is sufficient. For some applications a reduction in output voltage ripple can be achieved by increasing the output capacitor. Local bypassing for the input is needed on the AME5130. Multilayer ceramic capacitors are a good choice for this as well. A 4.7µF capacitor is sufficient for most applications. For additional bypassing, a 100nF ceramic capacitor can be used to shunt high frequency ripple on the input.

 

FEATURES 0.7Ω internal switch Uses small surface mount components Adjustable output voltage up to 20V 2V to 5.5V input range Input undervoltage lockout 0.01µA shutdown current Small 5-lead SOT-25 package

APPLICATIONS White LED backlighting Handheld devices Digital cameras Portable applications LCD bias power

 

 

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
AME5130AEEVADJZ		Micropower Step-up DC-to-DC Converter

refers to New Product Introduction