LTC3250ES6-1.5#TRM

LTC3250-1.5/LTC3250-1.2 High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO 2.7V to 5.5V Input Voltage Range No Inductors Li-Ion (3.6V) to 1.5V with 81% Efficiency Low Noise Constant Frequency Operation Output Voltages: 1.5V ±4%, 1.2V ±4% Output Current: 250mA Shutdown Disconnects Load from VIN Low Operating Current: IQ = 35µA Low Shutdown Current: ISD < 1µA Oscillator Frequency = 1.5MHz Soft-Start Limits Inrush Current at Turn-On Short-Circuit and Overtemperature Protected Low Profile (1mm) SOT-23 Package The LTC®3250-1.5/LTC3250-1.2 are charge pump stepdown DC/DC converters that produce a 1.5V or 1.2V regulated output from a 2.7V to 5.5V input. The parts use switched capacitor fractional conversion to achieve typical efficiency two times higher than that of a linear regulator. No inductors are required. A unique constant frequency architecture provides a low noise regulated output as well as lower input noise than conventional charge pump regulators.* High frequency operation (fOSC = 1.5MHz) simplifies filtering to further reduce conducted noise. The part also uses Burst Mode® operation to improve efficiency at light loads. Low operating current (35µA with no load, 350mA, and/or TJ > 140°C) should be avoided as it can degrade the performance of the part. Soft-Start To prevent excessive current flow at VIN during start-up, the LTC3250-1.5/LTC3250-1.2 have a built-in soft-start circuitry. Soft-start is achieved by increasing the amount of current available to the output charge storage capacitor linearly over a period of approximately 500µs. Soft-start is enabled whenever the device is brought out of shutdown, and is disabled shortly after regulation is achieved. Low Current “Burst Mode” Operation To improve efficiency at low output currents, Burst Mode operation was included in the design of the LTC3250-1.5/ LTC3250-1.2. An output current sense is used to detect when the required output current drops below an internally set threshold (30mA typ.). When this occurs, the part shuts down the internal oscillator and goes into a low current operating state. The LTC3250-1.5/LTC3250-1.2 will remain in the low current operating state until the output has dropped enough to require another burst of current. Unlike traditional charge pumps whose burst current is dependant on many factors (i.e. supply voltage, switch resistance, capacitor selection, etc.), the LTC32501.5/LTC3250-1.2’s burst current is set by the burst threshold and hysteresis. This means that the VOUT ripple voltage in Burst Mode will be fixed and is typically 12mV for a 4.7µF output capacitor. Power Efficiency The power efficiency (η) of the LTC3250-1.5/LTC32501.2 are approximately double that of a conventional linear regulator. This occurs because the input current for a 2 to 1 step-down charge pump is approximately half the output 3250fa 6 LTC3250-1.5/LTC3250-1.2 U OPERATIO (Refer to Simplified Block Diagram) current. For an ideal 2 to 1 step-down charge pump the power efficiency is given by: η≡ POUT VOUT • IOUT 2VOUT = = PIN VIN 1 VIN • IOUT 2 The switching losses and quiescent current of the LTC3250-1.5/LTC3250-1.2 are designed to minimize efficiency loss over the entire output current range, causing only a couple % error from the theoritical efficiency. For example with VIN = 3.6V, IOUT = 100mA and VOUT regulating to 1.5V the measured efficiency is 80.6% which is in close agreement with the theoretical 83.3% calculation. 0.15Ω for the LTC3250-1.5 and 0.12Ω for the LTC3250-1.2. For a 250mA load current change the output voltage will change by about 37mV for the LTC3250-1.5 and by 30mV for the LTC 3250-1.2. If the ESR of the output capacitor is greater than the closed-loop-output impedance the part will cease to roll-off in a simple one-pole fashion and poor load transient response or instability could result. Ceramic capacitors typically have exceptional ESR performance and combined with a tight board layout should yield excellent stability and load transient performance. Further output noise reduction can be achieved by filtering the LTC3250-1.5/LTC3250-1.2 output through a very small series inductor as shown in Figure 1. A 10nH inductor will VOUT Capacitor Selection The ESR and value of capacitors used with the LTC32501.5/LTC3250-1.2 determine several important parameters such as regulator control loop stability, output ripple, and charge pump strength. 10nH (TRACE INDUCTANCE) VOUT LTC3250-1.5/ LTC3250-1.2 VOUT 4.7µF 0.22µF GND 3250 F01 The value of COUT directly controls the amount of output ripple for a given load current. Increasing the size of COUT will reduce the output ripple. Figure 1. 10nH Inductor Used for Additional Output Noise Reduction To reduce output noise and ripple, it is suggested that a low ESR (