LTC3423/LTC3424 UUWUAPPLICATIO S I FOR ATIODETAILED DESCRIPTIONZero Current Amp . The zero current amplifier monitors the inductor current to the output and shuts off the synchro- The LTC3423/LTC3424 provides high efficiency, low noise nous rectifier once the current is below 50mA, preventing power for applications such as portable instrumentation negative inductor current. and are ideal for applications that require an output voltage between 1.5V and 2.6V from a single cell. These products Burst Mode Operation are an addition to the LTC3401 and LTC3402 family of synchronous boost converters, with the differences being Burst Mode operation is when the IC delivers energy to the the omission of the power good function (PGOOD) and the output until it is regulated and then goes into a sleep mode addition of a V where the outputs are off and the IC is consuming only DD input to provide internal power. The IC will not start up until the applied voltage on the V 38µA. In this mode, the output ripple has a variable DD pin is above 2.7V. frequency component with load current and the steady state ripple will be typically below 3%. The current mode architecture with adaptive slope compensation provides ease of loop compensation with During the period where the device is delivering energy to excellent transient load response. The low R the output, the peak current will be equal to 400mA and the DS(ON), low gate charge synchronous switches provides the pulse inductor current will terminate at zero current for each cycle. width modulation control at high efficiency. In this mode the maximum output current is given by: V Low Noise Fixed Frequency Operation I IN ≈ Amps OUT MAXBURST ( ) 6 • VOUT Oscillator . The frequency of operation is set through a resistor from the R Burst Mode operation is user controlled by driving the t pin to ground where f = 3 • 1010/Rt. An internally trimmed timing capacitor resides inside the IC. MODE/SYNC pin high to enable and low to disable. It is The oscillator can be synchronized with an external clock recommended that Burst Mode operation be entered after inserted on the MODE/SYNC pin. When synchronizing the the part has started up. oscillator, the free running frequency must be set to approximately 30% lower than the desired synchronized COMPONENT SELECTION frequency. Keeping the sync pulse width below 2µs will ensure that Burst Mode operation is disabled. Inductor SelectionCurrent Sensing . Lossless current sensing converts the The high frequency operation of the LTC3423/LTC3424 peak current signal to a voltage to sum in with the internal allows the use of small surface mount inductors. The slope compensation. This summed signal is compared to minimum inductance value is proportional to the operat- the error amplifier output to provide a peak current control ing frequency and is limited by the following constraints: command for the PWM. The slope compensation in the IC is adaptive to the input and output voltage. Therefore, the ( ) • ( ) – ( )) k V V V IN MIN OUT MAX IN MIN L > H µ and L > H converter provides the proper amount of slope compensa- f f •Ripple • VOUT MAX ( ) tion to ensure stability and not an excess causing a loss of where phase margin in the converter. k = 3 for LTC3423, 2 for LTC3424 Error Amp . The error amplifier is a transconductance amplifier with gm = 85µmhos. A simple compensation f = Operating Frequency (Hz) network is placed from the VC pin to ground. Ripple = Allowable Inductor Current Ripple (A) Current Limit . The current limit amplifier will shut the VIN(MIN) = Minimum Input Voltage (V) NMOS switch off once the current exceeds its threshold. VOUT(MAX) = Maximum Output Voltage (V) The current amplifier delay to output is typically 50ns. 34234f 7
