DESCRIPCIÓN

The LTM4657 is a complete 8A step-down switching mode µModule (micromodule) regulator in a tiny 6.25mm × 6.25mm × 3.87mm BGA package. Included in the package are the switching controller, power FETs, inductor and support components. Operating over an input voltage range of 3.1V to 20V, the LTM4657 supports an output voltage range of 0.5V to 5.5V, set by a single external resistor. Its high efficiency design delivers up to 8A continuous output current. Only bulk input and output capacitors are needed. The LTM4657 supports selectable discontinuous mode operation and output voltage tracking for supply rail sequencing. Its high switching frequency and current mode control enable a very fast transient response to line and load changes without sacrificing stability. Fault protection features incl0ude overvoltage, overcurrent and overtemperature protection. The LTM4657 is available with SnPb or RoHS compliant terminal finish.

 

CARACTERÍSTICAS

Complete Solution in <1cm2 (Single-Sided PCB) or 0.5cm2 (Dual-Sided PCB)

6.25mm × 6.25mm × 3.87mm BGA Package

Amplio rango de tensión de entrada: 3,1 V a 20 V

0.5V to 5.5V Output Voltage

8A DC Output Current

±1.5% Maximum Total DC Output Voltage Error Over Line, Load and Temperature

Differential Remote Sensing Amp

Control en modo corriente, respuesta transitoria rápida

Sincronización de frecuencia externa

Multiphase Parallel Current Sharing with Multiple LTM4657s

Seguimiento de la tensión de salida

Selectable Discontinuous Mode

Indicador de buena alimentación

Overvoltage, Overcurrent and Overtemperature  Protection

 

APLICACIONES

Telecom, Datacom, Networking and  Industrial Equipment

Equipos de diagnóstico médico

Data Storage Rack Units and Cards

Sistemas de prueba y depuración

 

INFORMACIÓN SOBRE APLICACIONES

External component selection is primarily determined by the input voltage, the output voltage and the maximum load current.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN and VOUT stepdown ratios that can be achieved for a given input voltage due to the minimum off-time and minimum on-time limits of the regulator.  The minimum off-time limit imposes a maximum duty cycle which can be calculated as:

DMAX = 1 – (tOFF(MIN) • fSW)

where tOFF(MIN) is the minimum off-time, typically 50ns for LTM4657, and fSW (Hz) is the switching frequency. Conversely the minimum on-time limit imposes a minimum duty cycle of the converter which can be calculated as:

DMIN = tON(MIN) • fSW

where tON(MIN) is the minimum on-time, typically 25ns for LTM4657. In the rare cases where the minimum duty cycle is surpassed, the output voltage will still remain in regulation, but the switching frequency will decrease from its programmed value. Note that additional thermal derating may be applied. See the Thermal Considerations and Output Current Derating section in this data sheet.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth design, only a single low ESR output ceramic capacitor is required for the LTM4657 to achieve low output ripple voltage and very good transient response. In extreme cold or hot temperature or high output voltage case, additional ceramic capacitor or tantalum-polymer capacitor is required due to variation of actual capacitance over bias voltage and temperature. Table 7 shows a matrix of different output voltages and output capacitors to minimize the voltage droop and overshoot during a 2A load-step transient. Additional output filtering may be required by the system designer if further reduction of output ripple or dynamic transient spikes is required. The Analog Devices  LTpowerCAD design tool is available to download online for output ripple, stability and transient response analysis for further optimization.

Discontinuous Current Mode (DCM)

In applications where low output ripple and high efficiency at intermediate current are desired, discontinuous current mode (DCM) should be used by connecting the MODE/ CLKIN pin to GND.At light loads the internal current comparator may remain tripped for several cycles and force the top MOSFET to stay off for several cycles, thus skipping cycles. The inductor current does not reverse in this mode.

Forced Continuous Current Mode (CCM)

In applications where fixed frequency operation is more critical than low current efficiency, and where the lowest output ripple is desired, forced continuous operation should be used. Forced continuous operation can be enabled by tying the MODE/CLKIN pin to INTVCC. In this mode, inductor current is allowed to reverse during low output loads, the COMP voltage is in control of the current comparator threshold throughout, and the top MOSFET always turns on with each oscillator pulse. During start-up, forced continuous mode is disabled and inductor current is prevented from reversing until the LTM4657’s output voltage is in regulation.