DESCRIPTION

The LT3437 is a 200kHz monolithic buck switching regulator that accepts input voltages up to 80V. A high efficiency 500mA, 0.8Ω switch is included on the die along with all the necessary oscillator, control and logic circuitry. Current mode topology is used for fast transient response and good loop stability.
Innovative design techniques along with a new high voltage process achieve high efficiency over a wide input range. Efficiency is maintained over a wide output current range by employing Burst Mode operation at low currents, utilizing the output to bias the internal circuitry, and by using a supply boost capacitor to fully saturate the power switch. Burst Mode operation can be defeated by a logic high signal on the SYNC pin which results in lower light load ripple at the expense of light load efficiency. Patented circuitry maintains peak switch current over the full duty cycle range. Shutdown reduces input supply current to less than 1µA. External synchronization can be implemented by driving the SYNC pin with logic-level inputs. A single capacitor from the CSS pin to the output provides a controlled output voltage ramp (soft-start).
The LT3437 is available in a low profile (0.75mm) 3mm ×3mm 10-pin DFN package or a 16-Pin TSSOP Package,both with exposed pad leadframes for low thermal resistance.

 

FEATURES

■ Wide Input Range: 3.3V to 60V
■ Load Dump (Input Transient) Protection to 80V
■ 500mA Peak Switch Current
■ Burst Mode Operation: 100µA Quiescent Current
■ Low Shutdown Current: IQ < 1µA
■ Defeatable Burst Mode Operation
■ 200kHz Switching Frequency
■ Saturating Switch Design: 0.8Ω On-Resistance
■ Peak Switch Current Maintained Over Full Duty Cycle Range
■ 1.25V Feedback Reference Voltage
■ Easily Synchronizable
■ Soft-Start Capability
■ Small 10-Pin Thermally Enhanced DFN Packag

 

APPLICATIONS

■ High Voltage Power Conversion
■ 14V and 42V Automotive Systems
■ Industrial Power Systems
■ Distributed Power Systems
■ Battery-Powered Systems
■ Powered Ethernet

 

APPLICATIONS INFORMATION

In addition to lowering switching frequency, the soft-start ramp rate is also affected by the feedback voltage. Large capacitive loads or high input voltages can cause a high input current surge during start-up. The soft-start function reduces input current surge by regulating switch current via the VC pin to maintain a constant voltage ramp rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from the CSS pin to the output determines the maximum output dV/dt. When the feedback voltage is below 0.4V, the VC pin will rise, resulting in an increase in switch current and output voltage. If the dV/dt of the output causes the current through the CSS capacitor to exceed ICSS, the VC voltage is reduced resulting in a constant dV/dt at the output. As the feedback voltage increases, ICSS increases, resulting in an increased dV/dt until the soft-start function is defeated with 0.9V present at the FB pin. The soft-start function does not affect operation during normal load conditions.
However, if a momentary short (brown out condition) is present at the output which causes the FB voltage to drop below 0.9V, the soft-start circuitry will become active.

 

Short-Circuit Considerations

The LT3437 is a current mode controller. It uses the VC node voltage as an input to a current comparator which turns off the output switch on a cycle-by-cycle basis as this peak current is reached. The internal clamp on the VC node, nominally 1.5V, then acts as an output switch peak current limit. This action becomes the switch current limit specification. The maximum available output power is then determined by the switch current limit.
A potential controllability problem could occur under short-circuit conditions. If the power supply output is short circuited, the feedback amplifier responds to the low output voltage by raising the control voltage, VC, to its peak current limit value. Ideally, the output switch would be turned on, and then turned off as its current exceeded the value indicated by VC. However, there is finite response time involved in both the current comparator and turn-off of the output switch.