TPS2346

SLUS529 – MAY 2002

10

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detailed description (continued)

The ramp generator consists of a series of multiplexers feeding a digital-to-analog converter (DAC) circuit which

sets the magnitude of an internal current source. When active, the current source is used to establish a constant

value source or sink current at the IRAMP pin. This current is used to alternately charge and discharge a

capacitor connected between IRAMP and ground, generating a series of sawtooth timing pulses. During turn-on

of the back-end voltages, the capacitor is charged with a 58-

µA current, then subsequently discharged with a

1.8-

µA load. A comparator in the ramp generator circuit monitors the IRAMP voltage against two alternating

thresholds, such that the voltage charges up to 1.5 V and back down to 0 V. During the rising edge of this

waveform, the voltage at IRAMP is used to develop the reference threshold at the inverting input of the active

channel’s LCA. The other input is connected to the channel’s current sense (CSx) input. The LCA slews the

GATEx output to maintain the CSx pin at the reference value. Since the CSx voltage is developed as the drop

across the external sense resistor due to load current, the current to the load is therefore ramped at a linear rate

set by the dV/dt on the IRAMP pin. Therefore, inrush slew rate limiting is easily programmed by the user with

the programmed curve, particularly at initial turn-on. However, the maximum sourcing limit imposed by the

VMAX4 threshold still applies.

During a load turn-on, the current sense voltage, if required in order to fully charge the back-end plane, can track

the IRAMP waveform up to approximately 1.35 V on the IRAMP pin. If the charging current achieves that level,

the LCA reference is switched automatically to the fixed IMAX reference. Load charging then continues at that

fixed level until complete or until timer expiration, whichever occurs first. For Channels 1, 2 and 3, the IMAX level

has been set to 20 mV at the CSx pin, referenced to VINx. For Channel 4, the IMAX level is 150 mV.

The precharge circuitry is powered from the VIN2 supply input. Therefore, when the 5-V supply and GND pins

mate during an insertion, or until they break contact during an extraction, the precharge actively biases the bus

signal lines to 1.0 V. The precharge block consists of a 1-V reference generator and a unity gain amplifier. The

amplifier provides source and sink capability up to 5 mA.

The ROM data cell sets the configuration information for the TPS2346. These parameters include the order of

channel sequencing, the magnitude of the charge and discharge currents at IRAMP, and the nominal voltage

range of each channel. This information is all pre-programmed at the factory; no programming by the user is

necessary.