TIME

SIMULTANEOUS STARTUP

EN

OUT

OUT1

V

0.8

V

´

trkb

trkt

OUT

V

R

R

V

V

2

0.8

0.8

=

´

-

LMZ10503EXT

www.ti.com

SNVS668H – JUNE 2010 – REVISED SEPTEMBER 2015

Feature Description (continued)

7.3.7 Tracking - Equal Slew Rates

Alternatively, the tracking feature can be used to have similar output voltage ramp rates. This is referred to as

simultaneous start-up. In this case, the tracking resistors can be determined based on Equation 5:

(5)

and to ensure proper overdrive of the SS pin:

(6)

Equation 5 to be 15.5 k

Ω. Figure 20 shows an example of tracking using equal slew rates.

Figure 20. Timing Diagram for Tracking Using Equal Slew Rates

7.3.8 Current Limit

the low-side MOSFET is activated. The low-side MOSFET stays on for the entire next four switching cycles.

During these skipped pulses, the voltage on the soft-start pin is reduced by discharging the soft-start capacitor by

a current sink on the soft-start pin of nominally 14 µA. Subsequent overcurrent events will drain more and more

charge from the soft-start capacitor, effectively decreasing the reference voltage as the output droops due to the

pulse skipping. Reactivation of the soft-start circuitry ensures that when the overcurrent situation is removed, the

part will resume normal operation smoothly.

7.3.9 Overtemperature Protection

When the LMZ10503EXT senses a junction temperature greater than 145°C (typical), both switching MOSFETs

are turned off and the part enters a standby state. Upon sensing a junction temperature below 135°C (typical),

the part will re-initiate the soft-start sequence and begin switching once again.

7.4 Device Functional Modes

7.4.1 Prebias Start-Up Capability

At start-up, the LMZ10503EXT is in a prebiased state when the output voltage is greater than zero. This often

occurs in many multi-rail applications such as when powering an ASIC, FPGA, or DSP. The output can be

prebiased in these applications through parasitic conduction paths from one supply rail to another. Even though

the LMZ10503EXT is a synchronous converter, it will not pull the output low when a prebias condition exists. The

LMZ10503EXT will not sink current during start-up until the soft-start voltage exceeds the voltage on the FB pin.

Because the device does not sink current it protects the load from damage that might otherwise occur if current

is conducted through the parasitic paths of the load.

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