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LMZ10505EXT
SNVS669G – JUNE 2010 – REVISED OCTOBER 2013
www.ti.com
TRACKING - EQUAL SLEW RATES
Alternatively, the tracking feature can be used to have similar output voltage ramp rates. This is referred to as
simultaneous startup. In this case, the tracking resistors can be determined based on the following equation
R trkb =
0.8V
V OUT2 - 0.8V
x R trkt
(23)
and to ensure proper overdrive of the SS pin
V OUT2 < 0.8 x V
OUT1
(24)
For the example case of V OUT1 = 5V and V OUT2 = 2.5V, with R trkt set to 33 k ? as before, R trkb is calculated from
the above equation to be 15.5 k ? . A timing diagram for the case of equal slew rates is shown below.
SIMULTANEOUS STARTUP
V OUT1
V OUT2
EN
TIME
PRE-BIAS STARTUP CAPABILITY
At startup, the LMZ10505EXT is in a pre-biased 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 pre-
biased in these applications through parasitic conduction paths from one supply rail to another. Even though the
LMZ10505EXT is a synchronous converter, it will not pull the output low when a pre-bias condition exists. The
LMZ10505EXT will not sink current during startup until the soft-start voltage exceeds the voltage on the FB pin.
Since 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.
CURRENT LIMIT
When a current greater than the output current limit (I OCL ) is sensed, the on-time is immediately terminated and
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 over-current 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 over-current situation is removed, the
part will resume normal operation smoothly.
OVER-TEMPERATURE PROTECTION
When the LMZ10505EXT senses a junction temperature greater than 145°C (typ.), both switching MOSFETs are
turned off and the part enters a standby state. Upon sensing a junction temperature below 135°C (typ.), the part
will re-initiate the soft-start sequence and begin switching once again.
LMZ10505EXT Application Circuit Schematic and BOMs
This section provides several application solutions with an associated bill of materials. The compensation for
each solution was optimized to work over the full input range. Many applications have a fixed input voltage rail. It
is possible to modify the compensation to obtain a faster transient response for a given input voltage operating
point.
16
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