Datasheet
VOLTAGE
TIME
SIMULTANEOUS STARTUP
V
OUT2
EN
V
OUT1
R
trkb
=
0.8V
V
OUT2
- 0.8V
x R
trkt
VOLTAGE
TIME
RATIOMETRIC STARTUP
V
OUT2
EN
V
OUT1
LMZ10503
SNVS641I –JANUARY 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
(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 kΩ as before, R
trkb
is calculated from
the above equation to be 15.5 kΩ. A timing diagram for the case of equal slew rates is shown below.
Pre-Bias Startup Capability
At startup, the LMZ10503 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
LMZ10503 is a synchronous converter, it will not pull the output low when a pre-bias condition exists. The
LMZ10503 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.
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