LMZ10505TZE-ADJ National Semiconductor Corporation, LMZ10505TZE-ADJ Datasheet - Page 14

• Manufacturer Part Number: LMZ10505TZE-ADJ
• Description: 5a Simple Switcher Power Module With 5.5v Maximum Input Voltage
• Manufacturer: National Semiconductor Corporation

www.national.com
R
would be 14.3 kΩ. A timing diagram for this example, the
equal soft-start time case, is shown below.
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
and to ensure proper overdrive of the SS pin
For the example case of V
R
equation to be 15.5 kΩ. A timing diagram for the case of equal
slew rates is shown below.
trkb
trkt
set to 33 kΩ as before, R
needed to give the two supplies identical soft-start times
V
OUT2
< 0.8 x V
OUT1
trkb
= 5V and V
is calculated from the above
OUT1
OUT2
= 2.5V, with
30107459
30107461
14
PRE-BIAS STARTUP CAPABILITY
At startup, the LMZ10505 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, FP-
GA, or DSP. The output can be pre-biased in these applica-
tions through parasitic conduction paths from one supply rail
to another. Even though the LMZ10505 is a synchronous
converter, it will not pull the output low when a pre-bias con-
dition exists. The LMZ10505 will not sink current during start-
up 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 conduct-
ed through the parasitic paths of the load.
CURRENT LIMIT
When a current greater than the output current limit (I
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 dis-
charging 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 re-
moved, the part will resume normal operation smoothly.
OVER-TEMPERATURE PROTECTION
When the LMZ10505 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.
OCL
) is