LMZ23608 Datasheet, PDF (20/26 Pages) Texas Instruments – 8A SIMPLE SWITCHER® Power Module with 36V Maximum Input Voltage and Current Sharing

15°C) the SS pin is released, V_OUT rises smoothly, and normal
operation resumes.
Applications requiring maximum output current especially
those at high input voltage may require additional derating at
elevated temperatures.
PRE-BIASED STARTUP
The LMZ23608 will properly start up into a pre-biased output.
This startup situation is common in multiple rail logic applica-
tions where current paths may exist between different power
rails during the startup sequence. The following scope cap-
ture shows proper behavior in this mode. Trace one is Enable
going high. Trace two is 1.8V pre-bias rising to 3.3V. Trace
three is the SS voltage with a C_SS= 0.47uF. Risetime deter-
mined by C_SS.
Pre-Biased Startup
In CCM, current flows through the inductor through the entire
switching cycle and never falls to zero during the off-time.
Following is a comparison pair of waveforms showing both
the CCM (upper) and DCM operating modes.
CCM and DCM Operating Modes
V_IN = 12V, V_O = 3.3V, I_O = 3A/0.3A
30151286
The approximate formula for determining the DCM/CCM
boundary is as follows:
30151285
DISCONTINUOUS CONDUCTION AND CONTINUOUS
CONDUCTION MODES
At light load the regulator will operate in discontinuous con-
duction mode (DCM). With load currents above the critical
conduction point, it will operate in continuous conduction
mode (CCM). When operating in DCM, inductor current is
maintained to an average value equaling Iout . In DCM the
low-side switch will turn off when the inductor current falls to
zero, this causes the inductor current to resonate. Although it
is in DCM, the current is allowed to go slightly negative to
charge the bootstrap capacitor.
(17)
The inductor internal to the module is 2.2 μH. This value was
chosen as a good balance between low and high input voltage
applications. The main parameter affected by the inductor is
the amplitude of the inductor ripple current (Δi_L). Δi_L can be
calculated with:
(18)
Where V_IN is the maximum input voltage and f_SW is typically
359 kHz.
If the output current I_OUT is determined by assuming that
I_OUT = I_L, the higher and lower peak of Δi_L can be determined.
19
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