Datasheet
Table Of Contents
- FEATURES
- APPLICATIONS
- DESCRIPTION
- Electrical Specifications
- Performance Benefits
- Absolute Maximum Ratings
- Operating Ratings
- Electrical Characteristics
- Typical Performance Characteristics
- Block Diagram
- Design Steps for the LMZ22005 Application
- ENABLE DIVIDER, RENT, RENB AND RENHSELECTION
- OUTPUT VOLTAGE SELECTION
- SOFT-START CAPACITOR SELECTION
- TRACKING SUPPLY DIVIDER OPTION
- CO SELECTION
- CIN SELECTION
- POWER DISSIPATION AND BOARD THERMAL REQUIREMENTS
- PC BOARD LAYOUT GUIDELINES
- Additional Features
- Typical Application Schematic Diagram
- Power Module SMT Guidelines
- Revision History
LMZ22005
SNVS686I –MARCH 2011–REVISED OCTOBER 2013
www.ti.com
THERMAL PROTECTION
The junction temperature of the LMZ22005 should not be allowed to exceed its maximum ratings. Thermal
protection is implemented by an internal thermal shutdown circuit which activates at 165 °C (typ) causing the
device to enter a low power standby state. In this state the main MOSFET remains off causing V
O
to fall, and
additionally the C
SS
capacitor is discharged to ground. Thermal protection helps prevent catastrophic failures for
accidental device overheating. When the junction temperature falls back below 150 °C (typ Hyst = 15°C) the SS
pin is released, V
O
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 LMZ22005 will properly start up into a pre-biased output. This startup situation is common in multiple rail
logic applications where current paths may exist between different power rails during the startup sequence. The
following scope capture shows proper behavior in this mode. Trace one is Enable going high. Trace two is 1.5V
pre-bias rising to 3.3V. Rise time determined by C
SS
, trace three.
Figure 49. Pre-Biased Startup
DISCONTINUOUS AND CONTINUOUS CONDUCTION MODES
At light load the regulator will operate in discontinuous conduction mode (DCM). With load currents above the
critical conduction point, it will operate in continuous conduction mode (CCM). In CCM, current flows through the
inductor through the entire switching cycle and never falls to zero during the off-time. When operating in DCM,
inductor current is maintained to an average value equaling Iout. Inductor current exhibits normal behavior for the
emulated current mode control method used. Output voltage ripple typically increases during this mode of
operation.
Following is a comparison pair of waveforms of the showing both CCM (upper) and DCM operating modes.
Figure 50. CCM and DCM Operating Modes
V
IN
= 12V, V
O
= 3.3V, I
O
= 3A/0.3A 2 μsec/div
The approximate formula for determining the DCM/CCM boundary is as follows:
I
DCB
≊V
O
*(V
IN
–V
O
)/(2*3.3 μH*f
SW(CCM)
*V
IN
) (13)
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