Texas Instruments | TPS65320C-Q1 Auto-Source Feature | Application notes | Texas Instruments TPS65320C-Q1 Auto-Source Feature Application notes

Texas Instruments TPS65320C-Q1 Auto-Source Feature Application notes
Application Report
SLVA808 – July 2016
TPS65320C-Q1 Auto-Source Feature
Frank Dehmelt.................................................................................................................... MSA-ASP
ABSTRACT
This application report provides a summary of the auto-source feature of the TPS65320C-Q1 device.
1
TPS65320C-Q1 Auto-Source Feature
In a variety of applications, during normal operation a supply current in the ampere-range is required,
while during stand-by, only minimal current is needed. For efficiency and thermal reasons, the normal
operation calls for a Buck-converter, while the switching losses (even in low-power mode) are undesired.
However, operating a linear regulator straight off the battery is very inefficient and creates high powerdissipation. This application note discusses how the TPS65320C-Q1 device addresses this controversy
with the auto-source function.
The TPS65320C-Q1 device is a combination of a 36-V, 3.2-A, DC-DC step-down converter and a lowdropout (LDO) regulator. The DC-DC step-down converter, referred to as Buck, has an integrated highside MOSFET.
A typical configuration uses the buck-converter to drive the power-hungry domains and peripherals, while
the LDO supports the real-time-functions, which generally need very low-supply currents. During normal
operation, the LDO uses the output of the buck-converter as an input supply (Figure 1, left side). Due to
the reduction in supply voltage by the buck, the drop-out of the linear regulator is reduced and thus the
losses are minimized. In case of standby, the domains driven by the buck regulator may not be required
and potentially can remain unpowered. Hence, the buck will be disabled. Without an auto-source feature,
the LDO would therefore be unsupplied and cannot serve its purpose to keep the real-time domains alive.
The auto-source feature, however, will automatically change the LDO-supply from the Buck output
(connected to VIN_LDO) to VIN as soon as the buck is disabled (or out of regulation) (Figure 1, right side).
VIN_LDO
Automotive Battery
BOOT
5-V Preregulator
VIN
KL15 ON
VIN_LDO BOOT
Automotive Battery
5-V Preregulator
VIN
SW
SW
FB1
EN1
KL15 OFF
FB1
EN1
LDO Input
MUX
LDO Input
MUX
KL30 Always ON
KL30 Always ON
EN2
EN2
3.3-V Standby MCU
LDO_OUT
LDO_OUT
Regulator
Control
Regulator
Control
GND
3.3-V Standby MCU
GND
FB2
Full Running Mode
FB2
Standby Mode
Figure 1. LDO Sourced by Buck-Output (Left), LDO Sourced by VIN (Right)
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SLVA808 – July 2016
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TPS65320C-Q1 Auto-Source Feature
Copyright © 2016, Texas Instruments Incorporated
1
Revision History
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In this mode of operation, the LDO has to drop the entire voltage difference between the battery voltage
and the output voltage, but this concern is minimal as the stand-by current is typically very low.
Furthermore, since the buck-regulator is disabled, there will be no switching losses and associated
dissipation during this operation mode, so the entire thermal dissipation path is available to the linear
regulator and its losses.
The LDO also features a tracking function: At low input voltages, the regulator drops out of regulation and
the output voltage tracks input minus a drop-out voltage. This feature allows for a smaller input capacitor
and possibly can eliminate the need to use a boost converter during cold-crank conditions.
One intrinsic short-coming of the cascading configuration of the buck supplying the LDO is that the buck
output voltage needs to be higher than the LDO output voltage. In the given example, this is the case and
no issue. However, in different scenarios, customers may want to use the buck to supply 3.3-V domains,
while using the LDO, for example, to supply the CAN-transceiver with 5 V, and the approach/solution with
TPS65320C-Q1 does not satisfy the system requirements. For such applications, TI developed the
TPS65321-Q1, which does not have the auto-source feature, but has a fully 36-V rated VIN_LDO input
that may attach directly to the battery. In this configuration, the buck and the linear regulator are likely
operating simultaneously and the LDO potentially has a high drop-out voltage, so the thermal
consideration is essential.
The TPS65320C-Q1 device is an ideal solution for applications that demand a high power-rail (up to 3.2
A) at a higher voltage and a low-power rail (up to 280 mA) at a lower voltage, in particular if the highpower rail may not be available at all times. In case the high-power rail has a lower voltage than the LDOoutput, the TPS65321-Q1 device can support those applications.
Revision History
2
DATE
REVISION
NOTES
July 2016
*
Initial release
Revision History
SLVA808 – July 2016
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