Texas Instruments | Enable Functionality and Adjusting Under Voltage Lockout for TPS57112-Q1 Device | Application notes | Texas Instruments Enable Functionality and Adjusting Under Voltage Lockout for TPS57112-Q1 Device Application notes

Texas Instruments Enable Functionality and Adjusting Under Voltage Lockout for TPS57112-Q1 Device Application notes
Application Report
SLVA784 – June 2016
Enable Functionality and Adjusting Undervoltage Lockout
for TPS57112-Q1
....................................................................................................................................... MSA-ASP
ABSTRACT
This document describes the TPS57112-Q1 device Enable pin functionality and provides possible risks
associated with driving the Enable (EN) pin voltage higher than VIN pin voltage. This application note is
also applicable for TPS57114-Q1 and TPS54388-Q1 devices as they belong to the same family as
TPS57112-Q1.
1
Introduction
The TPS57112-Q1 device is a full-featured 6-Vin, 2-A, synchronous step-down current-mode converter
with two integrated metal–oxide–semiconductor field-effect transistors (MOSFETs). When the EN pin is
left floating, the device turns off when the input voltage falls below 2.6 V. If an application requires a
higher undervoltage lockout (UVLO), use the EN pin as shown in Figure 1 to adjust the input voltage
UVLO by using two external resistors. The TPS57112-Q1 datasheet, SLVSAL8 (Enable Functionality and
Adjusting Undervoltage Lockout section) provides the detailed design procedure to calculate the values of
R1 and R2.
When the enable undervoltage lockout is set using this method, it ensures that the EN pin voltage is less
than the VIN voltage. This is the recommended method of driving the EN pin, if higher than 2.6 V
undervoltage lockout is desired.
TPS57112-Q1
I(hys)
VIN
1.6 mA
I(1)
R1
1.6 mA
R2
EN
+
-
Figure 1. Adjustable Undervoltage Lockout Using External Voltage Divider to VIN Pin
Some applications, like driving EN pin using CAN inhibit signal or driving EN pin through some external
logic, may require using digital control for the EN pin. In such cases, there is a possibility that the EN pin
voltage is higher than the VIN voltage. This application note explains the risk of driving the EN pin voltage
higher than the VIN voltage and provides recommendations to address this issue.
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Enable Functionality and Adjusting Undervoltage Lockout for TPS57112-Q1
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1
Interaction Between EN Pin and VIN Pin
2
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Interaction Between EN Pin and VIN Pin
Internally, the EN pin is connected via a 10-kΩ resistor and a p-channel metal-oxide-semiconductor
(PMOS) to the VIN supply, as shown in Figure 2. When the EN pin voltage is greater than a diode drop
above VIN (0.5 V, typical), the body diode of the PMOS becomes forward biased causing the EN pin to
provide the supply for the TPS57112-Q1 device. Due to this incorrect drive, when the EN pin voltage is
larger than VIN by around 1.3 V, typical, the device will shut down. When the VEN – VVIN voltage is
between 1.1 V and 1.3 V, the device may enter into hiccup mode due to the UVLO comparator hysteresis
as shown in Figure 3.
Therefore, it is always advisable to have the EN voltage to be below the VIN voltage under all conditions
to avoid incorrect biasing of the device.
VIN
Current flow when
VEN > VVIN + 0.5 V
EN
Voltage derived from VIN
generates PMOS gate
bias voltage
PMOS
Current bias to pull up
voltage on EN pin
Figure 2. Internal Drive Structure of EN and VIN Pins
C1 (Yellow trace): Output voltage
C2 (Red trace): SW pin voltage
C3 (Blue trace): Input voltage
C4 (Green trace): EN pin voltage
Figure 3. Vout Oscillation Observed on TPS57112-Q1 EVM With VVIN = 3 V, VEN = 4.1 V, Vout = 1.8 V
2
Enable Functionality and Adjusting Undervoltage Lockout for TPS57112-Q1
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Impact of Product Change Notification #PCN20140408000A on This Behavior
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3
Impact of Product Change Notification #PCN20140408000A on This Behavior
For device material which incorporates the product change notification PCN20140408000A, circuitry is
added to clamp the EN voltage to around 2.8 V internally as shown in Figure 4. With this change, it was
observed that the device does not shut down when VEN > VVIN by 1.3 V. Also, it does not show any
unstable behavior.
This was tested using the TPS57112-Q1 EVM with VVIN = 3 V, VEN = 4.1 V to 4.5 V, Vout = 1.8 V. Even
with material that includes this PCN, it is not recommended to drive the EN pin with a voltage higher than
the VIN pin.
VIN
Voltage derived from VIN
generates PMOS gate
bias voltage
PMOS
Current bias to pull up
voltage on EN pin
EN
Transistor to clamp
EN voltage to around
2.8 V
Figure 4. Internal Drive Structure of EN and VIN Pins With PCN20140408000A
4
Conclusion
When it is necessary to use the undervoltage lockout voltage higher than the default UVLO voltage, TI
recommends the use of a resistor divider to VIN as shown in Figure 1. If this method cannot be used,
customers should make sure that EN pin voltage is less than the VIN voltage.
SLVA784 – June 2016
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Enable Functionality and Adjusting Undervoltage Lockout for TPS57112-Q1
Copyright © 2016, Texas Instruments Incorporated
3
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