Datasheet

www.ti.com
APPLICATION INFORMATION
EXTERNAL CAPACITOR REQUIREMENTS
OVERCURRENT
OC RESPONSE
TPS2070
TPS2071
SLVS287B – SEPTEMBER 2000 – REVISED SEPTEMBER 2007
A 0.1- μ F ceramic bypass capacitor and a 10- μ F bulk capacitor between BP and AGND, close to the device, are
recommended. Similarly, a 0.1- μ F ceramic and a 68- μ F bulk capacitor, from SP to AGND, and from VEXT to
AGND if an external 5-V LDO is required, are recommended because of much higher current in the self-powered
mode.
From each of the outputs (OUTx) to ground, a 33- μ F or higher-valued bulk capacitor is recommended when the
output load is heavy. This precaution reduces power-supply transients. Additionally, bypassing the outputs with a
0.1- μ F ceramic capacitor improves the immunity of the device to short-circuit transients.
An output capacitor connected between 3.3V_OUT and GND is required to stabilize the internal control loop. The
internal LDO is designed for a capacitor range of 4.7 μ F to 33 μ F with an ESR of 0.2 Ω to 10 Ω . Solid
tantalum-electrolytic, aluminum-electrolytic and multilayer ceramic capacitors are all suitable.
Ceramic capacitors have different types of dielectric material, each exhibiting different temperature and voltage
variations. The most common types are X5R, X7R, Y5U, Z5U, and NPO. The NPO-type ceramic capacitors are
generally the most stable over temperature. However, the X5R and X7R are also relatively stable over
temperature (with the X7R being the more stable of the two) and are therefore acceptable for use. The Y5U and
Z5U types provide high capacitance in a small geometry, but exhibit large variations over temperature. For this
reason, the Y5U and Z5U are not generally recommended.
A transient condition occurs because of a sudden increase in output current. The output capacitor reduces the
transient effect by providing the additional current needed by the load. Depending on the current demand at the
output, a voltage drop occurs across the internal resistance, ESR, of the capacitor. Using a low-ESR capacitor
helps minimize this voltage drop. A larger capacitor also reduces the voltage drop by supplying the current
demand for a longer time, versus that provided by a smaller capacitor.
An internal sense FET checks for overcurrent conditions. Unlike current-sense resistors, sense FETs do not
increase the series resistance of the current path. When an overcurrent condition is detected, the device
maintains a constant output current and reduces the output voltage accordingly. Complete shutdown occurs only
if the fault is present long enough to activate thermal limiting.
Three possible overload conditions can occur. In the first condition, the output has been shorted before the
device is enabled or before BP and SP have been applied. The TPS2070 and TPS2071 sense the short and
immediately switch into a constant-current output.
In the second condition, the short occurs while the device is enabled. At the instant the short occurs, very high
currents may flow for a very short time before the current-limit circuit can react. After the current-limit circuit has
tripped (reached the overcurrent trip threshold), the device switches into constant-current mode.
In the third condition, the load has been gradually increased beyond the recommended operating current. The
current is permitted to rise until the current-limit threshold is reached or until the thermal limit of the device is
exceeded. The TPS2070 and TPS2071 are capable of delivering current up to the current-limit threshold without
damaging the device. Once the threshold has been reached, the device switches into its constant-current mode.
The OCx output is asserted (active-low) when an overcurrent or overtemperature condition is encountered and
remains asserted until the overcurrent or overtemperature condition is removed. Connecting a heavy capacitive
load to an enabled device can cause momentary false overcurrent reporting from the inrush current flowing
through the device and charging the downstream capacitor. The TPS2070 and TPS2071 are designed to reduce
false overcurrent reporting by implementing an internal deglitch circuit. This circuit eliminates the need for an
external filter, which requires extra components. Also, using low-ESR electrolytic capacitors on the outputs can
reduce erroneous overcurrent reporting by providing a low-impedance energy source to lower the inrush current
flow through the device during hot-plug events. The OCx outputs are logic outputs, thereby requiring no pullup or
pulldown resistors.
22 Submit Documentation Feedback Copyright © 2000 – 2007, Texas Instruments Incorporated
Product Folder Link(s): TPS2070 TPS2071