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SLUS812 – FEBRUARY 2008
DETAILED DESCRIPTION
VO SINK/SOURCE REGULATOR
The TPS51200 is a sink/source tracking termination regulator specifically designed for low input voltage,
low-cost, and low external component count systems where space is a key application parameter. The
TPS51200 integrates a high-performance, low-dropout (LDO) linear regulator that is capable of both sourcing
and sinking current. The LDO regulator employs a fast feedback loop so that small ceramic capacitors can be
used to support the fast load transient response. To achieve tight regulation with minimum effect of trace
resistance, a remote sensing terminal, VOSNS, should be connected to the positive terminal of the output
capacitor(s) as a separate trace from the high current path from VO.
REFERENCE INPUT (REFIN)
The output voltage, VO, is regulated to REFOUT. When REFIN is configured for standard DDR termination
applications, REFIN can be set by an external equivalent ratio voltage divider connected to the memory supply
bus (VDDQ). The TPS51200 supports REFIN voltage from 0.5 V to 1.8 V, making it versatile and ideal for many
types of low-power LDO applications.
REFERENCE OUTPUT (REFOUT)
When it is configured for DDR termination applications, REFOUT generates the DDR VTT reference voltage for
the memory application. It is capable of supporting both a sourcing and sinking load of 10 mA. REFOUT
becomes active when REFIN voltage rises to 0.390 V and VIN is above the UVLO threshold. When REFOUT is
less than 0.375 V, it is disabled and subsequently discharges to GND through an internal 10-kΩ MOSFET.
REFOUT is independent of the EN pin state.
SOFT-START
The soft-start function of the VO pin is achieved via a current clamp. The current clamp allows the output
capacitors to be charged with low and constant current, providing a linear ramp-up of the output voltage. When
VO is outside of the powergood window, the current clamp level is one-half of the full overcurrent limit (OCL)
level. When VO rises or falls within the PGOOD window, the current clamp level switches to the full OCL level.
The soft-start function is completely symmetrical; it works not only from GND to the REFOUT voltage, but also
from VLDOIN to the REFOUT voltage.
EN CONTROL (EN)
When EN is driven high, the TPS51200 VO regulator begins normal operation. When EN is driven low, VO is
discharges to GND through an internal 18-Ω MOSFET. REFOUT remains on when EN is driven low.
POWERGOOD FUNCTION (PGOOD)
The TPS51200 provides an open-drain PGOOD output that goes high when the VO output is within ±20% of
REFOUT. PGOOD de-asserts within 10
µs
after the output exceeds the size of the powergood window. During
initial VO startup, PGOOD asserts high 2 ms (typ) after the VO enters power good window. Because PGOOD is
an open-drain output, a 100-kΩ, pull-up resistor between PGOOD and a stable active supply voltage rail is
required.
VO CURRENT PROTECTION
The LDO has a constant overcurrent limit (OCL). Note that the OCL level reduces by one-half when the output
voltage is not within the powergood window. This reduction is a non-latch protection.
VIN UVLO PROTECTION
For VIN undervoltage lockout (UVLO) protection, the TPS51200 monitors VIN voltage. When the VIN voltage is
lower than the UVLO threshold voltage, both the VO and REFOUT regulators are powered off. This shutdown is
a non-latch protection.
Copyright © 2008, Texas Instruments Incorporated
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