Global Mixed-mode Technology Inc.
Description
VTT SINK/SOURCE REGULATOR
The G2997 is a 3A sink/source tracking termination
regulator designed specially for low-cost, low external
components system where space is at premium such
as notebook PC applications. The G2997 integrates
high performance low-dropout linear regulator that is
capable of sourcing and sinking current up to 3 A. This
VTT linear regulator is implemented with ultimately
fast response feedback loop so that small ceramic
capacitors are enough to keep tracking to the VTTREF
within 40mV at all conditions including fast load tran-
sient. To achieve tight regulation with minimum effect
of trace resistance, a remote sensing terminal,
VTTSNS, should be connected to the positive node of
VTT output capacitor as a separate trace from the high
current line from VTT.
VTTREF REGULATOR
The VTTREF block consists of an on-chip 1/2 divider,
LPF and buffer. This regulator can source/sink current
up to 10mA. Bypass VTTREF to GND using a 0.1µF
ceramic capacitor to ensure stable operation. To en-
sure better start-up and transient performance, the
larger 1µF ceramic capacitors are recommended.
Power on sequence
To operate safely, the G2997 must keep VIN voltage
larger than other Input-PIN voltage .This condition is
due to the internal parasitic diodes between VIN to
others PIN for ESD issue. If the V
IN
voltage is lower
than the other pins voltage, the G2997 will consume
extra current through the parasitic diodes during the
power-on period.
Soft-Start
The soft-start function of the VTT is achieved via a
current clamp, allowing the output capacitors to be
charged with low and constant current that gives linear
ramp up of the output voltage. The current limit
threshold is changed in two stages using an internal
powergood signal. When VTT is outside the power-
good threshold, the current limit level is 2A. When VTT
rises above (VTTREF - 5%) or falls below (VTTREF +
5%) the current limit level switches to 4 A. The thresh-
olds are typically VTTREF
5% (from outside regula-
tion to inside) and 10% (when it falls outside). The
soft-start function is completely symmetrical and it
works not only from GND to VTTREF voltage, but also
from VDDQSNS to VTTREF voltage. Note that the
VTT output is in a high impedance state during the S3
state (S3 = low, S5 = high) and its voltage can be up
to VDDQSNS voltage depending on the external con-
dition. Note that VTT does not start under a full load
condition.
G2997
S3, S5 Control and Soft-Off
The S3 and S5 terminals should be connected to
SLP_S3 and SLP_S5 signals respectively. Both
VTTREF and VTT are turned on at normal state (S3 =
high, S5 = high). VTTREF is kept alive while VTT is
turned off and left high impedance in standby state (S3
= low, S5 = high). Both VTT and VTTREF outputs are
turned off and discharged to the ground through inter-
nal MOSFETs during shutdown state (S5 = low). The
control function is showed on the Table 1.
Table 1. S3 and S5 Control Table
STATE
Normal
Standby
Shutdown
Shutdown
S3
Hi
Lo
Lo
Hi
S5
Hi
Hi
Lo
Lo
VTT
1.25V/0.9V
12mV/6mV
(High-Z)
0V
(Discharge)
0V
(Discharge)
VREF
1.25V/0.9V
1.25V/0.9V
0V
(Discharge)
0V
(Discharge)
VTT Current Protection
The LDO has a constant overcurrent limit (OCL) at 4A.
This trip point is reduced to 2A before the outputvolt-
age comes within 5% of the target voltage or goes
outside of 10% of the target voltage.
Input Capacitor
Adding a capacitance close to VLDOIN pin can improve
the VTT performance when fast load- transient. In gen-
eral, 1/2 COUT is recommended for the VLDOIN ca-
pacitance. Separating the VDDQSNS and VLDOIN pins
will get better transient performance. The recommended
capacitor types are tabulated in the Table 2.
Output Capacitor
For stable operation, total capacitance of the VTT
output terminal can be equal or greater than 20µF.
The output capacitor should be located near VTT out-
put terminal as close as possible to minimize the effect
of ESR and ESL. The recommended capacitor types
are tabulated in the Table 2.
Table 2.
CAP
C1
C2
MANUF
TAYO
YUDEN
TAYO
YUDEN
PART NUMBER
EMK 325BJ106KD
UMK 212BJ104KG
Ver: 1.1
Jul 06, 2006
TEL: 886-3-5788833
http://www.gmt.com.tw
8
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
