LDO Regulator with Soft Start or Tracking

elektor - 7-8/2007
LDO Regulator
with Soft Start or Tracking
Dirk Gehrke
(Texas Instruments Germany)
The devices described here are tailored
to fit the needs of modern
DSPs, processors and FPGAs that
require low supply voltages at relatively
high currents, plus the capability
of supply voltage ramp-up and
sequencing in a defined manner. The
latter two requirements are not easily
solved using discrete components.
The TPS74x01 family fits the bill.
At the time of writing the family consists
of three parts, called TPS74201
(1.5 A with soft-start), TPS74301
(1.5 A with tracking) and TPS74401
(3 A with soft-start). The family is
expected to grow soon.
Figure 1 shows a simplified connection
diagram for the three
devices. These regulators require a
low power bias voltage, VBIAS, and a
power input voltage, VIN, from which
Vout will be derived. All three regulators
are capable of providing output
voltages down to 0.8 V and the
device in the QFN package also includes
an integrated supervisory circuit with opendrain
output that goes to high impedance
when the output voltage reaches regulation
(‘power good’ or PG). The TPS74301
can provide up to 1.5 A output current and
has a TRACK pin which allows the user to
input a ramp signal for the output voltage
to follow, effectively implementing either
simultaneous or ratiometric sequencing.
The relevant connections are shown separately.
The TPS74201 and TPS74401 can
provide up to 1.5 A and 3.0 A DC current,
respectively, and have an SS pin which
allows the user to set the linear ramp rate
of the output voltage.
For the TPS74301, the value of R4 (in a voltage
divider) allows the user to select either
simultaneous or ratiometric sequencing.
R4’s value is calculated from the equations
in the datasheet, assuming an external 3.3-
V ramp signal is applied to the TRACK IN
pin. When the value of R4 selected to be
10 kΩ in this particular example, the output
of the TPS74301 will follow the external
ramp signal within a few millivolts until
the TPS74301 reaches its regulated voltage.
This is called Simultaneous Sequencing
(Figure 2). The TPS74301 output voltage
will have the same rate of rise (dv/dt)
as the external ramp signal but a different
soft-start time.
Changing resistor R4 to a value of 1.78 kΩ
results in the TPS74301 output voltage
reaching its regulated voltage at the same
time the externally applied tracking signal
reaches its maximum voltage (e.g., 3.3 V).
This is called Ratiometric Sequencing (Figure
3). Although the external ramp signal
and the TPS74301 will have different rates
of rise, they will have the same softstart
time.
For the TPS74201 and TPS74401
the capacitor value can be used to
program the soft-start ramp. In this
example, values of 1 nF and 10 nF
for C4 were used to realize ramps
with 1 ms and 10 ms rise time
respectively (Figures 4 and 5 respectively).
With no capacitor attached
to this pin the default soft-start time
will be 500 μs. This 500-μs start up
time is also valid for the TPS74301
when applying a voltage greater than
800 mV to the TRACK pin.
(070231-I)
Literature
TPS74401 3.0A Ultra-Low Dropout Linear
Regulator, Texas Instruments Literature
# SBVS066C.
TPS74301 1.5A Ultra-Low Dropout Linear
Regulator with Programmable
Sequencing, Texas Instruments Literature
# SBVS065C.
TPS74201 1.5A LDO with Programmable
Soft-Start, Texas Instruments Literature
# SBVS064C
TPS74x01EVM-118 User’s Guide, Texas Instruments
Literature # SLVU143.
Sequencing Power Supplies in Multiple Voltage
Rail Environments by David Daniels, Dirk
Gehrke and Mike Segal, Texas Instruments
Literature # SLUP228 or http://focus.ti.com/lit/
ml/slup228/slup228.pdf
C3 C4
4μ7
C1
1μ
C2
1μ
TPS74201
TPS74401
IC1
BIAS
OUT
GND
FB
IN
EN
SS
5 3
4
7
6
1 2
R1
2k49
R2
4k99
UIN UOUT
UBIAS
*
C3
4μ7
C1
1μ
C2
1μ
R1
2k49
R2
4k99
UIN UOUT
UBIAS
*
R4
R3
4k99
TPS74301
TRACK
IC1
BIAS
OUT
GND
FB
IN
EN
5 3
4
7
6
1 2
TRACK
IN
070231 - 11
*see text
Figure 1. Schematic of TPS74201, TPS74301 and TPS74401
(see text for values of R4 and C4).
Figure 2. Tracking LDO TPS74301 with
simultaneous startup (R4 = 10kΩ).
Timebase: 2 mV/div.
Figure 3. Tracking LDO TPS74301with
ratiometric startup (R4 = 1.78kΩ).
Timebase: 2 mV/div.
Figure 4. Soft-Start LDO TPS74201 and TPS74401
with 1ms soft startup (C4 = 1nF).
Timebase: 2 mV/div.
Figure 5. Soft-Start LDO TPS74201 and TPS74401
with 10ms soft startup (C4 = 10nF).
Timebase: 2 mV/div.

Khai trương phòng thí nghiệm Edison

Chào mừng khai trương phòng thí nghiệm Edison