TPS54611, TPS54612
TPS54613, TPS54614
TPS54615, TPS54616
SLVS400C AUGUST 2001 REVISED APRIL 2005
www.ti.com
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DETAILED DESCRIPTION
Under Voltage Lock Out (UVLO)
The TPS5461x incorporates an under voltage lockout
circuit to keep the device disabled when the input voltage
(VIN) is insufficient. During power up, internal circuits are
held inactive until VIN exceeds the nominal UVLO
threshold voltage of 2.95 V. Once the UVLO start threshold
is reached, device start-up begins. The device operates
until VIN falls below the nominal UVLO stop threshold of
2.8 V. Hysteresis in the UVLO comparator, and a 2.5-s
rising and falling edge deglitch circuit, reduces the
likelihood of shutting the device down due to noise on VIN.
Slow-Start/Enable (SS/ENA)
The slow-start/enable pin provides two functions. First, the
pin acts as an enable (shutdown) control by keeping the
device turned off until the voltage exceeds the start
threshold voltage of approximately 1.2 V. When SS/ENA
exceeds the enable threshold, device start up begins. The
reference voltage fed to the error amplifier is linearly
ramped up from 0 V to 0.891 V in 3.35 ms. Similarly, the
converter output voltage reaches regulation in
approximately 3.35 ms. Voltage hysteresis and a 2.5-s
falling edge deglitch circuit reduce the likelihood of
triggering the enable due to noise. Refer to the following
table for start up times for each device
DEVICE
OUTPUT VOLTAGE
SLOW-START
TPS54611
0.9 V
3.3 ms
TPS54612
1.2 V
4.5 ms
TPS54613
1.5 V
5.6 ms
TPS54614
1.8 V
3.3 ms
TPS54615
2.5 V
4.7 ms
TPS54616
3.3 V
6.1 ms
The second function of the SS/ENA pin provides an
external means for extending the slow-start time with a
ceramic capacitor connected between SS/ENA and
AGND. Adding a capacitor to the SS/ENA pin has two
effects on start-up. First, a delay occurs between release
of the SS/ENA pin and start-up of the output. The delay is
proportional to the slow-start capacitor value and lasts until
the SS/ENA pin reaches the enable threshold. The
start-up delay is approximately:
td + C(SS)
1.2 V
5 mA
Second, as the output becomes active, a brief ramp up at
the internal slow-start rate may be observed before the
externally set slow-start rate takes control and the output
rises at a rate proportional to the slow-start capacitor. The
slow-start time set by the capacitor is approximately:
t(SS) + C(SS)
0.7 V
5 mA
The actual slow-start time is likely to be less than the above
approximation due to the brief ramp up at the internal rate.
VBIAS Regulator
The VBIAS regulator provides internal analog and digital
blocks with a stable supply voltage over variations in
junction temperature and input voltage. A high quality,
low-ESR, ceramic bypass capacitor is required on the
VBIAS pin. X7R or X5R grade dielectrics are
recommended because their values are more stable over
temperature. The bypass capacitor should be placed close
to the VBIAS pin and returned to AGND.
External loading on VBIAS is allowed, with the caution that
internal circuits require a minimum VBIAS of 2.7 V, and
external loads on VBIAS with ac or digital switching noise
may degrade performance. The VBIAS pin may be useful
as a reference voltage for external circuits.
Voltage Reference
The voltage reference system produces a precise,
temperature-stable voltage from a bandgap circuit. A
scaling amplifier and DAC are then used to produce the
reference voltages for each of the fixed output devices.
Oscillator and PWM Ramp
The oscillator frequency can be set to internally fixed
values of 350 kHz or 550 kHz using the FSEL pin as a static
digital input. If a different frequency of operation is required
for the application, the oscillator frequency can be
externally adjusted from 280 kHz to 700 kHz by connecting
a resistor from the RT pin to AGND and floating the FSEL
pin. The switching frequency is approximated by the
following equation, where R is the resistance from RT to
AGND:
Switching Frequency + 100 kW
R
500 [kHz]
The following table summarizes the frequency selection
configurations:
SWITCHING FREQUENCY
SYNC PIN
RT PIN
350 kHz, internally set
Float or AGND Float
550 kHz, internally set
≥2.5 V
Float
Externally set 280 kHz to
700 kHz
Float
R = 180 k to 68 k
Error Amplifier
The high performance, wide bandwidth, voltage error
amplifier is gain-limited to provide internal compensation
of the control loop. The user is given limited flexibility in
choosing output L and C filter components. Inductance
values of 4.7 H to 10 H are typical and available from
several vendors. The resulting designs exhibit good noise
(2)
(3)
(4)