TPS54120 Datasheet, PDF (19/31 Pages) Texas Instruments – Low-Noise, 1-A Power Supply

TPS54120
www.ti.com
SBVS180C – JANUARY 2012 – REVISED JUNE 2012
The EN pin has a small pull-up current (I_p) that sets the state of the pin to enable (default) when no external
components are connected. The pull-up current is also used to control the voltage hysteresis for the UVLO
function because it increases by I_h when the EN pin crosses the enable threshold. The UVLO thresholds can be
calculated using Equation 7 and Equation 8.
R1 =
V_START
æ V_ENFALLING ö
ç
è
V_ENRISING
÷
ø
-
V_STOP
I_p
æç_{1 -}
è
V_ENFALLING
V_ENRISING
ö
÷
+
I_h
ø
(7)
R2 =
R1´ V_ENFALLING
V_STOP - V_ENFALLING + R1(I_p + I_h )
(8)
LDO ENABLE AND UNDERVOLTAGE LOCKOUT
The LDO enable pin (LDOEN) is active-high and compatible with standard and low-voltage, TTL-CMOS levels.
When shutdown capability is not required, EN can be connected to IN.
The LDO also has a fixed UVLO to keep the output shut off until the LDO internal circuitry is working properly.
The LDO UVLO circuit has a deglitch feature that ignores undershoot transients on the LDO input if they are less
than 50 µs in duration.
SEQUENCING
The TPS54120 is easy to use and suited for applications that require tracking and sequencing. It has a built-in
power good function to indicate the status of the device, a soft-start circuit to control the output voltage slope
during start-up, noise reduction with start-up time for the LDO, and an enable function for independently
controlling the start-up of both the LDO and the switcher. Each of these functions is useful for tracking and
sequencing applications. See Application Report SLVA497, TPS54120 Sequencing and Tracking, for more
details regarding the sequencing application setup of the TPS54120.
SWITCHER PWM CONTROL AND CONTINUOUS CURRENT MODE OPERATION (CCM)
The integrated switcher of the TPS54120 uses adjustable, fixed-frequency, peak-current mode control. The
output voltage is compared through external resistors on the VSENSE pin to an internal voltage reference by an
error amplifier that drives the COMP pin. An internal oscillator turns on the high-side power switch. The error
amplifier output is converted into a current reference that is compared to the high-side power switch current.
When the power-switch current reaches the current reference generated by the COMP voltage level, the high-
side power switch is turned off and the low-side power switch is turned on. The device normally works in
continuous conduction mode (CCM) under all load conditions.
Copyright © 2012, Texas Instruments Incorporated
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