100 k

Switching Frequency

500 [kHz]

R

W

=

´

TPS54311, TPS54312, TPS54313

TPS54314, TPS54315, TPS54316

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SLVS416C – FEBRUARY 2002 – REVISED JANUARY 2015

7.3.5 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 to the RT pin to ground and floating the

FSEL pin. The switching frequency is approximated by the following equation, where R is the resistance from RT

to AGND:

(3)

Table 2. Summary of the Frequency Selection Configurations

SWITCHING FREQUENCY

FSEL 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 = 68 k

Ω to 180 kΩ

7.3.6 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 µH to 10 µH are typical and available from several vendors. The resulting designs exhibit good

noise and ripple characteristics, along with exceptional transient response. Transient recovery times are typically

in the range of 10 to 20 µs.

7.3.7 PWM Control

Signals from the error amplifier output, oscillator, and current limit circuit are processed by the PWM control logic.

Referring to the internal block diagram, the control logic includes the PWM comparator, OR gate, PWM latch,

and portions of the adaptive dead-time and control logic block. During steady-state operation below the current

limit threshold, the PWM comparator output and oscillator pulse train alternately reset and set the PWM latch.

Once the PWM latch is set, the low-side FET remains on for a minimum duration set by the oscillator pulse

duration. During this period, the PWM ramp discharges rapidly to its valley voltage. When the ramp begins to

charge back up, the low-side FET turns off and high-side FET turns on. As the PWM ramp voltage exceeds the

error amplifier output voltage, the PWM comparator resets the latch, thus turning off the high-side FET and

turning on the low-side FET. The low-side FET remains on until the next oscillator pulse discharges the PWM

ramp

During transient conditions, the error amplifier output could be below the PWM ramp valley voltage or above the

PWM peak voltage. If the error amplifier is high, the PWM latch is never reset and the high-side FET remains on

until the oscillator pulse signals the control logic to turn the high-side FET off and the low-side FET on. The

device operates at its maximum duty cycle until the output voltage rises to the regulation set-point, setting

continually reset and the high-side FET does not turn on. The low-side FET remains on until the VSENSE

voltage decreases to a range that allows the PWM comparator to change states. The TPS5431x is capable of

sinking current continuously until the output reaches the regulation set-point.

If the current limit comparator trips for longer than 100 ns, the PWM latch resets before the PWM ramp exceeds

the error amplifier output. The high-side FET turns off and low-side FET turns on to decrease the energy in the

output inductor and consequently the output current. This process is repeated each cycle in which the current

limit comparator is tripped.

7.3.8 Dead-Time Control and MOSFET Drivers

Adaptive dead-time control prevents shoot-through current from flowing in both N-channel power MOSFETs

during the switching transitions by actively controlling the turnon times of the MOSFET drivers. The high-side

driver does not turn on until the voltage at the gate of the low-side FET is below 2 V. The high-side and low-side

drivers are designed with 300 mA source and sink capability to quickly drive the power MOSFETs gates. The

low-side driver is supplied from VIN, while the high-side drive is supplied from the BOOT pin. A bootstrap circuit

uses an external BOOT capacitor and internal 2.5-

Ω bootstrap switch connected between the VIN and BOOT

pins. The integrated bootstrap switch improves drive efficiency and reduces external component count.

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