TPS54240_16 Datasheet, PDF (13/56 Pages) Texas Instruments – 3.5-V to 42-V Step-Down DC - DC Converter With Eco-Mode

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TPS54240
SLVSAA6B – APRIL 2010 – REVISED NOVEMBER 2014
7.2 Functional Block Diagram
VSENSE 7
Shutdown
PWRGD
6
UV
Logic
OV
Voltage
Reference
ERROR
AMPLIFIER
EN
3
Enable
Comparator
Thermal
Shutdown
UVLO
Shutdown
Shutdown
Logic
Enable
Threshold
Boot
Charge
Minimum
Clamp
Pulse
Skip
PWM
Comparator
Boot
UVLO
VIN
2
Current
Sense
1 BOOT
SS/TR 4
COMP 8
Shutdown
Logic
And
PWM Latch
Slope
Compensation
Frequency
Shift
10 PH
11 POWERPAD
Overload
Recovery
Maximum
Clamp
Oscillator TPS54240 Block Diagram
with PLL
9 GND
5
RT/CLK
7.3 Feature Description
7.3.1 Fixed Frequency PWM Control
The TPS54240 uses an 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 which drives
the COMP pin. An internal oscillator initiates the turn on of the high-side power switch. The error amplifier output
is compared to the high-side power switch current. When the power switch current reaches the level set by the
COMP voltage, the power switch is turned off. The COMP pin voltage will increase and decrease as the output
current increases and decreases. The device implements a current limit by clamping the COMP pin voltage to a
maximum level. The Eco-Mode is implemented with a minimum clamp on the COMP pin.
7.3.2 Slope Compensation Output Current
The TPS54240 adds a compensating ramp to the switch current signal. This slope compensation prevents sub-
harmonic oscillations. The available peak inductor current remains constant over the full duty cycle range.
7.3.3 Pulse Skip Eco-Mode
The TPS54240 operates in a pulse skip Eco-Mode at light load currents to improve efficiency by reducing
switching and gate drive losses. The TPS54240 is designed so that if the output voltage is within regulation and
the peak switch current at the end of any switching cycle is below the pulse-skipping current threshold, the
device enters Eco-Mode. This current threshold is the current level corresponding to a nominal COMP voltage or
500 mV.
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