Datasheet

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0.8 V
R2 R1
V 0.8 V
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TPS57112-Q1
SLVSAL8 –DECEMBER 2010 www.ti.com
DETAILED DESCRIPTION
FIXED FREQUENCY PWM CONTROL
The TPS57112-Q1 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
COMP voltage level the high side power switch is turned off and the low side power switch is turned on. The
COMP pin voltage increases and decreases as the output current increases and decreases. The device
implements a current limit by clamping the COMP pin voltage to a maximum level and also implements a
minimum clamp for improved transient response performance.
SLOPE COMPENSATION AND OUTPUT CURRENT
The TPS57112-Q1 adds a compensating ramp to the switch current signal. This slope compensation prevents
sub-harmonic oscillations as duty cycle increases. The available peak inductor current remains constant over the
full duty cycle range.
BOOTSTRAP VOLTAGE (BOOT) AND LOW DROPOUT OPERATION
The TPS57112-Q1 has an integrated boot regulator and requires a small ceramic capacitor between the BOOT
and PH pin to provide the gate drive voltage for the high side MOSFET. The value of the ceramic capacitor
should be 0.1 mF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher
is recommended because of the stable characteristics over temperature and voltage.
To improve drop out, the TPS57112-Q1 is designed to operate at 100% duty cycle as long as the BOOT to PH
pin voltage is greater than 2.2 V. The high side MOSFET is turned off using an UVLO circuit, allowing for the low
side MOSFET to conduct when the voltage from BOOT to PH drops below 2.2 V. Since the supply current
sourced from the BOOT pin is low, the high side MOSFET can remain on for more switching cycles than are
required to refresh the capacitor, thus the effective duty cycle of the switching regulator is high.
ERROR AMPLIFIER
The TPS57112-Q1 has a transconductance amplifier. The error amplifier compares the VSENSE voltage to the
lower of the SS/TR pin voltage or the internal 0.800 V voltage reference. The transconductance of the error
amplifier is 245mA/V during normal operation. When the voltage of VSENSE pin is below 0.800 V and the device
is regulating using the SS/TR voltage, the gm is typically greater than 79 mA/V, but less than 245 mA/V. The
frequency compensation components are placed between the COMP pin and ground.
VOLTAGE REFERENCE
The voltage reference system produces a precise ±1% voltage reference over temperature by scaling the output
of a temperature-stable bandgap circuit. The bandgap and scaling circuits produce 0.800 V at the non-inverting
input of the error amplifier.
ADJUSTING THE OUTPUT VOLTAGE
The output voltage is set with a resistor divider from the output node to the VSENSE pin. It is recommended to
use divider resistors with 1% tolerance or better. Start with a 100 kΩ for the R1 resistor and use the Equation 1
to calculate R2. To improve efficiency at light loads consider using larger value resistors. If the values are too
high the regulator is more susceptible to noise and voltage errors from the VSENSE input current are noticeable.
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