Datasheet

ss ref
ss
ss
C (nF) V (V)
t (ms)
I ( A)
´
=
m
TPS5402
www.ti.com SLVSBK4 –SEPTEMBER 2012
Pulse Skipping Mode
The TPS5402 is designed to operate in pulse skipping mode at light load currents to boost light load efficiency.
When the peak inductor current is lower than 300 mA typically, the COMP pin voltage falls to 0.5 V typically and
the device enters pulse skipping mode. When the device is in pulse skipping mode, the COMP pin voltage is
clamped at 0.5 V internally which prevents the high side integrated MOSFET from switching. The peak inductor
current must rise above 300 mA for the COMP pin voltage to rise above 0.5 V and exit pulse skipping mode.
Since the integrated current comparator catches the peak inductor current only, the average load current
entering pulse skipping mode varies with the applications and external output filters.
Voltage Reference (V
SENSE
)
The voltage reference system produces a ±3% accuracy voltage reference by scaling the output of a temperature
stable bandgap circuit. The typical voltage reference is designed at 0.8 V.
Bootstrap Voltage (BOOT)
The TPS5402 has an integrated boot regulator and requires a 0.1-µF ceramic capacitor between the BOOT and
LX pins to provide the gate drive voltage for the high-side MOSFET. A ceramic capacitor with an X7R or X5R
grade dielectric is recommended because of the stable characteristics over temperature and voltage. To improve
drop out, the TPS5402 is designed to operate at 100% duty cycle as long as the BOOT to LX pin voltage is
greater than 2.1 V typically.
Programmable Slow Start Using SS Pin
It is recommended to program the slow start time externally because no slow start time is implemented internally.
The TPS5402 effectively uses the lower voltage of the internal voltage reference or the SS pin voltage as the
power supply’s reference voltage fed into the error amplifier and will regulate the output accordingly. A capacitor
(C
SS
) on the SS pin to ground implements a slow start time. The TPS5402 has an internal pull-up current source
of 2 μA that charges the external slow start capacitor. The equation for the slow start time (10% to 90%) is
shown in Equation 2. The internal V
ref
is 0.8 V and the I
SS
current is 2 μA.
(2)
The slow start time should be set between 1 ms to 10 ms to ensure good start-up behavior. The slow start
capacitor should be no more than 27 nF.
If during normal operation, the input voltage drops below the VIN UVLO threshold, or a thermal shutdown event
occurs, the TPS5402 stops switching.
Error Amplifier
The TPS5402 has a transconductance amplifier for the error amplifier. The error amplifier compares the
VSENSE voltage to the internal effective voltage reference presented at the input of the error amplifier. The
transconductance of the error amplifier is 92 μA/V during normal operation. Frequency compensation
components are connected between the COMP pin and ground.
Slope Compensation
To prevent the sub-harmonic oscillations when operating the device at duty cycles greater than 50%, the
TPS5402 adds a built-in slope compensation which is a compensating ramp to the switch current signal.
Overcurrent Protection and Frequency Shift
The TPS5402 implements current mode control that uses the COMP pin voltage to turn off the high-side
MOSFET on a cycle by cycle basis. Every cycle the switch current and the COMP pin voltage are compared;
when the peak inductor current intersects the COMP pin voltage, the high-side switch is turned off. During
overcurrent conditions that pull the output voltage low, the error amplifier responds by driving the COMP pin high,
causing the switch current to increase. The COMP pin has a maximum clamp internally, which limits the output
current.
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