TPS55340
SLVSBD4 –MAY 2012
www.ti.com
SLOPE COMPENSATION
The TPS55340 has internal slope compensation to prevent sub-harmonic oscillations. The sensed current slope
of boost converter can be expressed as Equation 4:
V
IN
S
=
´R
SENSE
n
L
(4)
The slope compensation dv/dt can be calculated using Equation 5.
0.32 V R
0.5 mA
FREQ
+
16´(1- D)´ 6 pF
S
=
e
6 pF
(5)
In a converter with current mode control, in addition to the output voltage feedback loop, the inner current loop
including the inductor current sampling effect as well as the slope compensation on the small signal response
should be taken into account, which can be modeled as seen in Equation 6:
1
He(s) =
é
ê
ù
ú
æ
ö
S
S
e
n
ç
÷
÷
ø
s´ 1+
´(1- D) - 0.5
ç
ê
2
ú
s
è
ë
û
1+
+
2
f
sw
p´ f
( )
sw
(6)
Where RSENSE (15mΩ) is the equivalent current sense resistor, RFREQ is timing resistor used to set frequency,
and D is the duty cycle.
Note that if Sn << Se, the converter operates in voltage mode control rather than current mode control, and
Equation 6 is no longer valid.
OVER-CURRENT PROTECTION AND FREQUENCY FOLDBACK
The TPS55340 provides cycle-by-cycle over-current protection that turns off the power switch once the inductor
current reaches the over-current limit threshold. The PWM circuitry resets itself at the beginning of the next
switch cycle. During an over-current event, the output voltage begins to droop as a function of the load on the
output. When the FB voltage through the feedback resistors, drops lower than 0.9 V, the switching frequency is
automatically reduced to 1/4 of the normal value. Figure 7 shows the non-foldback frequency with an 80kΩ timing
resistor and the corresponding foldback frequency. The switching frequency does not return to normal until the
over-current condition is removed and the FB voltage increases above 0.9 V. The frequency foldback feature is
disabled during soft-start.
ENABLE AND THERMAL SHUTDOWN
The TPS55340 enters shutdown when the EN voltage is less than 0.68 V (min) for more than 1ms. In shutdown,
the input supply current for the device is less than 10µA (max). The EN pin has an internal 950kΩ pull down
resistor to disable the device if the pin is floating.
An internal thermal shutdown turns off the device when the junction temperature exceeds 165°C (typical). The
device will restart when the junction temperature drops by 15°C.
UNDER-VOLTAGE LOCKOUT (UVLO)
An under-voltage lockout circuit prevents mis-operation of the device at input voltages below 2.5 V (typical).
When the input voltage is below the UVLO threshold, the device remains off and the internal power MOSFET is
turned off. The UVLO threshold is set below minimum operating voltage of 2.9 V to ensure that a transient VIN
dip will not cause the device to reset. For the input voltages between UVLO threshold and 2.9 V, the device
attempts to operate, but the electrical specifications are not guaranteed.
10
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