TPS51518 Datasheet, PDF (18/34 Pages) Texas Instruments – Single-Phase, D-CAP™ and D-CAP2™ Controller with 2-Bit Flexible VID Control

TPS51518
SLUSAO8 – DECEMBER 2011
www.ti.com
Out-of-Bound Operation
When the output voltage rises to 8% above the target value, the out-of-bound operation starts. During the
out-of-bound condition, the controller operates in forced PWM-only mode. Turning on the low-side MOSFET
beyond the zero inductor current quickly discharges the output capacitor. During this operation, the
cycle-by-cycle negative overcurrent limit is also valid. Once the output voltage returns to within regulation range,
the controller resumes to auto-skip mode.
Current Sensing
In order to provide both cost effective solution and good accuracy, TPS51518 supports MOSFET R_DS(on) sensing.
For R_DS(on) sensing scheme, TRIP pin should be connected to GND through the trip voltage setting resistor,
R_TRIP. In this scheme, TRIP terminal sources 10µA of I_TRIP current (at T_A = 25°C) and the trip level is set to 1/8 of
the voltage across the R_TRIP. The inductor current is monitored by the voltage between the GND pin and the SW
pin so that the SW pin is connected to the drain terminal of the low-side MOSFET. I_TRIP has a 4700ppm/°C
temperature slope to compensate the temperature dependency of the R_DS(on). GND is used as the positive
current sensing node so that GND should be connected to the sense resistor or the source terminal of the
low-side MOSFET.
Overcurrent Protection
TPS51518 has cycle-by-cycle overcurrent limiting protection. The inductor current is monitored during the
off-state and the controller maintains the off-state when the inductor current is larger than the overcurrent trip
level. The overcurrent trip level, V_OCTRIP, is determined by Equation 8.
V_OCTRIP
=
R_TRIP
æ I_TRIP
´ç
è
8
ö
÷
ø
(8)
Because the comparison is made during the off-state, V_OCTRIP sets the valley level of the inductor current. The
load current OCL level, I_OCL, can be calculated by considering the inductor ripple current.
Overcurrent limiting using R_DS(on) sensing is shown in Equation 9.
I_OCL
=
æ
ç
V_OCTRIP
ç
è
R_DS(on)
ö _IIND(ripple)
÷+
÷
2
ø
=
æ
ç
V_OCTRIP
ç
è
R_DS(on)
ö
÷+
÷
ø
1
´
2
V_IN - V_OUT
L_X
V_OUT
´
f_SW ´ V_IN
where
• I_IND(ripple) is inductor ripple current
(9)
In an overcurrent condition, the current to the load exceeds the current to the output capacitor, thus the output
voltage tends to fall down. Eventually, it crosses the undervoltage protection threshold and shuts down.
Overvoltage and Undervoltage Protection
The TPS51518 sets the overvoltage protection (OVP) when VSNS voltage reaches a level 20% (typ) higher than
the target voltage. When an OV event is detected, the controller changes the output target voltage to 0 V. This
usually turns off DRVH and forces DRVL to be on. When the inductor current begins to flow through the low-side
MOSFET and reaches the negative OCL, DRVL is turned off and DRVH is turned on, for a minimum on-time.
After the minimum on-time expires, DRVH is turned off and DRVL is turned on again. This action minimizes the
output node undershoot due to LC resonance. When the VSNS reaches 0 V, the driver output is latched as
DRVH off, DRVL on.
The undervoltage protection (UVP) latch is set when the VSNS voltage remains lower than 68% (typ) of the
REFIN voltage for 1 ms or longer. In this fault condition, the controller latches DRVH low and DRVL low and
discharges the V_OUT. UVP detection function is enabled after 1.2 ms of SMPS operation to ensure startup.
To release the OVP and UVP latches, toggle EN or adjust the V5IN voltage down and up beyond the
undervoltage lockout threshold.
V5IN Undervoltage Lockout Protection
TPS51518 has a 5-V supply undervoltage lockout protection (UVLO) threshold. When the V5IN voltage is lower
than UVLO threshold voltage, typically 4.0 V, V_OUT is shut off. This is a non-latch protection.
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