DS8813D Datasheet, PDF (19/23 Pages) Richtek Technology Corporation – Multi-Phase PWM Controller with PWM-VID Reference

RT8813D
Current Limit Setting
Current limit threshold can be set by a resistor (R_OCSET)
between ILIM and GND. Once PVCC exceeds the POR
threshold and chip is enabled, an internal current source
I_OCSET flows through R_OCSET. The voltage across R_OCSET is
stored as the current limit protection threshold V_OCSET.
The threshold range of V_OCSET is 50mV to 400mV.
R_OCSET can be determined using the following equation :
  _{ROCSET =}
I_VALLEY R_LGDS(ON)  40mV
I_OCSET
where I_VALLEY represents the desired inductor limit current
(valley inductor current) and I_OCSET is current limit setting
current which has a temperature coefficient to compensate
the temperature dependency of the R_DS(ON).
If R_OCSET is not present, there is no current path for I_OCSET
to build the current limit threshold. In this situation, the
current limit threshold is internally preset to 400mV
(typical).
Negative Current Limit
The RT8813D supports cycle-by-cycle negative current
limiting. The absolute value of negative current limiting
threshold is the same with the positive current limit
threshold. If negative inductor current is rising to trigger
negative current limit, the low side MOSFET will be turned
off and the current will flow to input side through the body
diode of the high side MOSFET. At this time, output voltage
tends to rise because this protection limits current to
discharge the output capacitor. In order to prevent shutdown
because of over voltage protection, the low side MOSFET
is turned on again 400ns after it is turned off. If the device
hits the negative over current threshold again before output
voltage is discharged to the target level, the low side
MOSFET is turned off and process repeats. It ensures
maximum allowable discharge capability when output
voltage continues to rise. On the other hand, if the output
is discharged to the target level before negative current
threshold is reached, the low side MOSFET is turned off,
the high side MOSFET is then turned on, and the device
keeps normal operation.
Current Balance
The RT8813D implements current balance mechanism in
the current loop. The RT8813D senses per phase current
signal and compares it with the average current. If the
sensed current of any particular phase is higher than the
average current, the on-time of this phase will be
decreased.
The current balance accuracy is major related with on-
resistance of low side MOSFET (R_LG,DS(ON)). That is, in
practical application, using lower R_LG,DS(ON) will reduce
the current balance accuracy.
Output Over Voltage Protection (OVP)
The output voltage can be continuously monitored for over
voltage protection. If REFIN voltage is lower than 1.33V,
the over voltage threshold follows to absolute over voltage
2V. If REFIN voltage is higher than 1.33V, the over voltage
threshold follows relative over voltage 1.5 x V_REFIN. When
OVP is triggered, UGATE goes low and LGATE is forced
high. The RT8813D is latched once OVP is triggered and
can only be released by PVCC or EN power on reset. A
5μs delay is used in OVP detection circuit to prevent false
trigger.
Output Under Voltage Protection (UVP)
The output voltage can be continuously monitored for under
voltage protection. When the output voltage is less than
40% of its set voltage, under voltage protection is triggered
and then all UGATEx and LGATEx gate drivers are forced
low. There is a 3μs delay built in the UVP circuit to prevent
false transitions. During soft-start, the UVP blanking time
is equal to PGOOD blanking time.
Thermal Monitoring and Temperature Reporting
The RT8813D provides thermal monitoring function in 2/1
phase operation via sensing the TSNS pin voltage, and
which can indicate ambient temperature through the voltage
divider R_OTSET and R_NTC shown in Figure 13. The voltage
of V_TSNS is typically set to be higher than 1V. When ambient
temperature rises, V_TSNS will fall and the TALERT signal
will be pulled to low level if TSNS voltage drops below 1V.
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
DS8813D-00 September 2016
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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