DS8817A Datasheet, PDF (17/21 Pages) Richtek Technology Corporation – Dual-Phase PWM Controller with PWM-VID Reference

I_L
I_L,PEAK
I_LOAD
I_L,VALLEY
t
0
Figure 12. “Valley” Current Limit
In an over-current condition, the current to the load exceeds
the average output inductor current. Thus, the output
voltage falls and eventually crosses the under-voltage
protection threshold, inducing IC shutdown.
Current Limit Setting
RT8817A adopts per-phase current limiting protection. The
current limit threshold can be set by a resistor (R_OCSET)
between OCSET/SS 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 30mV to 200mV.
It can be calculated according to the following equation :
V_OCSET
=
I_OCSET R_OCSET
12
R_OCSET can be determined using the following equation :
ROCSET

IVALLEY
 RDS _ ON  12
IOCSET
where I_VALLEY represents the desired Per-phase 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).
During soft-start period (EN is pulled high to SSOK), the
I_OCSET is 50μA. Once soft-start complete, the I_OCSET will
switch to 10μA.
For ensure the soft-start and current limit function works
normally. Below setting limitation must be followed.
R_OCSET x 50μA > 1.2 x V_REFIN
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 200mV.
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
DS8817A-00 September 2016
RT8817A
Negative Current Limit
The RT8817A supports cycle-by-cycle negative current
limit. The absolute value of negative current limit
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 current limit
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 limit 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 RT8817A implements current balance mechanism in
the current loop. The RT8817A 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 RT8817A is latched once OVP is triggered and
can only be released by PVCC or EN power on reset. A
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