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RT8856 View Datasheet(PDF) - Richtek Technology

Part NameDescriptionManufacturer
RT8856 Multi-Phase PWM Controller for CPU Core Power Supply Richtek
Richtek Technology Richtek
RT8856 Datasheet PDF : 23 Pages
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RT8856
The RT8856 provides 2 slew rates for deeper sleep mode
entry/ exit. For standard deeper sleep exit, the RT8856
immediately activates all enabled phases and ramps the
output voltage to the DAC code provided by the processor
at the slew rate of 100μA/CSOFT. The RT8856 remains in
1-phase ringing free mode and ramps the output voltage
to the DAC code provided by the processor at the slew
rate of 20μA/CSOFT.
Current Limit Setting
The RT8856 compares a programmable current limit set
point to the voltage from the current sense amplifier output
for Over Current Protection (OCP). The voltage applied to
OCSET pin defines the desired current limit threshold,
ILIM :
VOCSET = 25 x ILIM x RSENSE
(16)
Connect a resistive voltage divider from VCC to GND, with
the joint of the voltage divider connected to OCSET pin as
shown in Figure 6. For a given ROC2,
ROC1
=
ROC2
×
⎜⎝
VCC
VOCSET
1⎟⎠⎞
(17)
RT8856
VCC
OCSET
ROC1
ROC2
Figure 6. OCP Setting Without Temperature
Compensation
The OCP works in two stages :
` Stage 1 : Average inductor current exceeds the current
limit threshold, ILIM, defined by VOCSET, but remains
smaller than 150% of ILIM If the over current condition
remains valid for 16 cycles, the OCP latches and the
system shuts down.
` Stage 2 : Any inductor current exceeds 150% of ILIM
then OCP latches instantaneously.
Latched OCP forces driver high impedance with
UGATEx = 0 and LGATEx = 0. After latched OCP happens,
VVSEN will be monitored. When VVSEN falls below 200mV,
a discharging resistor at VSEN will be enabled.
If inductor DCR is used as current sense component, then
DS8856-03 June 2011
temperature compensation is recommended to protect
under all conditions. Figure 7 shows a typical OCP setting
with temperature compensation.
VCC
RT8856 ROC1a
NTC
OCSET
ROC1b
ROC2
Figure 7. OCP Setting with Temperature Compensation
Usually, select ROC1a equal to thermistor's nominal
resistance at room temperature. Ideally, VOCSET should
have same temperature coefficient as RSENSE (Inductor
DCR) :
VOCSET, HOT = RSENSE, HOT
VOCSET, COLD RSENSE, COLD
(18)
According to the basic circuit calculation, VOCSET can be
obtained at any temperature :
VOCSET, T
=
ROC1a
ROC2
// RNTC, T + ROC1b
+ ROC2
(19)
Re-write Equation (18) from (19), and get VOCSET at room
temperature
ROC1a // RNTC, COLD + ROC1b + ROC2
ROC1a // RNTC, HOT + ROC1b + ROC2
=
RSENSE, HOT
RSENSE, COLD
(20)
VOCSET,
25
=
ROC1a
ROC2
// RNTC, 25 + ROC1b
+ ROC2
(21)
Solving Equation (20) and (21) yields ROC1b and ROC2
ROC2 =
α × REQU, HOT REQU, COLD + (1− α) × REQU, 25
(22)
VCC × (1− α)
VOCSET, 25
ROC1b =
(α − 1) × ROC2 + α × REQU, HOT REQU, COLD
(23)
(1− α)
where
α=
RSENSE, HOT
RSENSE, COLD
=
DCR25 × [1+ 0.00393 × (THOT 25)]
DCR25 × [1+ 0.00393 × (TCOLD 25)]
(24)
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