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RT8856 View Datasheet(PDF) - Richtek Technology

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RT8856 Datasheet PDF : 23 Pages
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RT8856
Since the DCR of inductor is highly temperature dependent,
it affects the output accuracy at hot conditions.
Temperature compensation is recommended for the
lossless inductor DCR current sense method. Figure 4
shows a simple but effective way of compensating the
temperature variations of the sense resistor using an NTC
thermistor placed in the feedback path.
RT8856
C2
C1
EA-
+
VDAC
COMP
FB
SOFT
RGND
R2 R1b R1a
NTC
VCC_SENSE
CSOFT
10nF VSS_SENSE
Figure 4. Loop Setting with Temperature Compensation
Usually, R1a is set to equal RNTC (25°C). R1b is selected
to linearize the NTC's temperature characteristic. For a
given NTC, design is to get R1b and R2 and then C1 and
C2. According to Equation (4), to compensate the
temperature variations of the sense resistor, the error
amplifier gain (AV) should have the same temperature
coefficient with RSENSE. Hence,
A V, HOT = RSENSE, HOT
(5)
A V, COLD RSENSE, COLD
From Equation (4), Av can be obtained at any temperature
(T) as shown below :
AV, T
=
R2
R1a // RNTC, T
+ R1b
(6)
The standard formula for the resistance of NTC thermistor
as a function of temperature compensation is given by :
{ ( ) ( ) } RNTC, T
= R25
e
β⎡⎢⎣
1
T+273
1
298
⎥⎦
(7)
where R25 is the thermistor's nominal resistance at room
temperature, β (beta) is the thermistor's material constant
in Kelvins, and T is the thermistor's actual temperature in
Celsius.
To calculate DCR value at different temperature, use the
equation below :
DCRT = DCR25 x [1 + 0.00393 x (T 25)]
(8)
where the 0.00393 is the temperature coefficient of the
copper. For a given NTC thermistor, solving Equation (6)
at room temperature (25°C) yields
R2 = AV, 25 x (R1b + R1a // RNTC, 25)
(9)
where AV, 25 is the error amplifier gain at room temperature
and can be obtained from Equation (4). R1b can be obtained
by substituting Equation (9) to (5),
R1b =
RSENSE, HOT
RSENSE, COLD
× (R1a // RNTC,
HOT
)
(R1a // RNTC,
HOT
)
⎛⎜1
RSENSE, HOT
RSENSE, COLD
(10)
Loop Compensation
Optimized compensation of the RT8856 allows for best
possible load step response of the regulator's output. A
type-II compensator with one pole and one zero is
adequate for a proper compensation. Figure 4 shows the
compensation circuit. Prior design procedure shows how
to select the resistive feedback components for the error
amplifier gain. Next, the C1 and C2 must be calculated for
the compensation. The target is to achieve constant
resistive output impedance over the widest possible
frequency range.
The pole frequency of the compensator must be set to
compensate the output capacitor ESR zero :
fP
=
1
2 × π × C × RC
(11)
where C is the capacitance of output capacitor, and RC is
the ESR of output capacitor. C2 can be calculated as
follows :
C2 = C × RC
R2
(12)
The zero of compensator has to be placed at half of the
switching frequency to filter the switching related noise.
such that,
( ) C1 =
1
R1b + R1a // RNTC, 25 × π × fSW
(13)
DS8856-03 June 2011
www.richtek.com
17
 

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