ISL6263D Datasheet, PDF (14/17 Pages) Intersil Corporation – Pre-biased Output Start-Up Capability

ISL6263D
Sensing the time varying inductor current accurately
requires that the parallel R-C network time constant match
the inductor L/DCR time constant. Equation 10 shows this
relationship:
D^-----C^-L----R^---
=



R^-R----_N^-N---^---T^-T---^{---+----R-}R^----S-_S^--_{}
 C_N
(EQ. 10)
Solution of C_N yields:
C_N
=
---------^---^{D----------C--L--------R-------}----------



R^-R----_N^-N---^---T^-T---^{---+----R-}R^----S-_S^--_{}
(EQ. 11)
The first step is to adjust R_N(T) and R_S such that the correct
current information appears between the ISP and VO pins
even at light loads. Assume V_N is the voltage drop across
R_N(T). The V_N to V_DCR gain G₁(T) provides a reasonable
amount of light load signal from which to derive the current
information. G₁(T) is given by Equation 12:
G₁T = -_R----_N---^R-_---_T^N----_-^--^T_+----^-_R-----_S--
(EQ. 12)
The gain of the current sense amplifier circuit is expressed in
Equation 13:
K_ISENSE = 1 + ^RR-----^II--^SS----²¹-
(EQ. 13)
The current sense amplifier output voltage is given by
Equation 14:
V_ICOMP = V_O + V_N  K_ISENSE
(EQ. 14)
The inductor DCR is a function of temperature T and is
approximately given by Equation 15:
DCRT = DCR_25C  1 + 0.00393  T – 25C
(EQ. 15)
0.00393 is the temperature coefficient of the copper. To
make V_ICOMP independent of the inductor temperature, the
NTC characteristic is desired to satisfy:
G₁T  1 + 0.00393  T – 25C  G_{1t arget}
(EQ. 16)
where G_1target is the desired ratio of V_N /V_DCR. Therefore,
the temperature characteristics G₁, which determines
parameters selection, is described by Equation 17:
G₁T = -₁----+-----₀----_.-₀----₀--^G-₃---₉-¹---₃^t---^a-_--^r-^g_---_T-^e---^t-–-----₂---₅----_---_C----_-
(EQ. 17)
It is recommended to begin the DCR current sense design
using the R_NTC, R_NTCS, and R_P component values of the
evaluation board available from Intersil.
Given the inductor DCR and the overcurrent set point I_OC, the
maximum voltage of ICOMP pin is given by Equation 18:
VICOMP_{max} – VO
=
IOC  DCR+25C 
-_R----_N---^R--_--₊-^N---₂-^---₅₊---_-²--_C-⁵----^---^C-+----^_R-----_S--
 1 +

RR_{-----II--SS-----21- }
(EQ. 18)
R_N, R_S, R_IS1, R_IS2 should be adjusted to meet the
requirement (V_ICOMP(max) - V_O) > 25mV and the time
constant matching according to Equation 10.
Given the output current I_O, the current monitor IMON
voltage can be determined by Equation 19:
VIMON
=
31  IO
 DCR+25C 
-_R----_N---^R--_--₊-^N---₂-^---₅₊---_-²--_C-⁵----^---^C-+----^_R-----_S--

 1

+
RR_{-----II--SS-----21- }
(EQ. 19)
The effectiveness of the R_N network is sensitive to the
coupling coefficient between the NTC thermistor and the
inductor. The NTC thermistor should be placed in the closet
proximity of the inductor.
Resistor Current Sense
Figure 3 shows a detailed schematic using discrete resistor
sense of the inductor current. Figure 9 shows the equivalent
circuit. Since the current sense resistor voltage represents the
VDD
-
OCP
+
+
-
10µA _
OCSET

Isense

ISP
ISN
ICOMP
VO
R_OCSET
R_S
V_DCR
FIGURE 8. EQUIVALENT MODEL OF CURRENT SENSE USING INDUCTOR DCR CURRENT SENSE
14
FN6753.1
July 8, 2010