ISL62884C Datasheet, PDF (17/30 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6™ Mobile CPUs

ISL62884C
overshoot, forcing the inductor current to freewheel
through the low-side MOSFET body diode. Since the body
diode voltage drop is much higher than MOSFET r_DS(ON)
voltage drop, more energy is dissipated on the low-side
MOSFET therefore the output voltage overshoot is lower.
If the overshoot reduction function is enabled, the
ISL62884C monitors the COMP pin voltage to determine
the output voltage overshoot condition. The COMP
voltage will fall and hit the clamp voltage when the
output voltage overshoots. The ISL62884C will turn off
LGATE when COMP is being clamped. The low-side
MOSFET in the power stage will be turned off. When the
output voltage has reached its peak and starts to come
down, the COMP voltage starts to rise and is no longer
clamped. The ISL62884C will resume normal PWM
operation.
While the overshoot reduction function reduces the
output voltage overshoot, energy is dissipated on the
low-side MOSFET, causing additional power loss. The
more frequent the transient event, the more the power
loss dissipated on the low-side MOSFET. The MOSFET
may face severe thermal stress when transient events
occur at a high repetitive rate. User discretion is advised
when this function is enabled.
Key Component Selection
R_BIAS
The ISL62884C uses a resistor (1% or better tolerance is
recommended) from the RBIAS pin to GND to establish
highly accurate reference current sources inside the IC.
Using R_BIAS = 147kΩ. Do not connect any other
components to this pin. Do not connect any capacitor to
the RBIAS pin as it will create instability.
Care should be taken in layout that the resistor is placed
very close to the RBIAS pin and that a good quality
signal ground is connected to the opposite side of the
R_BIAS resistor.
Figure 10 shows the inductor DCR current-sensing
network. An inductor current flows through the DCR and
creates a voltage drop. The inductor has a resistors in
R_sum connected to the phase-node-side pad and a PCB
trace connected to the output-side pad to accurately
sense the inductor current by sensing the DCR voltage
drop. The sensed current information is fed to the NTC
network (consisting of R_ntcs, R_ntc and R_p) and capacitor
C_n. R_ntc is a negative temperature coefficient (NTC)
thermistor, used to temperature-compensate the
inductor DCR change. The inductor current information is
presented to the capacitor C_n.
Inductor DCR Current-Sensing Network
PHASE
RSUM
ISUM+
L
DCR
RNTCS
RP
RNTC
+
CN VCN
-
RI ^ISUM-
IO
FIGURE 10. DCR CURRENT-SENSING NETWORK
Equations 6 through 10 describe the frequency-domain
relationship between inductor total current I_o(s) and C_n
voltage V_Cn(s):
V_Cn(s)
=
⎛
⎜
⎝
-----------^R-----ⁿ---^t--^c---ⁿ---^e----^t----------
R_ntcnet + R_sum
×
D
C
⎞
R⎟
⎠
× I_o(s) × A_cs(s)
(EQ. 6)
R_ntcnet
=
(_---R_{-----n---t--c---s----+-----}R_{----n----t--c---})_---×_-----R_----p--
R_ntcs + R_ntc + R_p
A_cs(s)
=
----¹----+------^ω-----^s----L-----
1
+
------^s------
ω_sns
(EQ. 7)
(EQ. 8)
ω_L
=
D_-----C_-----R_---
L
(EQ. 9)
ω_sns
=
---------------------------¹----------------------------
-^R----ⁿ---^t--^c---ⁿ----^e---^t---^×-----^R----^s---^u----^m--
R_ntcnet + R_sum
×
C_n
(EQ. 10)
Transfer function A_cs(s) always has unity gain at DC. The
inductor DCR value increases as the winding temperature
increases, giving higher reading of the inductor DC
current. The NTC R_ntc values decreases as its
temperature decreases. Proper selections of R_sum, R_ntcs,
R_p and R_ntc parameters ensure that V_Cn represents the
inductor total DC current over the temperature range of
interest.
There are many sets of parameters that can properly
temperature-compensate the DCR change. Since the
NTC network and the R_sum resistors form a voltage
divider, V_cn is always a fraction of the inductor DCR
voltage. It is recommended to have a higher ratio of V_cn
to the inductor DCR voltage, so the droop circuit has
higher signal level to work with.
17
FN7591.0
March 16, 2010