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APW7098 Datasheet(PDF) 19 Page - Anpec Electronics Coropration

Part No. APW7098
Description  Two- Phase Buck PWM Controller with Integrated MOSFET Drivers
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Maker  ANPEC [Anpec Electronics Coropration]
Homepage  http://www.anpec.com.tw
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APW7098 Datasheet(HTML) 19 Page - Anpec Electronics Coropration

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Copyright
© ANPEC Electronics Corp.
Rev. A.7 - Oct., 2011
APW7098
www.anpec.com.tw
19
Function Description (Cont.)
Figure 3 shows the circuit of sensing inductor current.
Connecting a series resistor (R
S) and a capacitor (CS)
network in parallel with the inductor and measuring
the voltage (V
C) across the capacitor can sense the in-
ductor current.
Figure 3. Illustration of Inductor Current Sensing Circuit
The equations of the sensing network are:
Take
for example, if the above equation is true, the voltage
across the capacitor C
S is equal to voltage drop across
the inductor DCR, and the voltage V
C is proportional to
the current I
L. The sensing current through the resistor
R2 can be expressed as the following equation:
where
I
CS is the sensed current
I
L is the inductor current
DCR is the inductor resistance
R2 is the sense resistor
LL
V (s)=I (s) (SL+DCR)
×
Over-Current Protection (OCP)
L
DCR
R
s
C
s
R2
CSP
CSN
PHASE
I
L
V
C
V
L
R2
DCR
I
I
L
CS
×
=
The APW7098 is a two-phase PWM controller; therefore,
the IC has two sensed current parts, I
CS1 and ICS2. When
I
CS1 plus ICS2 is greater than 120µA, the over current occurs.
In over-current protection, the IC shuts off the converter
and then initials a new soft-start process. After 3 over-
current events are counted, the device turns off both high-
side and low-side MOSFETs and the converter’s output
is latched to be floating.
Current Sharing
The APW7098 uses inductor’s DCRs and external net-
works to sense the both currents flowing through the in-
ductors of the PWM1 and PWM2 channels. The current
sharing circuit, with closed-loop control, uses the sensed
currents to adjust the two-phase inductor currents. For
example, if the sensed current of PWM1 is bigger than
PWM2, the duty of PWM1 will decrease and the duty of
PWM2 will increase. Then, the device will reduce I
L1
current and increase I
L2 current for current sharing.
DROOP
In some high current applications, a requirement on
precisely controlled output impedance is imposed. This
dependence of output voltage on load current is often
termed droop regulation.
As shown in figure 4, the droop control block generates
a voltage through external resistor R
DROOP and then
set the droop voltage. The droop voltage, V
DROOP, is
proportional to the total current in two channels. As
shown in the following equation:
The V
DROOP voltage is used the regulator to adjust the out-
put voltage, therefore, it is equal to the reference voltage
minus the droop voltage.
S
S
L
S
S
L
C
C
SR
1
)
DCR
SL
(
)
S
(
I
C
SR
1
1
(S)
V
(S)
V
+
+
×
=
+
×
=
DCR
L
C
R
S
S
=
Figure 4. Illustration of Droop Setting Function
Droop Control
V
R
V
REFIN/EN or 0.6V
R
DROOP
V
DROOP
]
R
)
I
I
[(
05
.
0
V
DROOP
2
CS
1
CS
DROOP
×
+
×
=


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