APW7098
Function Description (Cont.)
Current Sharing
Over-Current Protection (OCP)
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 IL1
current and increase IL2 current for current sharing.
Figure 3 shows the circuit of sensing inductor current.
Connecting a series resistor (RS) and a capacitor (CS)
network in parallel with the inductor and measuring
the voltage (VC) across the capacitor can sense the in-
ductor current.
VL
L
DCR
PHASE
IL
Rs
Cs
VC
DROOP
CSP
CSN
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.
R2
Figure 3. Illustration of Inductor Current Sensing Circuit
The equations of the sensing network are:
VL(s)=IL(s)´ (SL+DCR)
As shown in figure 4, the droop control block generates
a voltage through external resistor RDROOP and then
set the droop voltage. The droop voltage, VDROOP, is
proportional to the total current in two channels. As
shown in the following equation:
1
IL(S)´ (SL + DCR)
1+ SRSCS
VC(S) = VL(S)´
=
1+ SRSCS
VDROOP = 0.05´ [(ICS1 +ICS2)´ RDROOP]
Take
L
The VDROOP voltage is used the regulator to adjust the out-
put voltage, therefore, it is equal to the reference voltage
minus the droop voltage.
RSCS =
DCR
for example, if the above equation is true, the voltage
across the capacitor CS is equal to voltage drop across
the inductor DCR, and the voltage VC is proportional to
the current IL. The sensing current through the resistor
R2 can be expressed as the following equation:
Droop Control
IL ´ DCR
ICS =
VDROOP
R2
RDROOP
where
VR
ICS is the sensed current
IL is the inductor current
DCR is the inductor resistance
R2 is the sense resistor
VREFIN/EN or 0.6V
Figure 4. Illustration of Droop Setting Function
The APW7098 is a two-phase PWM controller; therefore,
the IC has two sensed current parts, ICS1 and ICS2. When
ICS1 plus ICS2 is greater than 120mA, 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.
Copyright ã ANPEC Electronics Corp.
19
www.anpec.com.tw
Rev. A.6 - Oct., 2009
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
