Ap3595, Application information, Vr i – Diodes AP3595 User Manual
Page 12: Ri v, Cr r l / 2, Ri i 2
AP3595
Document number: DS36749 Rev. 1 - 2
12 of 23
January 2014
© Diodes Incorporated
AP3595
A Product Line of
Diodes Incorporated
Application Information
(Cont.)
Where I
LX
is the phase x current in Ampere, R
DS(ON)
is the on-resistance of low side MOSFET, 12
A is a constant current to compensate the
offset voltage of the current sensing circuit.
AP3595 tunes the duty cycle of each channel for current balance according to the sensed inductor current signals as shown in Figure 4. If the
current of channel 1 is smaller than the current of channel 2, the IC increases the duty cycle of the corresponding phase to increase its phase
current accordingly, vice versa.
Figure 4. Current Balance Scheme of AP3595
7. Power Saving Interface (PSI)
The AP3595 supports dual phase or single phase which is controlled by V
PSI
. If V
PSI
>1.2V, AP3595 will operate in dual phase mode. If V
PSI
<0.4V,
AP3595 will operate in single phase mode. There is 2ms delay at the transient from dual phase to single phase, and no delay time from single
phase to dual phase.
8. Current Sense by DCR
Figure 5. Output Current Sensing Block
The above figure shows the output current sensing block of AP3595. The voltage V
CS
across the current sensing capacitor C
CS
can be expressed
as
2
/
DC
OUT
CS
R
I
V
,
If the following condition is true:
CS
CSP
DC
C
R
R
L
/
2
Where L is the output inductor of the buck converter, R
DC
is the parasitic resistance of the inductor, R
CSP
and C
CS
are the external RC network for
current sensing.
The GM amplifier will source a current I
CSN
to the CSN pin to let its inputs virtually short circuit.
CS
CSN
CSN
V
R
I
Therefore the output current signal I
CSN
can be expressed as:
CSN
DC
OUT
CSN
R
R
I
I
2
The output current signal I
CSN
is used to droop tuning and output over current protection.
+
_
+
_
PWM1
PWM2
COMP
I
CS2
I
CS1
+
_
+
RAMP1
RAMP2
AP3595
+
-
CSP
CSN
R
CSN
R
CSP
C
CS
R
DC
R
DC
L
L
R
CSP
PHASE1
PHASE2
V
OUT
GM
Amplifier
I
CSN
I
DRP
I
PSI
I
OCP