AP3598A Datasheet, PDF (13/28 Pages) Diodes Incorporated – COMPACT DUAL-PHASE SYNCHRONOUS-RECTIFIED BUCK CONTROLLER

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AP3598A
Application Information (Cont.)
6. Current Balance
The AP3598A extracts phase currents for current balance by parasitic on-resistance of the lower switches when turned on as shown in Figure 2.
PHASE
_
+
+_ Reference
Voltage
IOFS
ICS1
Sample
&Hold
ICS2
Current
Balance
Figure 2. RDS(ON) Current Sensing Scheme
The GM amplifier senses the voltage drop across the lower switch and converts it into current signal when it turns on. The sampled and held
current is expressed as:
I_CSX  I_LX  R_DS(ON) 10^3 12A
Where ILX is the phase x current in Ampere, RDS(ON) is the on-resistance of low side MOSFET (Ω), 12µA is a constant current to compensate the
offset voltage of the current sensing circuit.
The AP3598A tunes the duty cycle of each channel for current balance according to the sensed inductor current signals as shown in Figure 3. If
the current of channel 1 is smaller than the current of channel 2, the AP3598A increases the duty cycle of the corresponding phase to increase its
phase current accordingly, vice versa.
RAMP1
COMP
+
_
PWM1
RAMP2
I_CS2
+
+
I_CS1
_
+
PWM2
_
Figure 3. Current Balance Scheme of AP3598A
7. Power Saving Interface (PSI)
This is a multilevel input to support Power Saving features. The AP3598A supports dual phase with FCCM and single phase with DCM and FCCM.
Mode
DCM
FCCM
Descriptions
Discontinuous Conduction Mode decreases the switching frequency to improve the efficiency at light load
Forced Continuous Conduction Mode does not change the switching frequency when the inductor current
goes to negative at light load. This mode is used to disable Power Saving features
Table 2. Description of Operating Modes
As shown in Table 2, an input high voltage (>1.5V) will set the controller to dual phase with FCCM mode; an input of intermediate level (between
0.8V and 1.1V) will set the controller to single phase with FCCM mode; an input low voltage will set the controller to single phase with DCM mode.
AP3598A
Document number: DS37261 Rev. 2 - 2
13 of 28
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November 2014
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