AND8079 Datasheet, PDF (5/20 Pages) ON Semiconductor – A Low Cost DDR Memory Power Supply Using the NCP1571 Synchronous Buck Converter and a LM358 Based Linear Voltage Regulator

AND8079/D
Schottky Diode for Synchronous MOSFET
The efficiency of the buck converter can be improved
slightly by placing a Schottky diode (D₁) in parallel with the
bottom MOSFET (Q₃). The body diode of Q₃ is used to
conduct current during the non-overlap time when both the
top (Q₂) and bottom (Q₃) MOSFETs are turned OFF. But
because the non-overlap time is only approximately 50 ns
for the NCP1571’s 200 kHz switching speed, the efficiency
savings will be only approximately 1.0%. The
demonstration board included a provision for D₁; however,
the performance of the circuit was not testing with this
diode.
Experimental Results
The experimental results of the demonstration PCB are
shown in Figures 4 through 20. Figure 3 shows the test setup
used to create the current load transients for the V_tt supply
voltage. The transient current load tests for the V_dd and V_tt
supply voltages were created using a Kikusui Electronic
Load Controller. Unless noted, the standard test conditions
are as listed below:
1. Ambient Temperature = 23_C
2. V_dd Current Load (I_Vdd) = 8.0 A
3. V_tt Current Load (I_Vtt) = 1.25 A source load
4. V_REF Current Load (I_REF) = 2.5 mA
5. 5.0 V Input Voltage = 5.00 V
6. 12 V Input Voltage = 12.00 V
7. -12 V Input Voltage = -12.00 V
V_tt DDR Circuit
2.5 V
V_dd
Q4
- _{2.0 A}
+ ^{DC Current Source}
-
R7
OUT
V_REF
+
U2A
+
C22
Q5
+
C23
V_tt
+ _{Pulsating Current Source}
-
Pulse Width = 100 ms
Period = 200 ms
l1 = 0 A
l2 = 4.0 A
Rise Time = 50 ms
Fall Time = 50 ms
DC Current Source
2.0 A
V_tt Circuit
2 .0 A
V_tt
0A
Pulsating Current Source
2.0 A Current Sink Test
DC Current Source
2.0 A
V_tt Circuit
2 .0 A
V_tt
4A
Pulsating Current Source
2.0 A Current Source Test
Figure 3. V_tt Transient Load Test Setup for a 2.0 A Sink to 2.0 A Source Test
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