NCP1589A, NCP1589B
www.onsemi.com
8
Figure 8. Typical Power Good Function
G
G
0.88V
0.4V
GOOD
0.88V
0.8V
0.72V
0.8V
0.6V
VOS
Overvoltage Undervoltage
Feedback and Compensation
The NCP1589A/B allows the output voltage to be
adjusted from 0.8 V to 5.0 V via an external resistor divider
network. The controller will try to maintain 0.8 V at
feedback pin. Thus, if a resistor divider circuit was placed
across the feedback pin to V
OUT
, the controller will regulate
the output voltage proportional to the resistor divider
network in order to maintain 0.8 V at the FB pin. The same
formula applies to the VOS pin and the controller will
maintain 0.8 V at the VOS pin.
V
OUT
R1
R4
FB
Figure 9.
The relationship between the resistor divider network
above and the output voltage is shown in the following
equation:
R
4
+ R
1
ǒ
V
REF
V
OUT
* V
REF
Ǔ
The same formula can be applied to the feedback resistors
at VOS.
R
9
+ R
10
ǒ
V
REF
V
OUT
* V
REF
Ǔ
Design Example
Voltage Mode Control Loop with TYPE III
Compensation
Converter Parameters:
Input Voltage: V
IN
= 5 V
Output Voltage: V
OUT
= 1.65 V
Switching Frequency: 300 kHz
Total Output Capacitance: C
OUT
= 3600 mF
Total ESR: ESR = 6 mW
Output Inductance: L
OUT
: 1 mH
Ramp Amplitude: V
RAMP
= 1.1 V
−
+
Figure 10.
C3R3
R1
C1
C2R2
V
OUT
V
COMP
V
ref
E/A
R4
a.. Set a target for the close loop bandwidth at 1/6
th
of
the switching frequency.
F
cross_over
:+ 50 kHz
