nx2155h Microsemi Corporation, nx2155h Datasheet - Page 16

• Manufacturer Part Number: nx2155h
• Description: Single Power Supply Synchronous Pwm Controller
• Manufacturer: Microsemi Corporation

Output Voltage Calculation
ternal voltage divider. The reference voltage is fixed at
0.8V. The divider consists of two ratioed resistors so
that the output voltage applied at the Fb pin is 0.8V when
the output voltage is at the desired value. The following
equation and picture show the relationship between
of R
Rev.1.1
04/16/09
V
OUT
Figure 22 - Type II compensator with
1
value can be set by voltage divider.
Output voltage is set by reference voltage and ex-
where R
See compensator design for R
R =
,
R2
R1
R2
R1
V
1
Vout
Vout
REF
Figure 23 - Voltage divider
Voltage divider
V
R
and voltage divider. .
O U T
2
2
transconductance amplifier
Vref
Vref
is part of the compensator, and the value
Fb
Fb
-V
V
R E F
R E F
gm
...(10)
R3
C1
1
Ve
and R
C2
2
selection.
Over Current Protection
rent through the low side MOSFET. A typical internal
current source of 37uA flowing through an external resis-
tor connected from OCP pin to SW node sets the over
current protection threshold. When synchronous FET is
on, the voltage at node SW is given as
occurs.
equation
Frequency Selection
The relationship between frequency and RT pin is shown
as follows.
2500
2000
1500
1000
500
Over current protection is achieved by sensing cur-
The voltage at pin OCP is given as
When the voltage is below zero, the over current
The over current limit can be set by the following
The frequency can be set by external Rt resistor.
I
I
V =-I
Figure 24 - Over current protection
Figure 25 - Frequency versus Rt resistor
0
OCP
SET
OCP
comparator
SW
3
R
I
OCP
L
K R
OCP
8
R
+V
DSON
R
DSON
NX2155H Frequency vs Rt
SW
OCP
13
I
OCP
18
Rt(kohm)
OCP
23
R
OCP
NX2155H
28
vbus
SW
33
38
16