CS5171: Tracking boost regulator

DN06022/D
Design Note – DN06022/D
Tracking boost Regulator
Device
Application
Input Voltage
Output Power
Topology
I/O Isolation
CS5171
General
Up to 30V
Circuit
dependant
Modified
Boost
no
Other Specifications
Output Voltage
Output 1
Output 2
Output 3
Output 4
Vin +10V
N/A
N/A
N/A
Minimum Efficiency
70%
Operating Temp. Range
-40C to +125 °C
(Vout − Vin − Vbe(Q1A))
= R2
1mA
Circuit Description
Adding a current mirror circuit to a typical boost
circuit allows the user to select the amount of boost
voltage and ensure a constant difference between
input and output voltage. This is useful for high side
drive applications where a simple voltage doubling
circuit is not practical, either because of the voltage
range of the components involved or where the
input voltage can vary widely. This circuit can also
be used at the front end of a power supply to
ensure that the PWM controller has enough voltage
to start correctly in low input voltage conditions.
The schematic shown will maintain a 10V
difference between Vin and Vout and is easily
changed to provide other voltages.
The PWM in the design example is the CS5171.
However, this circuit can be used with any boost
controller or regulator. The current mirror circuit,
comprising the dual PNP transistor Q1 and the
associated resistors, establishes a current that
depends on the voltage difference between Vin and
Vout. The dual PNP transistor NST30010MXV6 has
a Vceo of 30 V so it is used in this example. If
higher output voltages are required a device such
as the BC856BDW1T1G has a Vceo of 65 V.
In this example Vin =14V nominal, so Vout is 10V
greater, i.e., 24V nominal. We begin by selecting a
reference current in Q1A. We can assign an
arbitrary value of 1 mA. Next calculate a value for
R2 to establish the reference current. With a
reference current of 1mA we get:
May 2007, Rev. 2
( 24V − 14V − 0.6V )
= 9 .4 K Ω
1mA
Since the output voltage is not critical we use a 10K
Ohm resistor.
This current gets mirrored in Q1B and sets up the
feedback voltage to the PWM. The CS5171 has an
internal voltage of 1.27V typical, so we choose R3 to
give the correct feedback voltage when the current
flowing through it is 1mA. In this case, R3 is 1.27K.
This gives an output voltage of 24V with Vin =14V.
As Vin varies, Vout will track it while maintaining a
10V difference between the input and output. R4 is
added to help reduce the power dissipation in Q1B.
As the input voltage varies, Vout will remain a
constant 10V above the input voltage. The only
limitation is the breakdown voltage of the regulator
and transistors used. In this case, since the CS5171
has a Vsw (switch input voltage) maximum of 40V, the
VRRM of the Schottky rectifier used is 40V, and the
Vceo of the transistor pair is 30 V the input voltage
should be kept below 20V.
Key Features
• Simple, low cost circuit to ensure a constant boost
voltage
• Easily adaptable for any boost circuit. Easy to
modify on demo board
• Uses off the shelf components.
www.onsemi.com
1
DN06022/D
Schematic
Q1A
May 2007, Rev. 2
www.onsemi.com
Q1B
2
DN06022/D
1
ID
C1
C2
R1
R2
R3
R4
Q1A &Q1B
L1
U1
D1
Bill of Material
Part Number
Description
Capacitor
Capacitor
Resistor
Resistor
Resistor
Resistor
Dual PNP
Inductor
Boost Controller
Schottky Rectifier
NST30010MXV6
TP4_220
CS5171
MBRS140T3
Value
22uF
22uF
1k
10k
1.27k
8.2k
30V Transistor
22uH
40V 1A
Vendor
ON Semiconductor
Coiltronics
ON Semiconductor
ON Semiconductor
1