Operation of NCP5252 in Low Dropout Applications

AND9095/D
Operation of NCP5252 in
Low Dropout Applications
http://onsemi.com
APPLICATION NOTE
Introduction
Low Dropout PWM Operation
The NCP5252 is a single−phase buck regulator with
integrated Power MOSFETs. The device is able to deliver up
to 2 A current at an externally adjustable output voltage,
which range is from 0.6 V to 5.0 V. The operation range of
the input supply voltage is from 4.5 V to 13.2 V. To provide
a design guide for low dropout applications, detailed
operation behavior of the NCP5252 is described in this
application note. Both measurement and calculation results
of a 5 V−output application are provided.
As a non−isolated buck converter, the input supply
voltage VIN of the NCP5252 needs to be higher than the
output voltage VOUT to provide a sustainable current to a
load. In applications where VIN is much higher than VOUT,
the NCP5252 operates in a selected fixed−frequency in
CCM. When the input voltage VIN drops to be close to the
output voltage VOUT, it is possible to see other two operation
modes with the NCP5252. Figure 1 is a plot that illustrates
key signals in these operation modes.
VIN
V IN_MIN1
Mode 1
V OUT
V IN_MIN2
Mode 2
COMP
RAMP
Mode 3
V COMP _MAX
0
t ON
PWM
TOFF _MIN
0
T
1
D
D MAX
0
Time
Figure 1. PWM Regulation Modes with the NCP5252
© Semiconductor Components Industries, LLC, 2012
July, 2012 − Rev. 0
1
Publication Order Number:
AND9095/D
AND9095/D
Mode 1 – Fixed−Frequency Operation
switching frequency until the off time of the PWM signal
reaches its minimum limit TOFF_MIN (225 ns typical in
datasheet, but it would be better to use 260 ns in calculation
to cover dead times of gate drivers.). The minimum required
voltage VIN_MIN1 can be obtained by
As shown in Figure 1, the NCP5252 operates in Mode 1
when the input supply voltage VIN is higher than a minimum
required voltage VIN_MIN1. It is a fixed−frequency
operation mode with an externally programmed frequency
at FREQ_SET pin. The duty ratio of the PWM signal can be
estimated by
D+
V IN_MIN1 +
V OUT ) I OUT @ ǒR DS_L ) DCRǓ
V IN * I OUT @ ǒR DS_H * R DS_LǓ
where IOUT is the load current, DCR is the DC resistance of
the output filter inductor L, RDS_H is the conduction
resistance of the integrated high−side MOSFET, and RDS_L
is the conduction resistance of the integrated low−side
MOSFET. To simplify analysis, the diode voltage drops of
MOSFETs during dead times have not been taken into
account in this calculation.
To ensure good stability and line transient response, a VIN
feedforward function has been implemented in the
NCP5252’s ramp generator. The slew rate SRRAMP of the
ramp signal is proportional to both the input voltage VIN and
the nominal switching frequency FSW.
If the input voltage drops below VIN_MIN1, the off time of
the PWM signal cannot be reduced further due to TOFF_MIN.
This forces the NCP5252 to reduce switching frequency to
maintain the output regulation. The switching frequency in
Mode 2 operation is
V COMP_M2 + T ON_M2 @ SR RAMP ) V RAMP_OFFSET (eq. 6)
T ON_M2 +
T OFF_MIN
Ǔ
*1
V
)I
@ ǒR
)DCRǓ
OUT OUT
DS_L
V
(eq. 3)
)I
IN
ǒ
@ R
OUT
*R
DS_L
DS_H
1*
0.25@F
V
@T
SW
*V
COMP_MAX
OFF_MIN
RAMP_OFFSET
ǒ
@ V OUT ) I OUT @ ǒR DS_L ) DCRǓ
Mode 3 – Maximum−Duty Operation
SR RAMP
0.25@V
1)V
Ǔ
@F
IN_MIN2
*V
COMP_MAX
@T
SW
OFF_MIN
(eq. 11)
RAMP_OFFSET
In Mode 3, the output voltage drops as input voltage drops.
The output voltage can be calculated by
(eq. 9)
The switching frequency FSW_M3 in Mode 3 does not
decrease as much as that in Mode 2 when VIN drops.
1
T ON_M3 ) T OFF_MIN
(eq. 8)
1
D MAX +
If the input voltage drops below VIN_MIN2, the COMP
signal is clamped to VCOMP_MAX, and thus the NCP5252
reaches its limitation in the on time TON_M3.
V COMP_MAX * T RAMP_OFFSET
(eq. 7)
The maximum on time is limited by the maximum voltage
level VCOMP_MAX of the COMP signal, which typical value
is 3.5 V. A corresponding minimum input voltage VIN_MIN2
to maintain the target regulation voltage can be calculated
by:
V OUT ) I OUT @ ǒR DS_H ) DCRǓ
V IN_MIN2 +
(eq. 5)
The control loop pushes the COMP voltage to a higher
level VCOMP_M2 to get a wider on time TON_M2.
where VRAMP_OFFSET is the offset/valley voltage (0.4 V
typical) of the ramp signal.
When the input voltage VIN drops, the duty ratio D
increases to maintain the output regulation with the fixed
F SW_M3 +
1*D
.
T OFF_MIN
F SW_M2 +
(eq. 2)
V COMP_M1 + D @ T @ SR RAMP ) V RAMP_OFFSET,
(eq. 4)
Mode 2 – Frequency−Deduction Operation
As a result of the input feedforward function, the voltage
level VCOMP_M1 of the error signal COMP does not change
much with the input voltage VIN, having a value of
T ON_M3 +
1 * F SW @ T OFF_MIN
) I OUT @ ǒR DS_H * R DS_LǓ.
(eq. 1)
SR RAMP + 0.25 @ V IN @ F SW
V OUT ) I OUT @ ǒR DS_L ) DCRǓ
V OUT_M3 +
V IN * I OUT @ ǒR DS_H * R DS_LǓ
1)
(eq. 10)
0.25@V
V
@F
IN
COMP_MAX
@T
SW
*V
OFF_MIN
(eq. 12)
RAMP_OFFSET
* I OUT @ ǒR DS_L ) DCRǓ
The NCP5252 reaches its maximum limitation in the duty
ratio, which has a value DMAX when the input voltage is
equal to VIN_MIN2.
The voltage drop between VIN and VOUT increases as
IOUT increases.
http://onsemi.com
2
AND9095/D
5 V−Output Application
Figure 3 shows measurement results with a 1 A load
current and three nominal switching frequency options. The
higher the nominal frequency is selected, the higher
VIN_MIN2 is required to maintain the 5 V output voltage. VIN
Figure 4 shows measurement results of the switching
frequency changing with the input voltage for three nominal
frequency options. The NCP5252 is able to have about 60%
off in switching frequency in this application. The 333 kHz
frequency option is able to support the lowest minimum
input voltage in the low dropout applications.
To verify the operation of the NCP5252 in a low dropout
application, a 5 V−output regulator has been built on a
NCP5252 demo board. Three plots in Figure 2 show
waveforms of three key signals (VOUT, COMP, and LX)
under the three operation modes when VIN is close to VOUT.
The load current is 1 A and the selected nominal frequency
is 500 kHz in this test. The switching frequency drops
smoothly as VIN drops. The minimum required input
voltage for the 5 V/1 A output is about 5.5 V. The NCP5252
still be able to provide current to the output when VIN drops
further, but the output voltage starts to drop below the 5 V
target.
http://onsemi.com
3
AND9095/D
VOUT (1.0V/div)
COMP (2.0V/div)
LX (5.0V/div)
Time (1us/div)
(a) VIN = 7.0 V (Mode 1)
VOUT (1.0V/div)
COMP (2.0V/div)
LX (5.0V/div)
Time (1us/div)
(b) VIN = 5.75 V (Mode 2)
VOUT (1.0V/div)
COMP (2.0V/div)
LX (5.0V/div)
Time (1us/div)
(c) VIN = 5.0 V (Mode 3)
Figure 2. PWM Operation Waveforms vs. Input Voltage (VOUT = 5 V, IOUT = 1 A, 500 kHz Option)
http://onsemi.com
4
AND9095/D
Figure 3. Output Regulation VOUT vs. Input
Voltage VIN
Figure 4. Switching Frequency FSW vs. Input
Voltage VIN
measurements and the calculations grows with an increasing
in the load current, which may be caused by some omitted
factors in the calculations such as body diode forward
voltage during dead times, conduction resistance variations
regarding to temperatures of inductor and MOSFETs, and
other power lose factors.
As shown in Figure 5, a result comparison between
measurements and calculations has been done on the
minimum required input voltage VIN_MIN2 to maintain 5 V
output voltage under different load current conditions. Both
results address that higher minimum input voltage is
required for higher load current. Difference between the
http://onsemi.com
5
AND9095/D
Figure 5. Minimum Input Voltage VIN_MIN2 vs. Load Current IOUT (333 kHz Option)
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks,
copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC
reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any
particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without
limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications
and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC
does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for
surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where
personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and
its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly,
any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture
of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT:
Literature Distribution Center for ON Semiconductor
P.O. Box 5163, Denver, Colorado 80217 USA
Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada
Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada
Email: [email protected]
N. American Technical Support: 800−282−9855 Toll Free
USA/Canada
Europe, Middle East and Africa Technical Support:
Phone: 421 33 790 2910
Japan Customer Focus Center
Phone: 81−3−5817−1050
http://onsemi.com
6
ON Semiconductor Website: www.onsemi.com
Order Literature: http://www.onsemi.com/orderlit
For additional information, please contact your local
Sales Representative
AND9095/D