LMR70503_15 [TI]

SIMPLE SWITCHER Buck-Boost Converter For Negative Output Voltage in;
LMR70503_15
型号: LMR70503_15
厂家: TEXAS INSTRUMENTS    TEXAS INSTRUMENTS
描述:

SIMPLE SWITCHER Buck-Boost Converter For Negative Output Voltage in

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LMR70503  
www.ti.com  
SNVS850A JUNE 2012REVISED APRIL 2013  
®
LMR70503 SIMPLE SWITCHER Buck-Boost Converter For Negative Output Voltage in  
µSMD  
Check for Samples: LMR70503  
System Performance  
FEATURES  
1
23  
Tiny 8-Bump Thin DSBGA Package: 0.84 mm ×  
1.615 mm × 0.6 mm  
80  
70  
60  
50  
40  
30  
2.8 V to 5.5 V Input Voltage Range  
Adjustable Output Voltage: -0.9 V to -5.5 V  
320 mA Switch Current Limit  
500 kHz Minimum Switching Frequency  
Ground Referred Enable Input  
Under Voltage Lock Out (UVLO)  
No External Compensation  
VIN = 2.8V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
0
10 20 30 40 50 60 70 80 90 100  
LOAD (mA)  
Internal Soft Start  
1 µA Shutdown Supply Current  
Small Output Voltage Ripple  
WEBENCH® Enabled  
Figure 1. Efficiency, VOUT= -5.0 V  
80  
70  
60  
50  
APPLICATIONS  
General Purpose Negative Voltage Supply  
Negative Rail / Bias Supply For Op-amp And  
Data Converters  
LCD Biasing  
VIN = 2.8V  
VIN = 3.3V  
VIN = 4.0V  
40  
PERFORMANCE BENEFITS  
VIN = 5.0V  
VIN = 5.5V  
Easy To Use  
30  
Tiny Overall Solution Size Reduces System  
Cost  
0
30  
60  
90  
120 150 180  
LOAD (mA)  
Figure 2. Efficiency, VOUT= -2.5 V  
DESCRIPTION  
The LMR70503 is a buck-boost converter with  
adjustable negative output voltage in a tiny 8-bump  
thin DSBGA package. Its unique control method is  
designed to provide fast transient response, low  
output noise, high efficiency, and tight regulation in  
the smallest possible PCB area. The LMR70503 has  
built in soft start, peak current limit, minimum  
switching frequency, and Under Voltage Lock Out  
(UVLO), with no external compensation required. For  
ease of use, the Enable pin is referred to the IC  
ground, instead of the lowest potential of the IC: the  
negative output voltage.  
Typical Application Circuit  
L
VIN (2.8V to 5.5V)  
VIN  
SW  
Cout  
Cin  
D
VOUT  
VOUT  
(-0.9V to -5.5V)  
LMR70503  
1.8V  
Cff  
Rb  
Rt  
EN  
0V  
FB  
Refer to GND  
GND  
VREF  
1
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of  
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.  
SIMPLE SWITCHER, WEBENCH are registered trademarks of Texas Instruments.  
2
3
All other trademarks are the property of their respective owners.  
PRODUCTION DATA information is current as of publication date.  
Products conform to specifications per the terms of the Texas  
Instruments standard warranty. Production processing does not  
necessarily include testing of all parameters.  
Copyright © 2012–2013, Texas Instruments Incorporated  
LMR70503  
SNVS850A JUNE 2012REVISED APRIL 2013  
www.ti.com  
Connection Diagram  
1
2
VREF  
FB  
A
B
VOUT  
EN  
GND  
SW  
GND  
VIN  
C
D
Figure 3. LMR70503 Bump Locations - Top View  
1
2
A
B
C
D
Figure 4. LMR70503 Package Marking - Top View  
(Diamond Denotes Bump A1)  
PIN DESCRIPTIONS  
Pin Number  
Name  
Description  
A1  
VREF  
Reference voltage output; connect to the bottom feedback resistor.  
Active high enable input for the device. Enable voltage level is referred to GND. Device must be enabled only  
with the presence of valid VIN (2.8 V to 5.5 V). The peak of the Enable input voltage must always lower than  
VIN voltage.  
B1  
C1, C2  
D1  
EN  
GND  
SW  
Analog ground for internal bias circuitry.  
Switch node pin, connected to the internal high side MOSFET. The cathode of the external Schottky diode  
must be connected as close as possible to this pin, in order to reduce inductance in the discontinuous current  
path.  
FB is connected to VOUT and VREF through two feedback resistors. It is compared to GND to regulate the  
output voltage.  
A2  
FB  
Output voltage. The anode of the external Schottky diode and output filter capacitor(s) should be connected to  
this pin.  
B2  
D2  
VOUT  
VIN  
Power supply input pin, connected to the internal high side MOSFET and powers the internal circuity.  
2
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SNVS850A JUNE 2012REVISED APRIL 2013  
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam  
during storage or handling to prevent electrostatic damage to the MOS gates.  
ABSOLUTE MAXIMUM RATINGS(1)(2)  
VIN to GND  
-0.5 V to 6.0 V  
-6.5 V to 0.5 V  
-6.5 V to VIN +0.2 V  
-0.5 V to VIN  
-0.5V to 5.5V  
±2 kV  
VOUT to GND  
SW to GND  
EN to GND  
FB to GND  
ESD Rating(3)  
Junction Temperature  
Storage Temperature Range  
150 °C  
-65 °C to 150 °C  
For Soldering Specs see:  
SNOA549  
(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of  
device reliability and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or  
other conditions beyond those indicated in the recommended Operating Ratings is not implied. The recommended Operating Ratings  
indicate conditions at which the device is functional and should not be operated beyond such conditions.  
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and  
specifications.  
(3) ESD using the human body model which is a 100 pF capacitor discharged through a 1.5 kresistor into each pin. Test method is per  
JESD22–A114.  
OPERATING RATINGS  
Input Voltage Range (VIN  
)
2.8 V to 5.5 V  
-0.9 V to -5.5 V  
-40°C to 125°C  
Output Voltage Range (VOUT  
)
Junction Temperature Range (TJ)  
ELECTRICAL CHARACTERISTICS  
Specifications with standard typeface are for TJ = 25°C only; limits in bold face type apply over the operating junction  
temperature (TJ) range of -40 °C to +125 °C. Typical values represent the most likely parametric norm at TJ = 25°C, and are  
provided for reference purposes only. VIN = 3.3 V, VOUT = -5.0 V, VEN = 1.8 V, unless otherwise indicated in the conditions  
column.  
Min  
Typ  
Max  
Symbol  
VREF  
ISD  
Parameter  
Conditions  
Units  
V
(1)  
(2)  
(1)  
Reference Voltage  
Shutdown Current  
RREF=100 kto GND  
1.166  
1.19  
0.01  
1.214  
1
EN = 0 V  
VIN = 5.5 V  
µA  
EN = 1.8 V, VIN = 5.5 V,  
No Switching  
IQ  
Quiescent Current  
245  
2.55  
0.13  
300  
2.7  
µA  
V
VIN Under Voltage Lock Out Threshold -  
Rising  
UVLORISE  
UVLOHYS  
VIN Under Voltage Lock Out Hysteresis  
Band  
0.1  
0.1  
V
VEN-RISE  
VEN-HYS  
IEN  
EN Input Voltage Rising Threshold  
EN Input Voltage Threshold Hysteresis  
Enable Current  
VIN = 5.5 V  
VIN = 5.5 V  
1.05  
0.15  
30  
1.2  
V
V
nA  
nA  
kHz  
ns  
IFB  
FB pin current  
10  
FSW-MIN  
TON-MIN  
RDSON  
Minimum Switching frequency  
Minimum High Side Switch On Time  
Switch On State Resistance  
400  
500  
70  
Load = 0 A  
VIN = 2.8V  
1.1  
2
(1) Min and Max limits are 100% production tested at an ambient temperature (TA) of 25 °C. Limits over the operating temperature range  
are specified through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate Average Outgoing Quality  
Level (AOQL).  
(2) Typical specifications represent the most likely parametric norm at 25°C operation.  
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ELECTRICAL CHARACTERISTICS (continued)  
Specifications with standard typeface are for TJ = 25°C only; limits in bold face type apply over the operating junction  
temperature (TJ) range of -40 °C to +125 °C. Typical values represent the most likely parametric norm at TJ = 25°C, and are  
provided for reference purposes only. VIN = 3.3 V, VOUT = -5.0 V, VEN = 1.8 V, unless otherwise indicated in the conditions  
column.  
Min  
Typ  
Max  
Symbol  
Parameter  
Conditions  
Units  
(1)  
(2)  
(1)  
IPEAK-CL  
Switch Peak Current limit(3)  
270  
320  
165  
10  
370  
mA  
°C  
TSDTH-HIGH  
TSDHYS  
Thermal Shutdown Threshold - Rising  
Thermal Shutdown Hysteresis Band  
Junction Temperature  
Junction Temperature  
°C  
(3) The switch peak current limit is internally trimmed. The actual peak current limit observed on the applications are dependant on the input  
voltage VIN, inductance value L and junction temperature TJ.  
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TYPICAL PERFORMANCE CHARACTERISTICS  
Unless otherwise specified, the following conditions apply: VIN = 3.3 V, VOUT = -5.0 V, VEN = 1.8 V, CIN = 10 µF 6.3 V X5R  
ceramic capacitor; COUT = 2 × 22 µF 6.3 V X5R ceramic capacitor; L = 6.8 µH (VLS2012ET-6R8M); TAMBIENT = 25 °C.  
Efficiency, VOUT = -5.0 V  
Output Regulation, VOUT = -5.0 V  
80  
70  
60  
50  
40  
30  
5.10  
5.08  
5.06  
5.04  
5.02  
5.00  
4.99  
4.96  
4.95  
4.92  
4.90  
VIN = 2.8V  
VIN = 2.8V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
0
0
0
10 20 30 40 50 60 70 80 90 100  
LOAD (mA)  
0
0
0
10 20 30 40 50 60 70 80 90 100  
LOAD (mA)  
Figure 5.  
Figure 6.  
Efficiency, VOUT = -3.3 V  
Output Regulation, VOUT = -3.3 V  
80  
70  
60  
50  
40  
30  
3.40  
3.38  
3.36  
3.34  
3.32  
3.30  
3.28  
3.26  
3.24  
3.22  
3.20  
VIN = 2.8V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
VIN = 2.8V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
20  
40  
60  
80 100 120 140  
20 40 60 80 100 120 140  
LOAD (mA)  
LOAD (mA)  
Figure 7.  
Figure 8.  
Efficiency, VOUT = -2.5 V  
Output Regulation, VOUT = -2.5 V  
80  
70  
60  
50  
40  
30  
2.60  
2.58  
2.56  
2.54  
2.52  
2.50  
2.48  
2.46  
2.44  
2.42  
2.40  
VIN = 2.8V  
VIN = 2.8V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
30  
60  
90  
120 150 180  
30  
60  
90  
120 150 180  
LOAD (mA)  
LOAD (mA)  
Figure 9.  
Figure 10.  
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)  
Unless otherwise specified, the following conditions apply: VIN = 3.3 V, VOUT = -5.0 V, VEN = 1.8 V, CIN = 10 µF 6.3 V X5R  
ceramic capacitor; COUT = 2 × 22 µF 6.3 V X5R ceramic capacitor; L = 6.8 µH (VLS2012ET-6R8M); TAMBIENT = 25 °C.  
Efficiency, VOUT = -1.5 V  
Output Regulation, VOUT = -1.5 V  
80  
70  
60  
50  
40  
30  
1.60  
1.58  
1.56  
1.54  
1.52  
1.50  
1.48  
1.46  
1.44  
1.42  
1.40  
VIN = 2.8V  
VIN = 2.8V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
0
30  
60  
90 120 150 180 210  
LOAD (mA)  
0
30 60 90 120 150 180 210  
LOAD (mA)  
Figure 11.  
Figure 12.  
Efficiency, VOUT = -0.9 V  
Output Regulation, VOUT = -0.9 V  
80  
70  
60  
50  
40  
30  
1.00  
0.98  
0.96  
0.94  
0.92  
0.90  
0.88  
0.86  
0.84  
0.82  
0.80  
VIN = 2.8V  
VIN = 2.8V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
VIN = 3.3V  
VIN = 4.0V  
VIN = 5.0V  
VIN = 5.5V  
0
50  
100  
150  
200  
250  
0
50  
100  
150  
200  
250  
LOAD (mA)  
LOAD (mA)  
Figure 13.  
Figure 14.  
Maximum Load Current  
Minimum Switching Frequency  
250  
200  
150  
100  
50  
600  
580  
560  
540  
520  
500  
480  
460  
VOUT = -5V  
VOUT = -3.3V  
VOUT = -2.5V  
VOUT = -1.5V  
VOUT = -0.9V  
Temp = -40°C  
Temp = 25°C  
Temp = 125°C  
0
2.8 3.2 3.6 4.0 4.4 4.8 5.2  
VIN (V)  
2.5  
3.0  
3.5  
4.0  
VIN (V)  
4.5  
5.0  
5.5  
Figure 15.  
Figure 16.  
6
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LMR70503  
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SNVS850A JUNE 2012REVISED APRIL 2013  
TYPICAL PERFORMANCE CHARACTERISTICS (continued)  
Unless otherwise specified, the following conditions apply: VIN = 3.3 V, VOUT = -5.0 V, VEN = 1.8 V, CIN = 10 µF 6.3 V X5R  
ceramic capacitor; COUT = 2 × 22 µF 6.3 V X5R ceramic capacitor; L = 6.8 µH (VLS2012ET-6R8M); TAMBIENT = 25 °C.  
No Load Supply Current  
Rds-on  
3.0  
2.5  
2.0  
1.5  
1.0  
0.5  
0.0  
2.0  
1.8  
1.6  
1.4  
1.2  
1.0  
0.8  
0.6  
0.4  
0.2  
0.0  
Vin = 2.8V  
Vin = 4.0V  
Vin = 5.5V  
2.8 3.2 3.6 4.0 4.4 4.8 5.2  
VIN (V)  
-40 -20  
0
20 40 60 80 100 120 140  
TEMPERATURE (°C)  
Figure 17.  
Figure 18.  
Enable Thresholds  
Soft Start Time (No Load)  
Vout = -5.0V  
800  
700  
600  
500  
400  
300  
200  
100  
0
Vout = -3.3V  
Vout = -2.5V  
Vout = -1.5V  
Vout = -0.9V  
1.1  
1.0  
0.9  
0.8  
0.7  
0.6  
0.5  
0.4  
Rising TH -40°C  
Falling TH -40°C  
Rising TH 25°C  
Falling TH 25°C  
Rising TH 125°C  
Falling TH 125°C  
2.5  
3.0  
3.5  
4.0  
4.5  
5.0  
5.5  
2.5  
3.0  
3.5  
4.0  
4.5  
5.0  
5.5  
VIN (V)  
VIN (V)  
Figure 19.  
Figure 20.  
Soft Off Time, VOUT = -5.5 V (No Load, From EN Falling  
Edge)  
Soft Start Delay Time (From EN Rising Edge)  
160  
800  
700  
600  
500  
140  
120  
100  
80  
60  
40  
Temp = -40°C  
VIN = 2.8 V  
VIN = 3.0 V  
VIN = 4.0 V  
VIN = 5.0 V  
400  
300  
Temp = 25°C  
20  
Temp = 125°C  
0
2.5  
3.0  
3.5  
4.0  
VIN (V)  
4.5  
5.0  
5.5  
0
10 20 30 40 50 60 70 80 90  
TEMP (°C)  
Figure 21.  
Figure 22.  
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)  
Unless otherwise specified, the following conditions apply: VIN = 3.3 V, VOUT = -5.0 V, VEN = 1.8 V, CIN = 10 µF 6.3 V X5R  
ceramic capacitor; COUT = 2 × 22 µF 6.3 V X5R ceramic capacitor; L = 6.8 µH (VLS2012ET-6R8M); TAMBIENT = 25 °C.  
Soft Start And Soft Off Waveform  
VIN = 5.0 V, VOUT = -5.0 V, No Load  
Soft Start And Soft Off Waveform  
VIN = 5.0 V, VOUT = -5.0 V, Load = 50  
2V/Div  
1V/Div  
2V/Div  
V
SW  
V
SW  
EN  
EN  
1V/Div  
V
OUT  
V
OUT  
1V/Div  
1V/Div  
100 mA/Div  
500 µs/Div  
I
L
100 mA/Div  
500 µs/Div  
I
L
Figure 23.  
Figure 24.  
Typical Switching Waveform  
VIN = 5.0 V, VOUT = -5.0 V, No Load  
Typical Switching Waveform  
VIN = 5.0 V, VOUT = -5.0 V, IOUT = 70 mA  
5V/Div  
V
SW  
V
SW  
5V/Div  
10 mV/Div, AC coupled  
V
V
OUT  
OUT  
5 mV/Div  
AC coupled  
200 mA/Div  
2 µs/Div  
I
I
L
L
200 mA/Div  
2 µs/Div  
Figure 25.  
Figure 26.  
Load Transient, VIN = 4.0 V, VOUT = -5.5 V  
Load steps between 2 mA and 50 mA  
Short Circuit Waveform  
VIN = 5.0 V, VOUT = -5.5 V  
2V/Div  
V
SW  
V
SW  
0V  
-5.5V  
2V/Div  
20 mV/Div, AC coupled  
10 mA/Div  
0V  
V
OUT  
EN  
V
OUT  
OUT  
1V/Div  
-5.5V  
I
0.5V/Div  
100 mA/Div  
2 ms/Div  
I
L
I
L
100 mA/Div  
5 ms/Div  
Figure 27.  
Figure 28.  
8
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BLOCK DIAGRAM  
Peak Current Limit  
Modulation  
-
GND  
FB  
+
-
VIN  
VIN  
+
+
-
CIN  
UVLO  
Current  
Sensing  
shutdown  
R
R
GND  
FB  
-
LOGIC  
EN  
Driver  
S
S
Q
+
L
SW  
Minimal Frequency  
Detector  
Voltage  
Reference  
shutdown  
GND  
COUT  
TSD  
R
R
D
Min Off  
Timer  
Soft Pull  
Down  
VOUT  
VREF  
FB  
VOUT  
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OPERATING DESCRIPTION  
The LMR70503 integrates an inverting buck-boost controller and a high-side MOSFET in one tiny 8-bump thin  
DSBGA package. A simplified buck-boost converter schematic is shown in Figure 29.  
VIN  
SW  
VOUT  
Figure 29. Buck Boost Converter  
The LMR70503 controller incorporates a unique peak current mode control method with a minimum switching  
frequency limit. The integrated switch is turned off when its current crosses the peak current limit, while it is  
turned on when the magnitude of VOUT droops below a threshold. When the switch is off, the inductor current  
goes through the diode and charges the output capacitor(s). With fixed peak current limit, the switching  
frequency decreases with decreased load current. At light load, the switching frequency will decrease to the  
audible frequency range, which is not acceptable in many applications. The LMR70503 is designed to operate  
with peak current mode control and limit the switching frequency to 500 kHz (typical) minimum, to avoid audible  
frequency interference. At light load, when the switching frequency drops to the minimum, the inductor current  
limit is reduce instead of frequency to maintain regulation. The LMR70503 also incorporates an internal dummy  
load to compensate for the extra charges in the minimum ON-time (TON-MIN) condition. More details on the  
LMR70503 operation are described in the later sessions. Typical switching waveforms in discontinuous  
conduction mode (DCM) and continuous conduction mode (CCM), including the inductor current, the switch node  
voltage and the output voltage ripple (absolute value), are shown in Figure 30.  
DCM  
CCM  
Inductor  
Current  
Inductor  
Current  
VIN  
VIN  
Switch  
Node  
0V Switch  
Node  
0V  
VOUT  
VOUT  
|VOUT  
|
|VOUT|  
t
Ts  
2Ts  
3Ts  
t
Ts  
2Ts  
Figure 30. Typical Waveforms In Buck Boost Converter  
Figure 31 illustrates the switching frequency, the peak current limit, the output voltage and the dummy load with  
different load current. More details on each operation mode will be described later.  
1. No load to very light load: high side switch is turned on for TON-MIN; switching frequency is limited at the  
minimum switching frequency; and the dummy load is turned on.  
2. Light load: switching frequency is limited at the minimum switching frequency, peak current limit increases  
with increased load current; and the dummy load is off.  
3. Heavy load: peak current equals the maximum peak current limit; switching frequency increases with  
increased load current; and the dummy load is off.  
10  
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Switching  
frequency  
F
SW-MIN  
0
Load current  
Load current  
|VOUT  
|
0
I
Peak Inductor  
Current  
PEAK-MAX  
0
Load current  
Load current  
Dummy Load  
Ton-min0  
(1)  
Const Frequency  
(2)  
Const Peak Current  
(3)  
Figure 31. The LMR70503 Operation Modes vs. Load Current  
Minimum Switching Frequency Operation  
In a typical peak current mode controlled DC-DC converter, the peak current limit is constant and the switching  
frequency decreases when load current reduces. To maintain low noise operation and avoid audio frequency  
interference, the minimum switching frequency of the LMR70503 is limited at 500 kHz typically. At heavy load,  
the peak current limit remains constant and the switching frequency varies with the load to regulate the output  
voltage. With reduced loading, the absolute output voltage is going to be charged higher than regulation if the  
switching frequency cannot decrease accordingly. Therefore, to regulate the output voltage with minimum  
frequency at light load, the peak current limit is reduced, in proportional to the output voltage offset.  
In this mode, as shown in Figure 31, the switching frequency is fixed to the minimum switching frequency, the  
peak inductor current increases with load current, and the output voltage magnitude has a small offset above  
regulation.  
Minimum ON-Time and Dummy Load  
When load current is near zero, the peak inductor current can not reduce further due to TON-MIN of the high side  
switch. Under such conditions, an internal dummy load is turned on by sensing excessive output voltage offset,  
which removes the extra charge from the output capacitor(s). In this condition, the switching frequency is fixed to  
the minimum value. The peak inductor current value is at its minimum value, as shown in Figure 31. The dummy  
load current is zero when the LMR70503 operates with on time higher than TON-MIN  
.
The minimum peak inductor current is determined by  
IPEAK-MIN = TON-MIN × VIN / L  
where  
VIN is the supply voltage  
L is the inductance value  
(1)  
(2)  
The peak inductor current is higher with higher VIN. The inductor current falling slew rate is determined by  
SRFALLING = (|VOUT| + VF) / L  
where  
|VOUT| is the absolute value of the output voltage  
VF is the forward voltage drop of the power diode  
At lower |VOUT|, it takes longer time to discharge the inductor current to zero. Therefore, there is more energy to  
charge the output capacitor(s). The output voltage will have more offset at higher VIN and lower VOUT. The  
dummy load current is a function of the FB voltage: the more the offset at the FB node, the higher the dummy  
load current, as shown in Figure 32.  
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10  
9
8
7
6
5
4
3
2
1
0
VIN=5.5V -40°C  
VIN=5.5V 25°C  
VIN=5.5V 125°C  
VIN=2.8V -40°C  
VIN=2.8V 25°C  
VIN=2.8V 125°C  
-50  
-40  
-30  
-20  
-10  
0
FB VOLTAGE (mV)  
Figure 32. Dummy Load Current vs. FB Voltage  
Constant Peak Current Operation  
If the load current increases in the minimum switching frequency mode, the peak current limit will reach the  
maximum peak current limit (IPEAK-MAX). After this point, the LMR70503 behaves as a constant peak current  
converter with frequency modulation. The transition load level between the constant frequency mode and the  
constant peak current mode varies with VIN, VOUT and L.  
The IPEAK-MAX is trimmed to 320 mA in the LMR70503. Due to propagation delays in the comparator and gate  
drive, the measured peak inductor current will be higher than the trimmed value. The additional offset on the  
maximum peak current is proportional to the inductor current rising slope: VIN / L, approximately. For a typical  
inductor, the inductance will reduce at hot temperature. Therefore, IPEAK-MAX is the highest with 5.5 V input  
voltage at hot temperature.  
In the constant peak current operation mode, the switching frequency will increase with the increased load  
current, until the high side switch off time equals the minimum off-time (TOFF-MIN) limit. If the load keeps  
increasing when the switch operates with TOFF-MIN, VOUT will drop out of regulation due to loading limits of buck-  
boost type of converters. The maximum loading capability is higher with higher VIN, larger L, lower VOUT, and less  
losses in the converter. Figure 33 shows the measured maximum load current measured with the typical BOM  
shown in Table 1. To increase the maximum loading capability with given VIN and VOUT, one can choose a higher  
inductance value and a diode with lower forward voltage drop VF.  
250  
200  
150  
100  
VOUT = -5V  
VOUT = -3.3V  
VOUT = -2.5V  
VOUT = -1.5V  
VOUT = -0.9V  
50  
0
2.8 3.2 3.6 4.0 4.4 4.8 5.2  
VIN (V)  
Figure 33. LMR70503 Loading Capability vs. VIN, L = 6.8 µH  
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The built-in TOFF-MIN time is a function of both VIN and VOUT, as shown in Figure 34.  
900  
800  
700  
600  
500  
400  
300  
VOUT = -0.9V  
200  
VOUT = -1.5V  
VOUT = -2.5V  
VOUT = -3.3V  
VOUT = -5.0V  
100  
0
2.8 3.2 3.6 4.0 4.4 4.8 5.2  
VIN (V)  
Figure 34. Minimum Off Time vs. VIN at room temperature  
Enable And UVLO  
The LMR70503 features an enable (EN) pin and associated comparator to allow the user to easily sequence the  
LMR70503 from an external voltage rail, or to manually set the input UVLO threshold. Enable threshold levels  
are referred to the LMR70503 ground, instead of the lowest potential: the negative output voltage. Enable turning  
on (rising) and turning off (falling) thresholds are shown in Figure 35.  
1.1  
1.0  
0.9  
0.8  
0.7  
Rising TH -40°C  
Falling TH -40°C  
0.6  
Rising TH 25°C  
Falling TH 25°C  
0.5  
0.4  
Rising TH 125°C  
Falling TH 125°C  
2.5  
3.0  
3.5  
4.0  
4.5  
5.0  
5.5  
VIN (V)  
Figure 35. Enable Rising And Falling Thresholds vs. VIN  
It is important to ensure that a valid input voltage (2.8 V VIN5.5 V) is present on the VIN pin before the EN  
input is asserted. Also, as stated in the Absolute Maximum Ratings section of this data sheet, the voltage on the  
EN pin must always be less than VIN. This applies to both static and dynamic operation, and during start up and  
shut down sequences. If these precautions are not followed, an internal test mode may be activated; possibly  
damaging the regulator. The EN input must not be left floating. A resistor divider can be added from VIN to EN if  
an external enable signal is not available.  
An input under voltage lock-out (UVLO) circuit prevents the regulator from turning on when the input voltage is  
not great enough to properly bias the internal circuitry. The typical UVLO rising threshold is 2.55 V and typical  
hysteresis band is 0.13 V.  
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Soft Start And Soft Off  
The LMR70503 begins to operate when EN goes high with the presence of valid VIN, or VIN swings below UVLO  
level and back up with the presence of valid EN voltage. The soft start action is inherent with the maximum peak  
current limit and minimum off time. During start up, the inductor current rises to the maximum peak current limit,  
then the high-side switch is turned off for TOFF-MIN and the output capacitor(s) is charged during this time. Then  
the high-side turns on to repeat the cycle. After the output voltage is charged to the regulation level, the  
LMR70503 will operate in steady state. The soft start time will be longer with more output capacitance, and / or  
lower supply voltage VIN, and / or more loading during start up. Figure 36 shows soft start vs VIN with L= 6.8 µH  
and no load. Soft-start is reset any time the part is shut down or a thermal shutdown event occurs.  
800  
Vout = -5.0V  
Vout = -3.3V  
700  
600  
500  
400  
300  
200  
100  
0
Vout = -2.5V  
Vout = -1.5V  
Vout = -0.9V  
2.5  
3.0  
3.5  
4.0  
4.5  
5.0  
5.5  
VIN (V)  
Figure 36. Soft Start Time (No Load) vs. VIN  
The LMR70503 will shutdown when EN pin voltage goes below the falling threshold, or VIN goes below UVLO  
falling threshold. When shutdown, the LMR70503 incorporates an output voltage discharge feature to bring the  
output voltage to zero volts, regardless of the load current. When the EN input is taken below its lower threshold,  
an internal MOSFET turns on and discharges the output capacitors. Typical soft off times (from EN falling edge  
to 10% of Vout ) over VIN and temperature are shown in Figure 37. Figure 38 shows the typical off time from 90%  
to 10% of Vout.  
800  
700  
600  
500  
VIN = 2.8 V  
VIN = 3.0 V  
VIN = 4.0 V  
VIN = 5.0 V  
400  
300  
0
10 20 30 40 50 60 70 80 90  
TEMP (°C)  
Figure 37. Soft Off Time (EN Falling Edge To 10% Vout) vs. Temperature, VOUT = -5.5 V, No Load  
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800  
700  
600  
500  
400  
300  
VIN = 2.8 V  
VIN = 4.0 V  
VIN = 5.0 V  
0
10 20 30 40 50 60 70 80 90  
TEMPERATURE (°C)  
Figure 38. Soft Off Time (90% To 10% Vout) vs. Temperature, VOUT = -5.5 V, No Load  
Short Circuit Protection  
Peak current mode control has inherent short circuit protection. The protection level is the maximum inductor  
current limit level. It varies with VIN and temperature due to propagation delays. The minimum off-time limits the  
current going through the inductor during a short circuit condition.  
Over-Temperature Protection  
Internal thermal shutdown (TSD) circuitry protects the LMR70503 should the maximum junction temperature be  
exceeded. This protection is activated at 165 °C (typical), with the result that the regulator will shutdown until the  
junction temperature drops below 155 °C (typical). Of course the LMR70503 must not be operated continuously  
above 125 °C.  
Design Guide  
Output Voltage Setting  
The output voltage of the LMR70503 is programmable by the voltage divider resistors. The reference voltage is  
typically 1.19 V. To avoid overloading the VREF circuity, the resistor RT tied between VREF and FB is  
recommended to be between 20 kand 100 k. With a selected RT, RB tied between VOUT and FB can be found  
by  
RB = RT * |VOUT| / VREF  
(3)  
A feed-forward capacitor CFF can be used between VOUT and FB nodes to improve transient performance. 10 pF  
C0G, NP0 type of capacitor is recommended in LMR70503 applications.  
Input Capacitor And Output Capacitor Selection  
The input capacitor selection is based on both input voltage ripple and RMS current. Good quality input  
capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the regulator current  
during switch on-time. Low ESR ceramic capacitors are preferred. A minimum value of 10μF at 6.3 V, is required  
at the input of the LMR70503. Larger values of input capacitance are desirable to reduce voltage ripple and  
noise on the input supply.  
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The output capacitor is responsible for filtering the output voltage and suppling load current during transients and  
during the power diode off-time. Best performance is achieved with ceramic capacitors. For most applications, a  
minimum value of 22 μF, 6.3 V capacitor is required at the output of the LMR70503. The percentage of ripple  
coupled to the FB node can be found by  
RIPPLE PERCENTAGE = VREF / ( |VOUT| + VREF  
)
where  
|VOUT| is the magnitude of the output voltage  
VREF is the reference voltage  
(4)  
With lower magnitude VOUT, a higher percentage of output voltage ripple is coupled to the FB node. Output  
voltage ripple is also coupled to the FB node via the feed-forward capacitor CFF. Excessive ripple at the FB node  
may trigger peak current limit modulation causing unstable operation. Higher output capacitance is needed at  
lower magnitude output voltage. For VOUT = -0.9 V, a minimum of 44 μF, 6.3 V capacitor is required. Avoid using  
too much capacitance at CFF.  
A capacitor between VIN and VOUT also can be used to provide high frequency bypass. This capacitor is  
equivalent to the output capacitors in the small signal model. It also reduces the output voltage ripple if  
sufficiency capacitance is used. The voltage rating for this capacitor should be higher than VIN + |VOUT|.  
All ceramic capacitors have large voltage coefficients, in addition to normal tolerances and temperature  
coefficients. To help mitigate these effects, multiple capacitors can be used in parallel to bring the minimum  
capacitance up to the desired value. This may also help with RMS current constraints by sharing the current  
among several capacitors. With the LMR70503, ceramic capacitors rated at 6.3 V, or higher, are suitable for all  
input and output voltage combinations. Many times it is desirable to use an electrolytic capacitor on the input, in  
parallel with the ceramics. The moderate ESR of this capacitor can help to damp any ringing on the input supply  
caused by long power leads. This method can also help to reduce voltage spikes that may exceed the maximum  
input voltage rating of the LMR70503.  
Power Inductor Selection  
The power inductor selection is critical to the operation of the LMR70503. It affects the efficiency, the operation  
mode transition point, the maximum loading capability and the size / cost of the solution. A 4.7 μH or 6.8 μH  
inductor is recommended for most LMR70503 applications. The maximum loading capability is reduced with  
smaller inductance value. The no load VOUT offset is higher at low VOUT with smaller inductance value, due to  
higher peak current with the same TON-MIN. Higher inductance value usually comes with higher DCR with the  
same size and cost. Higher DCR will reduce the efficiency especially at heavy load.  
The inductor must be rated above the maximum peak current limit to prevent saturation. Good design practice  
requires that the inductor rating be adequate for the maximum IPEAK-MAX over VIN and temperature, plus some  
safety margin. If the inductor is not rated for the maximum expected current, saturation at high current may  
cause damage to the LMR70503 and/or the power diode. The DCR of the inductor should be as small as  
possible with given size / cost constrains to achieve optimal efficiency.  
Power Diode Selection  
A Schottky type power diode is required for all LMR70503 applications. The parameters of interests include the  
reverse voltage rating, the DC current rating, the repetitive peak current rating, the forward voltage drop, the  
reverse leakage current and the parasitic capacitance. In a buck-boost, this diode sees a reverse voltage of :  
VR-DIODE = |VOUT| + VIN  
(5)  
The reverse breakdown voltage rating of the diode should be selected for this value, plus safety margin. A good  
rule of thumb is to select a diode with a reverse voltage rating of 1.3 times this maximum. Select a diode with a  
DC current rating at least equal to the maximum load current that will be seen in the application and the  
repetitive peak current rating higher than IPEAK-MAX over VIN and temperature. The forward voltage drop of the  
power diode is a big part of the power loss in a buck-boost converter. It is preferred to be as low as possible. The  
reverse leakage current and the parasitic capacitance are also part of the power losses in the converter, but  
usually less pronounced than the forward voltage drop loss. Pay attention to the temperature coefficients of all  
the parameters. Some of them may vary greatly over temperature and may adversely affect the efficiency over  
temperature.  
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PC Board Layout Guidelines  
Board layout is critical for the proper operation of switching power converters. Switch mode converters are very  
fast switching devices. In such cases, the rapid increase of current combined with the parasitic trace inductance  
generates unwanted L·di/dt noise spikes. The magnitude of this noise tends to increase as the output current  
increases. This noise may turn into electromagnetic interference (EMI) and can also cause problems in device  
performance. Therefore, care must be taken in layout to minimize the effect of this switching noise. The most  
important layout rule is to keep the AC current loops as small as possible.  
Figure 29 shows the current flow in a buck-boost converter. The two dotted arrows indicate the current paths  
when the high side switch is on and when the power diode is on, respectively. The components and traces that  
contain discontinuous currents are critical in PCB layout design, since discontinuous currents contain high di/dt  
and high frequency noise. The components that carry critical discontinuous currents include the input  
capacitor(s), the high side switch, the power diode and the output capacitor(s). These components need to be  
placed as close as possible to each other and the traces between them must be made as short and wide as  
possible: place the input capacitor(s) as close as possible to the VIN pin of the LMR70503; place the cathode of  
the diode as close as possible to the SW pin; the anode of the diode should be as close as possible to the output  
capacitor(s); the GND end of the output capacitor(s) should be as close as possible to that of the input  
capacitor(s). Doing so will yield a small loop area, reducing the loop inductance and EMI.  
The feedback resistors RB and RT should be placed as close as possible to the FB pin. Since FB is a high  
impedance node, noise is likely be coupled to the FB node if the trace is long. The traces from VOUT to the  
resistor divider and from the divider to the FB pin should be far away from the discontinuous current path. It is  
recommended to use 4-layer board with ground plane as an internal layer, route the discontinuous current path  
on the top layer and the feedback path on the other side of the ground plane. Then the feedback path will be  
shielded from switching noise.  
To avoid functional problems due to layout, review the PCB layout example in Figure 39. It is also recommended  
to use 1oz copper boards or heavier to help reducing the parasitic inductances of board traces.  
PCB Layout Example  
Figure 39. PCB Layout Example (top layer and top overlay)  
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LMR70503 Typical Application Circuit  
L
VIN  
VIN  
SW  
Cin  
Cout1  
Cout2  
D
VOUT  
VOUT  
Ren1  
LMR70503  
Cff  
Rb  
Rt  
EN  
FB  
Ren2  
GND  
VREF  
LMR70503 Application Circuit Bill of Materials  
VIN = 2.8 V to 5.5 V, VOUT has options of -0.9 V, -1.5 V, -2.5 V, -3.3 V and -5.0 V. Optimized for minimum solution  
size.  
Table 1. Bill of Materials  
Designator  
Description  
Case Size  
Manufacturer  
Manufacturer P/N  
U1  
Inverting Buck-Boost  
8-bump thin DSBGA  
Texas Instruments  
LMR70503TM NOPB  
Ceramic 10 µF 10 V X5R  
0603  
CIN  
0603  
0603  
0402  
TDK  
C1608X5R1A106M  
C1608X5R0J226M  
Ceramic 22 µF 6.3 V X5R  
0603  
COUT1, COUT2  
Cff  
TDK  
CAP CER 10PF 50V 5%  
NP0 0402  
Murata  
GRM1555C1H100JZ01D  
D
L
Schottky 30 V 500 mA  
SOD882  
NXP Semi  
TDK  
PMEG3005EL  
6.8 µH, 0.76 A 362 mΩ  
2.0*2.0*1.2mm  
VLS2012ET-6R8M  
RES, 100k ohm, 1%,  
0.063W, 0402  
RT  
0402  
Vishay Dale  
CRCW0402100KFKED  
422 kFor Vout = -5.0V  
274 kFor Vout = -3.3V  
210 kFor Vout = -2.5V  
127 kFor Vout = -1.5V  
75 kFor Vout = -0.9V  
0402  
0402  
0402  
0402  
0402  
Vishay Dale  
Vishay Dale  
Vishay Dale  
Vishay Dale  
Vishay Dale  
CRCW0402422KFKED  
CRCW0402274KFKED  
CRCW0402210KFKED  
CRCW0402127KFKED  
CRCW040275K0FKED  
(1)  
RB  
RES, 20k ohm, 5%,  
0.063W, 0402  
REN1, REN2  
0402  
Vishay Dale  
CRCW040220K0JNED  
(1) RB is represented by R1 in Figure 39.  
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REVISION HISTORY  
Changes from Original (April 2013) to Revision A  
Page  
Changed layout of National Data Sheet to TI format .......................................................................................................... 18  
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PACKAGE OPTION ADDENDUM  
www.ti.com  
4-Apr-2013  
PACKAGING INFORMATION  
Orderable Device  
LMR70503TM/NOPB  
LMR70503TMX/NOPB  
Status Package Type Package Pins Package  
Eco Plan Lead/Ball Finish  
MSL Peak Temp  
Op Temp (°C)  
Top-Side Markings  
Samples  
Drawing  
Qty  
(1)  
(2)  
(3)  
(4)  
ACTIVE  
DSBGA  
DSBGA  
YFX  
8
8
250  
Green (RoHS  
& no Sb/Br)  
SNAGCU  
SNAGCU  
Level-1-260C-UNLIM  
S3  
S3  
ACTIVE  
YFX  
3000  
Green (RoHS  
& no Sb/Br)  
Level-1-260C-UNLIM  
(1) The marketing status values are defined as follows:  
ACTIVE: Product device recommended for new designs.  
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.  
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.  
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.  
OBSOLETE: TI has discontinued the production of the device.  
(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability  
information and additional product content details.  
TBD: The Pb-Free/Green conversion plan has not been defined.  
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that  
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.  
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between  
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.  
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight  
in homogeneous material)  
(3) MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.  
(4)  
Multiple Top-Side Markings will be inside parentheses. Only one Top-Side Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a  
continuation of the previous line and the two combined represent the entire Top-Side Marking for that device.  
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information  
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and  
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.  
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.  
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.  
Addendum-Page 1  
PACKAGE MATERIALS INFORMATION  
www.ti.com  
8-Apr-2013  
TAPE AND REEL INFORMATION  
*All dimensions are nominal  
Device  
Package Package Pins  
Type Drawing  
SPQ  
Reel  
Reel  
A0  
B0  
K0  
P1  
W
Pin1  
Diameter Width (mm) (mm) (mm) (mm) (mm) Quadrant  
(mm) W1 (mm)  
LMR70503TM/NOPB  
LMR70503TMX/NOPB  
DSBGA  
DSBGA  
YFX  
YFX  
8
8
250  
178.0  
178.0  
8.4  
8.4  
0.91  
0.91  
1.69  
1.69  
0.7  
0.7  
4.0  
4.0  
8.0  
8.0  
Q1  
Q1  
3000  
Pack Materials-Page 1  
PACKAGE MATERIALS INFORMATION  
www.ti.com  
8-Apr-2013  
*All dimensions are nominal  
Device  
Package Type Package Drawing Pins  
SPQ  
Length (mm) Width (mm) Height (mm)  
LMR70503TM/NOPB  
LMR70503TMX/NOPB  
DSBGA  
DSBGA  
YFX  
YFX  
8
8
250  
210.0  
210.0  
185.0  
185.0  
35.0  
35.0  
3000  
Pack Materials-Page 2  
MECHANICAL DATA  
YFX0008xxx  
D
0.600±0.075  
E
TOP SIDE OF PACKAGE  
BOTTOM SIDE OF PACKAGE  
TMP08XXX (Rev A)  
D: Max = 1.64 mm, Min = 1.58 mm  
E: Max = 0.855 mm, Min =0.795 mm  
4215093/A  
12/12  
A. All linear dimensions are in millimeters. Dimensioning and tolerancing per ASME Y14.5M-1994.  
B. This drawing is subject to change without notice.  
NOTES:  
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