DS7251AB 01

®
RT7251A/B
1.5A, 17V, 340/800kHz Synchronous Step-Down Converter
General Description
Features
The RT7251A/B is a high efficiency, monolithic
synchronous step-down DC/DC converter that can operate
at 340kHz/800kHz, while delivering up to 1.5A output
current from a 4V to 17V input supply. The RT7251A/B's
current mode architecture allows the transient response
to be optimized. Cycle-by-cycle current limit provides
protection against shorted outputs and soft-start eliminates
input current surge during start-up. Fault conditions also
include output under voltage protection, output over voltage
protection and thermal shutdown. The low current (<5μA)
shutdown mode provides output disconnection, enabling
easy power management in battery-powered systems. The
RT7251A/B is available in a WDFN-8L 2x2 package.
z
z
z
z
z
z
z
z
z
z
z
z
z
z
Ordering Information
z
RT7251A/B
z
Package Type
QW : WDFN-8L 2x2 (W-Type)
Lead Plating System
Z : ECO (Ecological Element with
Halogen Free and Pb free)
Note :
A : 340kHz
B : 800kHz
Richtek products are :
z
z
z
z
z
Industrial and Commercial Low Power Systems
Computer Peripherals
LCD Monitors and TVs
Green Electronics/Appliances
Point of Load Regulation for High-Performance DSPs,
FPGAs, and ASICs
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
`
Applications
Pin Configurations
Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
RT7251AZQW
03 : Product Code
03W
(TOP VIEW)
SW
VIN
BOOT
EN
1
2
3
4
GND
`
4V to 17V Input Voltage Range
1.5A Output Current
Internal N-MOSFETs
Current Mode Control
Fixed Frequency Operation : 340kHz/800kHz
Output Adjustable from 0.8V to 12V
Up to 95% Efficiency
Internal Compensation
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Over Current Protection
Input Under Voltage Lockout
Output Under Voltage Protection
Output Over Voltage Protection
Power Good Indicator
Thermal Shutdown Protection
RoHS Compliant and Halogen Free
9
8
7
6
5
GND
GND
PGOOD
FB
WDFN-8L 2x2
W : Date Code
RT7251BZQW
70 : Product Code
70W
W : Date Code
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT7251A/B
Typical Application Circuit
RT7251A
VIN
4V to 17V
2 VIN
BOOT 3
CIN
10µF
CBOOT
10nF
SW 1
6 PGOOD
PGOOD
Chip Enable
7, 8, 9 (Exposed Pad)
R1
110k
FB 5
4 EN
L
15µH
VOUT
3.3V
1.5A
COUT
22µF x 2
R2
36k
GND
RT7251B
VIN
4V to 17V
2 VIN
BOOT 3
CIN
10µF
CBOOT
10nF
SW 1
6 PGOOD
PGOOD
Chip Enable
7, 8, 9 (Exposed Pad)
R1
47k
FB 5
4 EN
L
6.8µH
VOUT
3.3V
1.5A
COUT
22µF x 2
R2
15k
GND
Table 1. Recommended Component Selection
RT7251A
V OUT (V)
L (μH)
R1 (kΩ)
R2 (kΩ)
COUT (μF)
1.2
4.7
110
220
22 x 2
2.5
10
110
51
22 x 2
3.3
15
110
36
22 x 2
5
22
120
22
22 x 2
V OUT (V)
L (μH)
R1 (kΩ)
R2 (kΩ)
COUT (μF)
1.2
3.6
47
91
22 x 2
2.5
4.7
47
22
22 x 2
3.3
6.8
47
15
22 x 2
5
10
62
12
22 x 2
RT7251B
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
2
is a registered trademark of Richtek Technology Corporation.
DS7251A/B-01 April 2012
RT7251A/B
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
SW
Switch Node. Connect to external L-C filter.
2
VIN
Input Supply Voltage. Must bypass with a suitably large ceramic capacitor.
3
BOOT
Bootstrap for High Side Gate Driver. Connect 0.01μF or greater ceramic
capacitor from BOOT to SW pin.
4
EN
5
FB
6
PGOOD
Chip Enable. A logic-high enables the converter; a logic-low forces the
RT7251A/B into shutdown mode, reducing the supply current to less than 5μA.
Attach this pin to VIN with a 100kΩ pull up resistor for automatic startup.
Feedback Input Pin. For an adjustable output, connect an external resistive
voltage divider to this pin.
Power Good Indicator. The output of this pin is low if the output voltage is
12.5% less than the nominal voltage. Otherwise, it is an open drain.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
7, 8,
GND
9 (Exposed Pad)
Function Block Diagram
VIN
Internal
Regulator
Enable
Comparator
+
2.5V
EN
5k
OSC
340kHz/800kHz
VA VCC
+
3V
0.4V
Current Sense
Amplifier
Foldback
Control
-
1V
Slope Comp
VA
+
BOOT
OV
OV Comparator
+
+
UV
-
-
UV Comparator
0.8V
FB
-
S
Q
R
Current
Comparator
Q
145m
SW
140m
+
Error Amp
PGOOD
Comparator
+
35pF 400k
-
1pF
GND
0.7V
FB
PGOOD
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT7251A/B
Absolute Maximum Ratings
z
z
z
z
z
z
z
z
z
z
(Note 1)
Supply Voltage, VIN ----------------------------------------------------------------------------------------------SW --------------------------------------------------------------------------------------------------------------------BOOT to SW -------------------------------------------------------------------------------------------------------All Other Pins ------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
WDFN-8L 2x2 ------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
WDFN-8L 2x2, θJA -------------------------------------------------------------------------------------------------WDFN-8L 2x2, θJC -------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------------Junction Temperature ---------------------------------------------------------------------------------------------Storage Temperature Range ------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Mode) --------------------------------------------------------------------------------------MM (Machine Mode) -----------------------------------------------------------------------------------------------
Recommended Operating Conditions
z
z
z
−0.3V to 19V
−0.3V to (VIN + 0.3V)
−0.3V to 6V
−0.3V to 6V
0.833W
120°C/W
8.2°C/W
260°C
150°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Input Voltage, VIN ---------------------------------------------------------------------------------------- 4V to 17V
Junction Temperature Range ------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range ------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VIN = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Shutdown Supply Current
ISHDN
VEN = 0V
--
1
5
μA
Supply Current
IOUT
VEN = 3V, VFB = 0.9V
--
0.6
1
mA
0.788
0.8
0.812
V
--
10
--
nA
RDS(ON)1
--
145
--
mΩ
RDS(ON)2
--
140
--
mΩ
Feedback Reference Voltage VFB
4V ≤ VIN ≤ 17V
Feedback Current
High Side Switch On
Resistance
Low Side Switch On
Resistance
VFB = 0.8V
IFB
Upper Switch Current Limit
Min. Duty Cycle, VBOOT − VSW = 4.8V
Maximum Loading = 1.5A
--
3
--
A
Lower Switch Current Limit
From Drain to Source
--
1
--
A
For RT7251A
300
340
380
For RT7251B
700
800
900
VFB = 0V, For RT7251A
--
95
--
VFB = 0V, For RT7251B
--
170
--
VFB = 0.7V, For RT7251A
--
93
--
VFB = 0.7V, For RT7251B
--
84
--
Oscillation Frequency
f OSC1
Short-Circuit Oscillation
Frequency
f OSC2
Maximum Duty Cycle
DMAX
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
4
kHz
kHz
%
is a registered trademark of Richtek Technology Corporation.
DS7251A/B-01 April 2012
RT7251A/B
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Minimum On-Time
Input Under Voltage Lockout
Threshold
Input Under Voltage Lockout
Threshold Hysteresis
t ON
--
100
--
ns
VUVLO
--
3.5
--
V
ΔVUVLO
--
200
--
mV
Logic-High
VIH
2.5
--
--
Logic-Low
VIL
--
--
0.4
--
1
--
μA
EN Threshold
Voltage
EN Pull Low Current
VEN = 2V, VFB = 1V
V
Soft-Start Period
t SS
--
1
--
ms
Thermal Shutdown
TSD
--
150
--
°C
Thermal Shutdown Hysteresis ΔTSD
--
15
--
°C
Power Good Threshold Rising
--
0.7
--
V
--
130
--
mV
--
12
--
Ω
--
125
--
%VREF
--
10
--
μs
Power Good Threshold
Hysteresis
Power Good Pull Down
Resistance
Output OVP Threshold
Output OVP Propagation
Delay
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
RT7251A/B
Typical Operating Characteristics
Efficiency vs. Load Current
Efficiency vs. Load Current
100
100
90
90
80
VOUT = 5V
VOUT = 3.3V
VOUT = 1.2V
70
60
Efficiency (%)
Efficiency (%)
80
50
40
30
70
VOUT = 5V
VOUT = 3.3V
VOUT = 1.2V
60
50
40
30
20
20
10
10
RT7251A, VIN = 12V
RT7251B, VIN = 12V
0
0
0.01
0.1
1
10
0.01
0.1
3.30
3.36
3.28
3.34
3.26
3.24
3.22
3.20
0.75
1
1.25
3.30
3.28
RT7251B, VIN = 12V, VOUT = 3.3V
3.18
0.5
3.32
3.26
RT7251A, VIN = 12V, VOUT = 3.3V
0.25
3.24
1.5
0
0.25
0.5
Load Current (A)
0.75
1
1.25
1.5
Load Current (A)
Reference Voltage vs. Temperature
Reference Voltage vs. Temperature
0.82
0.82
0.81
0.81
Reference Voltage (V)
Reference Voltage (V)
10
Output Voltage vs. Load Current
3.38
Output Voltage (V)
Output Voltage (V)
Output Voltage vs. Load Current
3.32
0
1
Load Current (A)
Load Current (A)
0.80
0.79
0.78
0.77
0.80
0.79
0.78
0.77
RT7251B, VIN = 12V, IOUT = 0.3A
RT7251A, VIN = 12V, IOUT = 0.3A
0.76
0.76
-50
-25
0
25
50
75
100
Temperature (°C)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
6
125
-50
-25
0
25
50
75
100
125
Temperature (°C)
is a registered trademark of Richtek Technology Corporation.
DS7251A/B-01 April 2012
RT7251A/B
Frequency vs. Input Voltage
820
360
810
Frequency (kHz)1
Frequency (kHz)1
Frequency vs. Input Voltage
370
350
340
330
320
800
790
780
770
760
310
RT7251B, VOUT = 3.3V, IOUT = 0.3A
RT7251A, VOUT = 3.3V, IOUT = 0.3A
750
300
4
6.6
9.2
11.8
14.4
4
17
6.6
9.2
Frequency vs. Temperature
17
Frequency vs. Temperature
380
900
370
880
360
860
Frequency (kHz)1
Frequency (kHz)1
14.4
Input Voltage (V)
Input Voltage (V)
350
340
330
320
310
840
820
800
780
760
740
300
290
720
RT7251A, VOUT = 3.3V, IOUT = 0.3A
RT7251B, VOUT = 3.3V, IOUT = 0.3A
700
280
-50
-25
0
25
50
75
100
-50
125
-25
0
25
50
75
100
125
Temperature (°C)
Temperature (°C)
Quiescent Current vs. Input Voltage
Quiescent Current vs. Input Voltage
0.80
0.80
0.76
0.76
Quiescent Current (mA)
Quiescent Current (mA)
11.8
0.72
0.68
0.64
0.72
0.68
0.64
RT7251B, VEN = 3V, VFB = 0.9V
RT7251A, VEN = 3V, VFB = 0.9V
0.60
0.60
4
6.6
9.2
11.8
14.4
Input Voltage (V)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
17
4
6.6
9.2
11.8
14.4
17
Input Voltage (V)
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
7
RT7251A/B
Quiescent Current vs. Temperature
0.90
0.82
0.82
Quiescent Current (mA)
Quiescent Current (mA)
Quiescent Current vs. Temperature
0.90
0.74
0.66
0.58
0.74
0.66
0.58
RT7251A, VIN = 12V, VEN = 3V, VFB = 0.9V
RT7251B, VIN = 12V, VEN = 3V, VFB = 0.9V
0.50
0.50
-50
-25
0
25
50
75
100
125
-50
-25
0
Temperature (°C)
50
1.90
1.82
1.82
Shutdown Current (μA)1
1.90
1.74
1.66
1.58
9.2
1.58
RT7251B, VEN = 0V
11.8
14.4
17
4
6.6
9.2
11.8
14.4
17
Input Voltage (V)
Input Voltage (V)
Shutdown Current vs. Temperature
Shutdown Current vs. Temperature
8
8
7
7
Shutdown Current (μA)1
Shutdown Current (μA)1
125
1.66
1.50
6.6
100
1.74
RT7251A, VEN = 0V
1.50
4
75
Shutdown Current vs. Input Voltage
Shutdown Current vs. Input Voltage
Shutdown Current (μA)1
25
Temperature (°C)
6
5
4
3
2
6
5
4
3
2
1
1
RT7251A, VEN = 0V
RT7251B, VEN = 0V
0
0
-50
-25
0
25
50
75
100
Temperature (°C)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
8
125
-50
-25
0
25
50
75
100
125
Temperature (°C)
is a registered trademark of Richtek Technology Corporation.
DS7251A/B-01 April 2012
RT7251A/B
Current Limit vs. Input Voltage
3.0
2.8
2.8
Current Limit (A)
Current Limit (A)
Current Limit vs. Input Voltage
3.0
2.6
2.4
2.2
2.6
2.4
2.2
RT7251A, VOUT = 3.3V
2.0
4
6.6
9.2
11.8
14.4
RT7251B, VOUT = 3.3V
2.0
17
4
6.6
9.2
Input Voltage (V)
14.4
17
Current Limit vs. Temperature
Current Limit vs. Temperature
3.0
3.0
2.8
2.8
Current Limit (A)
Current Limit (A)
11.8
Input Voltage (V)
2.6
2.4
2.6
2.4
2.2
2.2
RT7251A, VIN = 12V, VOUT = 3.3V
RT7251B, VIN = 12V, VOUT = 3.3V
2.0
2.0
-50
-25
0
25
50
75
100
125
-50
-25
Temperature (°C)
25
50
75
100
125
Temperature (°C)
Load Transient Response
Load Transient Response
RT7251A
RT7251B
VOUT
(200mV/Div)
VOUT
(100mV/Div)
IOUT
(1A/Div)
IOUT
(1A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 0.3A to 1.5A
Time (100μs/Div)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
0
VIN = 12V, VOUT = 3.3V, IOUT = 0.3A to 1.5A
Time (100μs/Div)
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
9
RT7251A/B
Switching
Switching
RT7251B
RT7251A
VSW
(10V/Div)
VSW
(10V/Div)
VOUT
(5mV/Div)
VOUT
(5mV/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.5A
VIN = 12V, VOUT = 3.3V, IOUT = 1.5A
Time (2.5μs/Div)
Time (500ns/Div)
Power On from EN
Power On from EN
VEN
(10V/Div)
VEN
(10V/Div)
VOUT
(5V/Div)
VOUT
(5V/Div)
PGOOD
(5V/Div)
PGOOD
(5V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
RT7251A, VIN = 12V, VOUT = 3.3V, IOUT = 1.5A
RT7251B, VIN = 12V, VOUT = 3.3V, IOUT = 1.5A
Time (500μs/Div)
Time (500μs/Div)
Power Off from EN
Power Off from EN
VEN
(10V/Div)
VEN
(10V/Div)
VOUT
(5V/Div)
VOUT
(5V/Div)
PGOOD
(5V/Div)
PGOOD
(5V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
RT7251A, VIN = 12V, VOUT = 3.3V, IOUT = 1.5A
Time (100μs/Div)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
10
RT7251B, VIN = 12V, VOUT = 3.3V, IOUT = 1.5A
Time (100μs/Div)
is a registered trademark of Richtek Technology Corporation.
DS7251A/B-01 April 2012
RT7251A/B
Application Information
The RT7251A/B is a synchronous high voltage buck
converter that can support the input voltage range from
4V to 17V and the output current can be up to 1.5A.
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
VOUT
R1
FB
R2
RT7251A/B
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive divider
according to the following equation :
R1 ⎞
⎛
VOUT = VFB ⎜ 1 +
⎟
⎝ R2 ⎠
where VFB is the feedback reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
driver voltage for the high side MOSFET. It is recommended
to add an external bootstrap diode between an external
5V and the BOOT pin for efficiency improvement when
input voltage is lower than 5.5V or duty ratio is higher
than 65%. The bootstrap diode can be a low cost one
such as 1N4148 or BAT54. The external 5V can be a 5V
fixed input from system or a 5V output of the RT7251A/B.
Note that the external boot voltage must be lower than
5.5V.
5V
10nF
SW
Figure 2. External Bootstrap Diode
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
The RT7251A/B provides over voltage protection function
when output voltage is over 125%. The internal MOS will
be turned off. The control will return to normal operation if
over voltage condition is removed.
Under Voltage Protection (UVP)
For the RT7251A/B, it provides Hiccup Mode Under
Voltage Protection (UVP). When the FB voltage drops
below 50% of the feedback reference voltage, the UVP
function will be triggered and the RT7251A/B will shut down
for a period of time and then recover automatically. The
Hiccup Mode UVP can reduce input current in short-circuit
conditions.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current £GIL increases with higher VIN
and decreases with higher inductance.
⎤
⎡V
⎤ ⎡ V
ΔIL = ⎢ OUT ⎥ × ⎢1− OUT ⎥
VIN ⎦
⎣ f ×L ⎦ ⎣
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
the highest efficiency operation. However, it requires a
large inductor to achieve this goal. For the ripple current
selection, the value of ΔIL = 0.2(IMAX) will be a reasonable
starting point. The largest ripple current occurs at the
highest VIN. To guarantee that the ripple current stays
below the specified maximum, the inductor value should
be chosen according to the following equation :
⎡ VOUT ⎤ ⎡
⎤
V
L= ⎢
⎥ × ⎢1− OUT ⎥
⎣⎢ f × ΔIL(MAX) ⎦⎥ ⎣⎢ VIN(MAX) ⎦⎥
Table 2. Suggested Inductors for Typical
Application Circuit
BOOT
RT7251A/B
Over Voltage Protection (OVP)
Component
Supplier
Series
Dimensions
(mm)
TDK
VLF10045
10 x 9.7 x 4.5
TDK
TAIYO
YUDEN
SLF12565
12.5 x 12.5 x 6.5
NR8040
8x8x4
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
RT7251A/B
CIN and COUT Selection
The input capacitance, C IN, is needed to filter the
trapezoidal current at the source of the high side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
RMS current is given by :
V
IRMS = IOUT(MAX) OUT
VIN
VIN
−1
VOUT
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do
not offer much relief. Choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be paralleled to meet size or height requirements in the
design. For the input capacitor, a 10μF low ESR ceramic
capacitor is recommended. For the recommended
capacitor, please refer to table 3 for more details. The
selection of COUT is determined by the required ESR to
minimize voltage ripple. Moreover, the amount of bulk
capacitance is also a key for COUT selection to ensure
that the control loop is stable. Loop stability can be
checked by viewing the load transient response as
described in a later section. The output ripple, ΔVOUT , is
determined by :
1
⎤
ΔVOUT ≤ ΔIL ⎡⎢ESR +
8fCOUT ⎥⎦
⎣
The output ripple will be highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Dry tantalum,
special polymer, aluminum electrolytic and ceramic
capacitors are all available in surface mount packages.
Special polymer capacitors offer very low ESR value.
However, it provides lower capacitance density than other
types. Although Tantalum capacitors have the highest
capacitance density, it is important to only use types that
pass the surge test for use in switching power supplies.
Aluminum electrolytic capacitors have significantly higher
ESR. However, it can be used in cost-sensitive applications
for ripple current rating and long term reliability
considerations. Ceramic capacitors have excellent low
ESR characteristics but can have a high voltage coefficient
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
12
and audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant
ringing.
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, VIN. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at VIN large enough to damage the
part.
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ΔILOAD (ESR) also begins to charge or discharge
COUT generating a feedback error signal for the regulator
to return VOUT to its steady-state value. During this
recovery time, VOUT can be monitored for overshoot or
ringing that would indicate a stability problem.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
is a registered trademark of Richtek Technology Corporation.
DS7251A/B-01 April 2012
RT7251A/B
PD(MAX) = (125°C − 25°C) / (120°C/W) = 0.833W for
WDFN-8L 2x2 package
Maximum Power Dissipation (W)1
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 3 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
0.9
Four-Layer PCB
Layout Consideration
Follow the PCB layout guidelines for optimal performance
of the RT7251A/B
`
Keep the traces of the main current paths as short and
wide as possible.
`
Put the input capacitor as close as possible to the device
pins (VIN and GND).
`
SW node is with high frequency voltage swing and should
be kept at small area. Keep sensitive components away
from the SW node to prevent stray capacitive noise
pickup.
`
Place the feedback components to the FB pin as close
as possible.
`
The GND and Exposed Pad should be connected to a
strong ground plane for heat sinking and noise protection.
0.8
0.7
VOUT
0.6
COUT
0.5
L
CIN
0.4
SW
VIN
BOOT
EN
Input capacitor must
be placed as close
to the IC as possible.
CBOOT
0.3
0.2
1
2
3
4
SW should be connected to
inductor by wide and short trace.
Keep sensitive components
away from this trace.
GND
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
WDFN-8L 2x2 package, the thermal resistance, θJA, is
120°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at TA = 25°C
can be calculated by the following formula :
9
8
7
6
5
GND
GND
PGOOD
FB
R1
R2
0.1
VOUT
SW
GND
The resistor divider must be
connected as close to the
device as possible.
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 3. Derating Curve of Maximum Power Dissipation
Figure 4. PCB Layout Guide
Table 3. Suggested Capacitors for CIN and COUT
Location
Component Supplier
Part No.
Capacitance (μF)
Case Size
CIN
MURATA
GRM31CR61E106K
10
1206
CIN
TDK
C3225X5R1E106K
10
1206
CIN
TAIYO YUDEN
TMK316BJ106ML
10
1206
COUT
MURATA
GRM32ER61E226M
22
1210
COUT
MURATA
GRM21BR60J226M
22
0805
COUT
TDK
C3225X5R0J226M
22
1210
COUT
TAIYO YUDEN
EMK325BJ226MM
22
1210
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS7251A/B-01 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
RT7251A/B
Outline Dimension
D2
D
L
E
E2
1
e
SEE DETAIL A
b
2
1
2
1
A
A1
A3
DETAIL A
Pin #1 ID and Tie Bar Mark Options
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.200
0.300
0.008
0.012
D
1.950
2.050
0.077
0.081
D2
1.000
1.250
0.039
0.049
E
1.950
2.050
0.077
0.081
E2
0.400
0.650
0.016
0.026
e
L
0.500
0.300
0.020
0.400
0.012
0.016
W-Type 8L DFN 2x2 Package
Richtek Technology Corporation
5F, No. 20, Taiyuen Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
www.richtek.com
14
DS7251A/B-01 April 2012