SKY87250_201922E.pdf

DATA SHEET
SKY87250: 400 mA Low-Noise Step-Down Converter
in a Micro-Inductor Package
Applications
Description
 Bluetooth headsets
The SKY87250 SwitcherTM step-down converter delivers up to
400 mA to support an adjustable 0.6 V to VIN output from 2.7 V to
5.5 V input supply. Its low supply current, small size, and high
switching frequency make the SKY87250 the ideal choice for
portable applications.
®
 Cellular phones
 Digital cameras
 Hard disk drives
 PDAs and handheld computers
 Portable media and MP3 players
 USB devices
Features
 VIN range: 2.5 V to 5.5 V
 Adjustable output voltage: 0.6 V to VIN
 Output current: 400 mA
 Up to 95% efficiency
 Low-noise, light-load architecture
 No load quiescent current: 40 A
 Switching frequency: 2.0 MHz
 Internal soft-start control
 Over-temperature and current-limit protection
The SKY87250 maintains a low 40 A no-load quiescent current.
The 2.0 MHz switching frequency minimizes the output
capacitance requirement while keeping switching losses low. The
SKY87250 feedback and control delivers excellent load regulation
and transient response with a small output capacitor.
The SKY87250 maintains high efficiency throughout the load
range. The unique low-noise, light-load architecture produces
reduced ripple and spectral noise. Over-temperature and shortcircuit protection safeguards the SKY87250 and system
components from damage.
The ultra-small, 8-pin, 2.4  2.4 mm DLN package footprint,
integrated inductor, and minimal capacitance requirements, make
the SKY87250 ideal for compact designs.
Figure 1 shows the typical application circuit size comparison. A
typical application circuit is shown in Figure 2. The pin
configuration is shown in Figure 3. Signal pin assignments and
functional pin descriptions are provided in Table 1.
 Shutdown current: <1 A
 Small 4.7 F output capacitor
 Temperature range: 40 C to +85 C
 Compact integrated micro-inductor (inductor) DLN (8-pin,
2.4 × 2.4 mm) package (MSL1, 260 °C per JEDEC-J-STD-020)
Skyworks Green™ products are compliant with
all applicable legislation and are halogen-free.
For additional information, refer to Skyworks
Definition of Green™, document number
SQ04-0074.
tc74
Figure 1. Typical Application Circuit Size Comparison
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
VIN
2.7 V to 5.5 V
U1
1
VOUT
5
OUT
VP
R1
adj
2
VIN
C1
4.7 μF
4
FB
R2
59 kΩ
SKY87250
6
EN
ON/OFF
8
N/C
3
GND
C2
4.7 μF
7
PGND
tc64
Figure 2. SKY87250 Typical Application Circuit
VP
1
8
N/C
VIN
2
7
GND
FB
3
6
PGND
EN
4
5
OUT
tc65
Figure 3. SKY87250 8-Pin DLN
(Top View)
Table 1. SKY87250 Signal Descriptions
Pin
Name
Description
1
VP
Input power pin; connect to the source of the P-channel MOSFET. Connect to the input capacitor.
2
VIN
Input bias voltage for the converter.
3
GND
Non-power signal ground pin.
4
FB
Feedback input pin. Connect this pin to an external resistive divider for adjustable output.
5
OUT
DC-DC converter output.
6
EN
Enable pin. A logic high enables normal operation. A logic low shuts down the converter.
7
PGND
Power ground. Input power return pin; connect to the source of the N-channel MOSFET. Connect to the output and input capacitor return.
8
N/C
Not internally connected.
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
Electrical and Mechanical Specifications
The absolute maximum ratings and thermal information of the
SKY87250 are provided in Tables 2 and 3, respectively. Electrical
specifications are provided in Table 4.
Typical performance characteristics of the SKY87250 are
illustrated in Figures 4 through 33.
Table 2. SKY87250 Absolute Maximum Ratings (Note 1)
Parameter
Symbol
Minimum
Typical
Maximum
Units
VIN, VP
0.3
+6.0
V
FB to GND
VFB
0.3
VIN + 0.3
V
EN to GND
VEN
0.3
VIN + 0.3
V
AGND to PGND
VGND
0.3
+0.3
V
OUT RMS current capability
IOUT
Input voltage and bias power to PGND
1
A
Note 1: Exposure to maximum rating conditions for extended periods may reduce device reliability. There is no damage to device with only one parameter set at the limit and all other
parameters set at or below their nominal value. Exceeding any of the limits listed may result in permanent damage to the device.
Table 3. SKY87250 Thermal Information
Parameter
Symbol
Minimum
Typical
Maximum
Units
TA
40
+85
°C
Operating junction temperature
TJ
40
+150
°C
Maximum soldering temperature (at leads, 10 seconds)
TLEAD
300
°C
Maximum power dissipation (Note 1, Note 2, Note 3)
PD
790
mW
Maximum junction-to-ambient thermal resistance
(Note 1, Note 3)
JA
158
°C/W
Ambient temperature
Note 1: Mounted on FR4 circuit board. Two-layer, 1 ounce copper.
Note 2: Derate 6.3 mW/C above 40 C ambient temperature.
Note 3: The thermal resistance is measured in accordance with EIA/JESD 51 series.
CAUTION: Although this device is designed to be as robust as possible, electrostatic discharge (ESD) can damage this device. This device
must be protected at all times from ESD. Static charges may easily produce potentials of several kilovolts on the human body
or equipment, which can discharge without detection. Industry-standard ESD precautions should be used at all times.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
Table 4. SKY87250 Electrical Specifications (Note 1)
(CIN = 4.7 F, COUT = 4.7 F , VIN = VP = 3.6 V, VEN = VIN, GND = PGND, TA = –40 °C to 85 °C [Typical Values are at TA = 25 °C], Unless
Otherwise Noted)
Parameter
Symbol
Test Condition
VP, VIN
Min
Input voltage
VIN
Output voltage
VOUT
Maximum continuous output current
capability
IOUT
No load supply current
IQ
No load current; not switching
Shutdown current
ISHDN
EN = GND
Input under-voltage lockout
VUVLO
VCC rising
VIN = 2.7 V to 5.5 V, 10 mA load, TA = 25 °C
588
FB regulation threshold
VFB
VIN = 2.7 V to 5.5 V, 10 mA load,
TA = –40 °C to 85 °C
582
IFB
VFB = 1 V
Line regulation
∆VOUT/VOUT
VIN = 2.7 V to 5.5 V
Load regulation
∆VOUT/VOUT
10 mA to 400 mA load
Oscillator frequency
fOSC
On-time resistance
RTON
Off-time resistance
Max
5.5
V
0.6
VIN
V
400
40
mA
100
A
1.0
A
1.8
2.5
V
600
612
mV
618
mV
200
nA
0.1
%
0.3
%
2.0
MHz
VP to OUT resistance (high-side MOSFET
on-resistance + DCR)
800
m
RTOFF
VP to PGND resistance (low-side MOSFET
on-resistance + DCR)
650
m
Soft-start period
tSS
Enable rising edge to output regulation
100
s
Over-temperature shutdown threshold
TSHDN
Hysteresis = 15 °C
140
°C
EN logic level high
VIH
EN logic level low
VIL
EN input current
IEN
1.4
VEN = 0 V or VIN
1.0
V
0.4
V
+1.0
A
Note 1: Performance is guaranteed only under the conditions listed in this table.
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4
Units
2.7
TA = 25 °C
FB leakage current
Typical
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
Typical Performance Characteristics
(CIN = 4.7 F, COUT = 4.7 F , VIN = VP = 3.6 V, VEN = VIN, GND = PGND, TA = –40 °C to 85 °C [Typical Values are at TA = 25 °C],
Unless Otherwise Noted)
100
1.0
0.8
Output Voltage Error (%)
90
Efficiency (%)
80
70
60
50
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
40
30
0.1
1
10
100
0.6
0.4
0.2
0.0
-0.2
-0.4
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
-0.6
-0.8
-1.0
0.1
1000
1
1.0
100
0.8
Output Voltage Error (%)
90
Efficiency (%)
80
70
60
50
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
40
1
10
100
0.6
0.4
0.2
0.0
-0.2
-0.4
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
-0.6
-0.8
-1.0
0.1
1000
1
Output Current (mA)
90
0.8
Output Voltage Error (%)
1.0
80
70
60
50
VIN = 2.7 V
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
40
30
10
100
1000
Figure 7. Load Regulation
(VOUT = 2.5 V, R3 = 0 , C3 = 0 pF)
100
1
10
Output Current (mA)
Figure 6. Efficiency vs Load
(VOUT = 2.5 V, R3 = 0 , C3 = 0 pF)
Efficiency (%)
1000
Figure 5. Load Regulation
(VOUT = 3.3 V, R3 = 0 , C3 = 0 pF)
Figure 4. Efficiency vs Load
(VOUT = 3.3 V, R3 = 0 , C3 = 0 pF)
20
0.1
100
Output Current (mA)
Output Current (mA)
30
0.1
10
100
Output Current (mA)
Figure 8. Efficiency vs Load
(VOUT = 1.8 V, R3 = 0 , C3 = 0 pF)
VIN = 2.7 V
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
1000
-1.0
0.1
1
10
100
1000
Output Current (mA)
Figure 9. Load Regulation
(VOUT = 1.8 V, R3 = 0 , C3 = 0 pF)
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
1.0
90
0.8
Output Voltage Error (%)
100
Efficiency (%)
80
70
60
50
VIN = 2.7 V
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
40
30
20
0.1
1
10
100
VIN = 2.7 V
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0.1
1000
1
Figure 10. Efficiency vs Load
(VOUT = 1.8 V, R3 = 500 , C3 = 100 pF)
90
0.8
Output Voltage Error (%)
1.0
Efficiency (%)
80
70
60
50
VIN = 2.7 V
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
40
30
10
100
VIN = 2.7 V
VIN = 3.6 V
VIN = 4.2 V
VIN = 5.0 V
VIN = 5.5 V
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0.1
1000
1
Figure 12. Efficiency vs Load
(VOUT = 1.2 V, R3 = 0 , C3 = 0 pF)
100
3.0
2.0
1.0
Output Voltage Error (%)
IOUT = 1 mA
IOUT = 10 mA
IOUT = 50 mA
IOUT = 100 mA
IOUT = 200 mA
IOUT = 400 mA
0.0
-1.0
-2.0
-25
0
25
50
75
Temperature (°C)
Figure 14. Output Voltage Error vs Temperature
(VIN = 3.6 V, VOUT = 1.8 V, R3 = 0 , C3 = 0 pF)
100
IOUT = 1 mA
IOUT = 10 mA
IOUT = 50 mA
IOUT = 100 mA
IOUT = 200 mA
IOUT = 400 mA
2.0
1.0
0.0
-1.0
-2.0
-3.0
-50
-25
0
25
50
75
Temperature (°C)
Figure 15. Output Voltage Error vs Temperature
(VIN = 3.6 V, VOUT = 1.8 V, R3 = 0 , C3 = 0 pF)
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1000
Figure 13. Load Regulation
(VOUT = 1.2 V, R3 = 0 , C3 = 0 pF)
3.0
Output Voltage Error (%)
10
Output Current (mA)
Output Current (mA)
-3.0
-50
1000
Figure 11. Load Regulation
(VOUT = 1.8 V, R3 = 500 , C3 = 100 pF)
100
1
100
Output Current (mA)
Output Current (mA)
20
0.1
10
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100
DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
3.0
IOUT = 0.1 mA
IOUT = 1 mA
IOUT = 10 mA
IOUT = 50 mA
IOUT = 100 mA
IOUT = 200 mA
IOUT = 400 mA
2.0
1.0
0.0
Output Voltage Error (%)
Output Voltage Error (%)
3.0
-1.0
-2.0
-3.0
-50
-25
0
25
50
75
1.0
0.0
-1.0
-2.0
-3.0
-50
100
IOUT = 0.1 mA
IOUT = 1 mA
IOUT = 10 mA
IOUT = 50 mA
IOUT = 100 mA
IOUT = 200 mA
IOUT = 400 mA
2.0
-25
0
Temperature (°C)
0.2
0.0
0.8
-0.2
-0.4
-0.6
0.6
0.4
0.2
0.0
-0.4
-0.6
-0.8
-1.0
2.5
-1.0
2.5
3.5
4.0
4.5
5.0
5.5
Input Voltage (V)
3.5
4.0
4.5
5.0
5.5
Figure 19. Line Regulation
(VOUT = 1.8 V, R3 = 0 , C3 = 0 pF)
1.0
4
0.6
0.4
IOUT = 100 mA
IOUT = 200 mA
IOUT = 400 mA
0.2
0.0
-0.2
-0.4
-0.6
-0.8
3.5
4.0
4.5
5.0
Input Voltage (V)
Figure 20. Line Regulation
(VOUT = 2.5 V, R3 = 0 , C3 = 0 pF)
3
2
1
0
0.6
0.4
Output Current
(bottom) (A)
IOUT = 0.1 mA
IOUT = 1 mA
IOUT = 10 mA
IOUT = 50 mA
Enable Voltage (top) (V)
Output Voltage (middle) (V)
0.8
Output Voltage Error (%)
3.0
Input Voltage (V)
Figure 18. Line Regulation
(VOUT = 1.8 V, R3 = 500 , C3 = 100 pF)
-1.0
3.0
IOUT = 1 mA
IOUT = 10 mA
IOUT = 50 mA
IOUT = 100 mA
IOUT = 200 mA
IOUT = 400 mA
-0.2
-0.8
3.0
100
1.0
IOUT = 0.1 mA
IOUT = 1 mA
IOUT = 10 mA
IOUT = 50 mA
IOUT = 100 mA
IOUT = 200 mA
IOUT = 400 mA
Output Voltage Error (%)
Output Voltage Error (%)
0.4
75
Figure 17. Output Voltage Error vs Temperature
(VIN = 4.2 V, VOUT = 3.3 V, R3 = 0 , C3 = 0 pF)
1.0
0.6
50
Temperature (°C)
Figure 16. Output Voltage Error vs Temperature
(VIN = 3.6 V, VOUT = 2.5 V, R3 = 0 , C3 = 0 pF)
0.8
25
0.2
0.0
5.5
Time (50 μs/div)
Figure 21. Soft Start
(VIN = 3.6 V, VOUT = 1.8 V, IOUT = 400 mA)
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
4
2.02
Frequency Variation (%)
Switching Frequency (MHz)
3
2.00
1.98
1.96
1.94
2
1
0
-1
-2
-3
1.92
-40
-20
0
20
40
60
80
-4
2.7
100
3.1
3.5
Temperature (°C)
4.7
5.5
1.2
70
1.1
VIH and VIL (V)
60
50
40
30
85 °C
25 °C
–40 °C
20
10
2.5
3.0
3.5
4.0
4.5
5.0
5.5
1.0
0.9
VIH
VIL
0.8
0.7
0.6
2.5
6.0
Input Voltage (V)
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
Figure 24. Input Current vs Input Voltage
Figure 25. Enable Threshold vs Input Voltage
1.4
2.2
1.3
2.0
1.2
1.6
1.4
0.6
0.4
0.2
0.2
0.0
0.0
Time (50 μs/div)
Time (50 μs/div)
Figure 26. Load Transient Response
(40 mA to 400 mA, VIN = 3.6 V, VOUT = 1.2 V,
COUT = 4.7 F, C3 = 0 pF, R3 = 0 )
Figure 27. Load Transient Response
(40 mA to 400 mA, VIN = 3.6 V, VOUT = 1.8 V,
COUT = 4.7 F, C3 = 100 pF, R3 = 500 )
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Output Current
(bottom) (A)
0.6
0.4
Output Current
(bottom) (A)
1.0
Output Voltage
(top) (V)
1.8
1.1
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5.1
Figure 23. Switching Frequency vs Input Voltage
(VOUT = 1.8 V, IOUT = 400 mA)
80
Input Current (μA)
4.3
Input Voltage (V)
Figure 22. Switching Frequency vs Temperature
(VIN = 3.6 V, VOUT = 1.8 V, IOUT = 400 mA)
Output Voltage
(top) (V)
3.9
4.8
2.0
4.2
1.8
3.6
0.6
1.4
0.4
3.0
1.30
1.25
0.2
1.20
0.0
1.15
Output Voltage
(bottom) (V)
1.6
Input Voltage
(top) (V)
2.2
Output Current
(bottom) (A)
Output Voltage
(top) (V)
DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
Time (50 μs/div)
Time (100 μs/div)
Figure 28. Load Transient Response
(40 mA to 400 mA, VIN = 3.6 V, VOUT = 1.8 V,
COUT = 4.7 F, C3 = 0 pF, R3 = 0 )
Figure 29. Line Transient Response
(VIN = 3.6 V to 4.2 V, VOUT = 1.2 V, IOUT = 400 mA,
COUT = 4.7 F, C3 = 0 pF, R3 = 0 )
4.8
4.2
4.2
3.6
3.6
2.0
1.9
3.0
2.0
1.9
1.8
1.8
1.7
1.7
1.6
1.6
Output Voltage
(bottom) (V)
3.0
Input Voltage
(top) (V)
4.8
Output Voltage
(bottom) (V)
Input Voltage
(top) (V)
1.10
Time (100 μs/div)
Time (100 μs/div)
Figure 30. Line Transient Response
(VIN = 3.6 V to 4.2 V, VOUT = 1.8 V, IOUT = 400 mA,
COUT = 4.7 F, C3 = 100 pF, R3 = 500 )
Figure 31. Line Transient Response
(VIN = 3.6 V to 4.2 V, VOUT = 1.8 V, IOUT = 400 mA,
COUT = 4.7 F, C3 = 0 pF, R3 = 0 )
1.84
1.84
1.78
0.10
0.05
Output Voltage
(top) (V)
Output Voltage
(top) (V)
1.80
1.78
0.8
0.6
Inductor Current
(bottom) (A)
1.82
1.80
Inductor Current
(bottom) (A)
1.82
0.4
0.00
0.2
0.0
Time (5 μs/div)
Time (200 ns/div)
Figure 32. Output Ripple
(VIN = 3.6 V, VOUT = 1.8 V, IOUT = 1 mA)
Figure 33. Output Ripple
(VIN = 3.6 V, VOUT = 1.8 V, IOUT = 400 mA)
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
VIN
VP
Slope
Comp
FB
Comp
DH
Error
Amp.
1 μH
Logic
OUT
Ref 0.6 V
DL
EN
Input
GND
PGND
tc66
Figure 34. SKY87250 Functional Block Diagram
Functional Description
The SKY87250 is a high performance 400 mA, 2.0 MHz
monolithic step-down converter with an integrated microinductor. It is designed with the goal of minimizing external
component requirements and optimizing efficiency over the
complete load range. The converter operates at 2.0 MHz, which
minimizes the ceramic output capacitor requirement.
The device is designed to operate with an output voltage as low
as 0.6 V over an input voltage range of 2.7 V to 5.5 V. Power
devices are sized for 400 mA current capability while
maintaining up to 95% efficiency. At dropout, the regulator's
duty cycle increases to 100% and the output voltage tracks the
input voltage minus the load drop across the on-time resistance
(RTON).
A high-DC gain error amplifier with internal compensation
controls the output. It provides excellent transient response and
load/line regulation. Soft start eliminates any output voltage
overshoot when the enable or the input voltage is applied.
A functional block diagram is shown in Figure 34.
Integrated Power Inductor
The SKY87250 integrates the power inductor within the
package. Therefore, no external power inductor is needed.
Control Loop
The SKY87250 is a 400 mA, current mode step-down converter.
The current through the P-channel MOSFET (high-side) is
sensed for current loop control, as well as short-circuit and
overload protection. A fixed slope compensation signal is added
to the sensed current to maintain stability for duty cycles over
50%. The peak current mode loop appears as a voltageprogrammed current source in parallel with the output
capacitor.
The output of the voltage error amplifier programs the current
mode loop for the necessary peak switch current to force a
constant output voltage for all load and line conditions. Internal
loop compensation terminates the transconductance voltage
error amplifier output. The error amplifier reference is fixed at
0.6 V.
Soft Start/Enable
Soft start ramps the reference voltage when the input voltage
and enable input are valid. The controlled output slew-rate
limits the current surge seen at the input and eliminates output
voltage overshoot. When the EN pin is pulled low, it forces the
SKY87250 into a low-power, non-switching state. The total
input current during shutdown is less than 1 A.
Current Limit and Over-Temperature Protection
For overload conditions, the peak input current is limited. As
load impedance decreases and the output voltage falls closer to
zero, more power is dissipated internally, raising the device
temperature. Thermal protection completely disables switching
when internal dissipation becomes excessive, protecting the
device from damage. The junction over-temperature threshold
is 140°C with 15°C of hysteresis.
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
Under-Voltage Lockout (UVLO)
Internal bias of all circuits is controlled using the VIN bias supply
input. UVLO guarantees sufficient VIN bias and proper operation
of all internal circuitry before activation.
Application Information
Input Capacitor Selection
Select a 4.7 F to 22 F X7R or X5R ceramic capacitor for the
input. To estimate the required input capacitor size, determine
the acceptable input voltage ripple level (VPP) and solve for CIN.
C IN 
D  1  D 
 VPP


 ESR   f SW
 I OUT

D
VOUT
VIN
The peak ripple voltage occurs when VIN = 2  VOUT (50% duty
cycle), resulting in a minimum output capacitance
recommendation:
1

V
4   PP  ESR   f SW
I

 OUT
The maximum input capacitor RMS current is:
I RMS  I OUT  D  1  D   I OUT 
VOUT  VOUT
 1 
VIN 
VIN
Output Capacitor Selection
The output voltage droop due to a load transient (ILOAD) is
dominated by the capacitance of the ceramic output capacitor.
During a step increase in load current, the ceramic output
capacitor alone supplies the load current until the loop
responds. Within two or three switching cycles, the loop
responds and the inductor current increases to match the load
current demand. The relationship of the output voltage droop
during the three switching cycles to the output capacitance can
be estimated by:
COUT 



The calculated value varies with input voltage and is a
maximum when VIN is twice the output voltage.
The input capacitor provides a low impedance loop for the
edges of pulsed current drawn by the SKY87250. X7R and X5R
ceramic capacitors are ideal for this function due to their low
Equivalent Series Resistance (ESR) and Equivalent Series
Inductance (ESL). To minimize stray parasitic inductance, the
capacitor should be placed as closely as possible to the
SKY87250. This keeps the high frequency content of the input
current localized, minimizing EMI and input voltage ripple.
The proper placement of the input capacitor can be seen in the
Evaluation Board layer detail (see C1 in Figure 35).
A laboratory test set-up typically consists of two long wires
running from the bench power supply to the Evaluation Board
input voltage pins. The inductance of these wires, along with
the low-ESR ceramic input capacitor, can create a high-Q
In applications where the input power source lead inductance
cannot be reduced to a level that does not affect the converter
performance, a high ESR tantalum or aluminum electrolytic
should be placed in parallel with the low ESR/ESL bypass
ceramic. This dampens the high Q network and stabilizes the
system.
The output capacitor limits the output ripple and provides
holdup during large load transitions. A 4.7 F to 10 F X5R or
X7R ceramic capacitor provides sufficient bulk capacitance to
stabilize the output during large load transitions and has the
ESR and ESL characteristics necessary for low output ripple.
Where D is the duty cycle and fSW is the switching frequency.
C IN 
network that may affect converter performance. This problem
often becomes apparent in the form of excessive ringing in the
output voltage during load transients. Errors in the loop phase
and gain measurements can also result.
3  I LOAD
VDROOP  f SW
Once the average inductor current increases to the DC load
level, the output voltage recovers. The above equation
establishes a limit on the minimum value for the output
capacitor with respect to load transients.
The internal voltage loop compensation also limits the minimum
output capacitor value to 4.7 F. This is due to its effect on the
loop crossover frequency (bandwidth), phase margin, and gain
margin. Increased output capacitance reduces the crossover
frequency with greater phase margin.
The maximum output capacitor RMS ripple current is given by:
I RMS ( MAX) 
1
2 3

VOUT  VIN ( MAX )  VOUT 
L  f SW  VIN ( MAX )
Where L = 1 H and fSW = 2.0 MHz.
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than a few
degrees rise in hot-spot temperature.
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11
DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
2
PTOTAL  I OUT
 RTON  D  RTOFF  1  D  
Feedback Resistor Selection
Resistors R1 and R2 in Figure 35 program the output to regulate
at a voltage higher than 0.6 V. To limit the bias current required
for the external feedback resistor string while maintaining good
noise immunity, the suggested value for R2 is 59 k.
Decreased resistor values are necessary to maintain noise
immunity on the FB pin, resulting in increased quiescent
current. Table 5 summarizes the resistor values for various
output voltages.
With an enhanced transient response for extreme pulsed load
applications, an external feed-forward capacitor and series
feedback resistor (C3 and R3 in Figure 4) can be added, and
usually use 100 pF capacitor as C3 and 500  resister as R3.
The feedback resistor value can be calculated using the
following equation:
V

 1.8V

R1   OUT  1  R 2  
 1  59 k  118 k
V
0
.
6
V


 REF

Table 5. Adjustable Resistor Values for Step-Down Converter
R1 (k)
(R2 = 59 k)
VOUT (V)
R1 (k)
(R2 = 221 k)
0.8
19.6
75
0.9
29.4
113
1.0
39.2
150
1.1
49.9
187
1.2
59
221
1.3
68.1
261
1.4
78.7
301
1.5
88.7
332
1.8
118
442
2.5
187
698
3.3
267
1000
Thermal Calculations
There are many types of losses in the internal micro-inductor
including the losses in the inductor core material, losses in the
inductor from skin effects, magnetic field losses of the
neighboring windings, and radiation losses. To simplify the
calculation, the total inductor losses can be combined into the
losses in the DC winding resistance of the inductor.
There are three types of losses associated with the SKY87250
step-down converter: switching losses, conduction losses, and
quiescent current losses. Conduction losses are associated with
the on-resistance characteristics of the power output switching
devices and the DC winding resistance (DCR) of the internal
micro-inductor. Switching losses are dominated by the gate
charge of the power output switching devices. At full load,
assuming a Continuous Conduction Mode (CCM), a simplified
form of the losses is given by:
t
DRV

 f SW  I OUT  I Q  VIN
Where D = VOUT/VIN, RTON is the high-side MOSFET onresistance plus inductor DCR, RTOFF is the low-side MOSFET onresistance plus inductor DCR, IQ is the step-down converter
quiescent current, and tDRV is used to estimate the full load
step-down converter switching losses.
Since RTON, RTOFF, quiescent current, and switching losses all
vary with the input voltage, the total losses should be
investigated over the complete input voltage range.
Given the total losses, the maximum junction temperature can
be derived from the JA for the package which is 158 °C/W.
TJ(MAX)  PTOTAL   JA  TA
Layout Considerations
The following guidelines should be used to help ensure a proper
layout:
 The input capacitor (C1) should be connected directly
between the VP and PGND pins.
 The output capacitor and analog ground should be connected
together to minimize any DC regulation errors caused by
ground potential differences.
 The output-sense connection to the FB pin should be
separated from any power trace. Route the output-sense
trace as close as possible to the load point to avoid additional
load regulation errors. Sensing along a high-current load
trace will degrade DC load regulation.
 Place the feedback components near the FB pin to minimize
the high-impedance feedback trace length. Avoid routing the
feedback trace directly under the package to avoid EMI
coupled noise.
 Connect the N/C pin to the power ground plane to enhance
thermal impedance, and use vias directly under the package
to ground planes on the bottom or internal PCB layers to help
distribute the heat.
Evaluation Board Description
The SKY87250 Evaluation Board schematic diagram is provided
in Figure 35. The PCB layer details are shown in Figure 36.
Component values for the SKY87250 Evaluation Board are listed
in Table 6.
Package Information
The PCB layout footprint for the SKY87250 is provided in
Figure 37. Package dimensions are shown in Figure 38, and
tape and reel dimensions are provided in Figure 39.
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DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
VIN
2.7 V to 5.5 V
VOUT
U1
1
2
C1
4.7 μF
EN
VP
OUT
5
R1
adj.
VIN
4
FB
R3
Short
SKY87250
3
2
6
EN
GND
N/C
PGND
3
C3
optional
R2
59 kΩ
C2
4.7 μF
1
8
7
tc67
Figure 35. SKY87250 Evaluation Board Schematic
(b) Bottom Side
(a) Top Side
(L1 not required; shown for size comparison only)
tc68
Figure 36. SKY87250 Evaluation Board Layer Details
Table 6. SKY87250 Evaluation Board Bill of Materials (BOM)
Component
Part Number
Description
Manufacturer
U1
SKY87250
400mA low noise step-down converter, 2.4 mm  2.4 mm
Skyworks
C1, C2
GRM188R60J475KE19D
MLCC cap, 4.7 F/6.3 V, 0603
Murata
C3
UMK105CG101JV-F
100 pF/50 V, 0402, optional
Taiyo Yuden
R1
Carbon film resistor
59 k, 1%, 0201
R2
Carbon film resistor
adjustable (see Table 5), 1%, 0201
R3
Carbon film resistor
0 , 1%, 0201
Yageo
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13
DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
8X 0.35 mm
Pin 8
8X 0.465 mm
2.7 mm
Package outline
Pin 1
0.6 mm pitch
tc69
Figure 37. SKY87250 PCB Layout Footprint
(Top View)
Top
2.4
8x SMT pad
A B C
0.1
B
C
Pin 8
C
2.4
B
0.9 ± 0.1
0.15 A B C
0.1
0.525 ± 0.050
(0.125)
0.45 ± 0.05
2x (0.05)
(0.05)
(0.05)
(0.05)
0.35 ± 0.05
Metal pad
edge
0.265 ± 0.100
0.35 ± 0.05
Detail B
Pad
Scale: 2x
1x this rotation
1x rotated 180º
Detail A
Pad
Scale: 2x
4x this rotation
4x rotated 180º
0.35 ± 0.05
(0.05)
Metal pad
edge
0.265 ± 0.100
0.265 ± 0.100
0.525 ± 0.050
(0.125)
Detail C
Pad
Scale: 2x
1x this rotation
1x rotated 180º
All measurements are in millimeters.
Figure 38. SKY87250 8-pin DLN2424 Package Dimensions
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Pin 1
Bottom View
Side View
Top View
A
4X 0.3
A
4X 0.9
Pin 1
Indicator
Metal pad
edge
8x 1.15
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tc70
DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
B’
±
4.00 ± 0.10
2.60 ± 0.05 (Bo)
3.50 ± 0.05
05
0.
0.
Ty
10
A
A’
B
Pin 1
0
ø1.0
R
p.
8.00 ± 0.30
5
.5
ø1
2.00 ± 0.05
1.75± 0.10
0.30 ± 0.05
Min.
4.00 ± 0.10
1.20 ± 0.10 (Ko)
SECTION B-B’
2.60 ± 0.05 (Ao)
SECTION A-A’
Notes:
1. Carrier tapes must meet all requirements of Skyworks GP01-D233 procurement Spec for tape and reel shipping.
2. Carrier tape shall be black conductive polystyrene.
3. Cover tape shall be transparent conductive material.
4. ESD-surface resistivity shall meet GP01-D233.
5. 10 sprocket hole pitch cumulative tolerance: ±0.20 mm.
6. Ao & Bo measured on plane 0.30 mm above the bottom of the pocket.
7. All measurements are in millimeters.
8. Part No. : 3M115401 (Please indicate on purchase order).
tc456
Figure 39. SKY87250 Carrier Tape Dimensions
SKY87250 Design Example
Specifications
VOUT = 1.8 V @ 400 mA
VIN = 5 V
fSW = 2.0 MHz
RTON = 800 m
RTOFF = 650 m
TA = 85 °C
SKY87250 Losses
All values assume 85°C ambient temperature and thermal resistance of 158 °C/W in the package.
PTOTAL 

2
I OUT
 RTON  VOUT  RTOFF  VIN  VOUT 
 t DRV  f SW  I OUT  I Q   VIN
VIN
0.4 A2  0.8 Ω  1.8V  0.65 Ω  5V  1.8V 
 5 ns  2 MHz  0.4 A  45 μA  5V
5V
 120 mW
TJ(MAX)  PLOSS   JA  TA  158C / W   120mW  85C  104C
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15
DATA SHEET • SKY87250: 400 mA LOW-NOISE STEP-DOWN CONVERTER IN A MICRO-INDUCTOR PACKAGE
Ordering Information
Model Name
SKY87250: 400 mA Low-Noise Step-Down Converter in a Micro-Inductor Package
Manufacturing Part Number
SKY87250-11
Evaluation Board Part Number
SKY87250-11-EVB
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