SEMTECH SC185BULTRT

SC185
4A Synchronous
Step-Down Regulator
POWER MANAGEMENT
Features
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Description
The SC185 is a 4A synchronous step-down regulator
designed to operate with an input voltage range of
2.9V to 5.5V. The device requires only three external
filter components for a complete a step down regulator
solution. The output voltage is factory predetermined
with an available range of 1.0V to 3.3V.
VIN Range: 2.9 – 5.5V
VOUT Options: 1.0V to 3.3V
Up to 4A Output Current
Ultra-Small Footprint, <1mm Height Solution
1.5MHz Switching Frequency
Optional Power Save Mode Operation
Efficiency Up to 95%
Low Output Noise Across Load Range
Excellent Transient Response
Start Up into Pre-Bias Output
100% Duty-Cycle Low Dropout Operation
<1µA Shutdown Current
Externally Programmable Soft Start Time
Power Good indicator
Input Under-Voltage Lockout
Output Over-Voltage, Current Limit Protection
Over-Temperature Protection
3mm x 3mm x 0.6mm thermally enhanced
MLPQ-UT16 package
-40 to +85°C Temperature Range
Pb-free, Halogen free, and RoHS/WEEE compliant
The SC185 is optimized for maximum efficiency over a
wide range of load currents. During full load operation,
the SC185 operates in PWM mode with fixed 1.5MHz
oscillator frequency, allowing the use of small surface
mount external components. As the load decreases, the
regulator has the option to transition into Power Save
mode maintaining high efficiency or stay in forced PWM
mode operation.
The SC185 offers output short circuit and thermal
protection to safe guard the device under extreme
operating conditions. The enable pin provides on/off
control of the regulator. When connected to logic
low, the device enters shutdown and consumes less
than 1uA of current. Other protection features include
programmable soft start with Power Good indicator, over
voltage protection and under voltage lockout.
Fixed VOUT version
The SC185 is available in a thermally-enhanced, 3mm
x 3mm x 0.6mm MLPQ-UT16 package and has a rated
temperature range of -40 to +85°C.
Applications
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Desktop Computing
Set-Top Box
LCD TV
Network Cards
Printer
Typical Application Circuit
1,2
VIN
CIN
22µF
LX
PVIN
14,15,16
SC185H
R1
1
4
C1
0.1µF
7
PGOOD
Enable
6
5
Mode
9
VOUT
1.5V
COUT
47µF
AVIN
VOUT
L, 1.0µH
10
PGOOD
EN
MODE
SS
PGND
AGND
11,12,13,T
3
CSS
June 18, 2010
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SC185
Pin Configuration
Ordering Information
LX
LX
LX
PGND
Evaluation Board
15
14
13
SC185xEVB(5)
2
11
PGND
AGND
3
10
VOUT
AVIN
4
9
SS
TOP VIEW
5
6
7
8
EN
PGOOD
NC
T
MODE
PVIN
3mm x 3mm x 0.6mm MLPQ-UT16
16
PGND
1
Package
SC185xULTRT(2)(3)(4)
12
PVIN
Device
Notes:
(1) Calculated from package in still air, mounted to 3” x 4.5”, 4 layer
FR4 PCB with thermal vias under the exposed pad per JESD51
standards.
(2) Available in tape and reel only. A reel contains 3,000 devices.
(3) Device is Pb-free, Halogen free, and RoHS/WEEE compliant.
(4) “x” is the code of the output voltage. See Table 1 for the code. For
example, the device number for VOUT= 1.50V is SC185HULTRT.
(5) “x” is the code of the output voltage. See Table 1 for the code. For
example, the EVB with VOUT= 1.50V is SC185HEVB.
3mm x 3mm x 0.6mm MLPQ-UT16
θJA = 40°C/W (1); θJC = 7°C/W
Table 1: Available Output Voltages
Marking Information
185x
yyww
nnnn
Code
VOUT(6)
B
1.00
E
1.20
H
1.50
J
1.80
L
2.50
Q
3.30
Notes:
(6) Contact factory for alternative output voltage options.
Marking for 3mm x 3mm MLPQ-UT 16 Lead Package:
x = Code of the output voltage (Example: H for VOUT=1.50V)
yyww = Datecode (Example: 0852)
nnnn = Semtech Lot number (Example: E901)
© 2010 Semtech Corp.
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SC185
Recommended Operating Conditions
Absolute Maximum Ratings
PVIN and AVIN Supply Voltages ………………… -0.3 to 6.0V
Supply Voltage PVIN and AVIN …………………… 2.9 to 5.5V
LX Voltage(9) ……………………
-0.3 to PVIN+0.3V, 6V Max
Maximum Output Current ………………………………… 4.0A
VOUT Voltage …………………………… -0.3 to AVIN+0.3V
Temperature Range …………………………… -40 to +85˚C
CTLx pins Voltages ……………………… -0.3 to AVIN+0.3V
Input Capacitor ………………………………………… 22uF
Peak IR Reflow Temperature …………………………. 260°C
Output Capacitor
……………………… 47uF (or 2 x 22uF)
ESD Protection Level(8) ………………………………….
Output Inductor
……………………………………… 1.0uH
3kV
Thermal Information
Thermal Resistance, Junction to Ambient(7) ………… 40 °C/W
Thermal Resistance, Junction to Case …………… 7 °C/W
Maximum Junction Temperature …………………… +150°C
Storage Temperature Range ………………… -65 to +150 °C
Exceeding the absolute maximum ratings may result in permanent damage to the device and/or device malfunction. Operation outside of the
parameters specified in the Electrical Characteristics section is not recommended.
Notes:
(7) Calculated from package in still air, mounted to 3” x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
(8) Tested according to JEDEC standard JESD22-A114-B.
(9) Due to parasitic board inductance, the transient LX pin voltage at the point of measurement may appear larger than that which exists on silicon. The device is
designed to tolerate the short duration transient voltages that will appear on the LX pin due to the deadtime diode conduction, for inductor currents up to the
current limit setting of the device.
Electrical Characteristics
Unless specified: PVIN= AVIN= 5.0V, VOUT= 1.50V, CIN= 22µF, COUT= 2 x 22µF; L= 1.0µH; -40°C≤ TJ ≤ +125 °C; Unless otherwise noted typical values are TA= +25 °C.
Parameter
Under-Voltage Lockout
Symbol
Conditions
Min
Typ
Max
Units
Rising AVIN, PVIN=AVIN
2.70
2.80
2.90
V
UVLO
Hysteresis
Output Voltage Tolerance(10)
Current Limit
300
ΔVOUT
PVIN= AVIN= 2.9 – 5.5V; IOUT=0A
-1.25
ILIMIT
Peak LX current
5.0
6.0
mV
+1.25
%
7.0
A
No load, MODE= High
12
mA
No load, MODE= Low
100
µA
ISHDN
EN= AGND
1
High Side Switch Resistance(11)
RDSON_P
ILX= 100mA, TJ= 25 °C
50
Low Side Switch Resistance(11)
RDSON_N
ILX= -100mA, TJ= 25 °C
35
PVIN= AVIN= 5.5V; LX= 0V; EN= AGND
1
Supply Current
Shutdown Current
LX Leakage Current(11)
Load Regulation
IQ
ILK(LX)
ΔVLOAD-REG
Oscillator Frequency
fOSC
Soft-Start Charging Current(11)
ISS
Foldback Holding Current
© 2010 Semtech Corp.
ICL_HOLD
PVIN= AVIN= 5.5V; LX= 5.0V; EN= AGND
-20
PVIN= AVIN= 5.0V, MODE=Hi, IOUT=1mA – 4A
10
-1
1.5
µA
mΩ
±0.3
1.275
Average LX Current
10
µA
%
1.725
MHz
+5
µA
1
A
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SC185
Electrical Characteristics (continued)
Parameter
Symbol
Impedence of PGOOD Low
RPGOOD_LO
Conditions
90
%
Asserted
2
ms
PGOOD= Low
20
µs
50
µs
VPG_DLY
EN Delay
tEN_DLY
From EN Input High to SS starts rising
IEN_
EN =AVIN or AGND
EN Input Low Threshold
VEN_LO
MODE Input Current(11)
IMODE_
Units
VOUT rising
PGOOD Delay
VEN_HI
Max
Ω
VPG_TH
EN Input High Threshold
Typ
10
PGOOD Threshold
EN Input Current(11)
Min
-2.0
2.0
1.2
MODE= AVIN or AGND
µA
V
-2.0
0.4
V
2.0
µA
MODE Input High Threshold
VMODE_HI
MODE Input Low Threshold
VMODE_LO
VOUT Over Voltage Protection
VOVP
Thermal Shutdown Temperature
TSD
160
°C
TSD_HYS
10
°C
Thermal Shutdown Hysteresis
1.2
110
V
115
0.4
V
120
%
Notes:
(10) The “Output Voltage Tolerance” includes output voltage accuracy, voltage drift over temperature and the line regulation.
(11) A negative current means the current flows into the pin and a positive current means the current flows out from the pin.
© 2010 Semtech Corp.
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SC185
Pin Descriptions
Pin #
Pin Name
1,2
Pvin
Input supply voltage for the converter power stage.
3
AGND
Ground connection for the internal circuitry. AGND needs to be connected to PGND directly.
4
AVIN
Power supply for the internal circuitry. AVIN is required to be connected to PVIN through an R-C filter of 1Ω
and 100nF.
MODE
MODE select pin. When connected to logic high, the device operates in forced PWM mode. When connected
to logic low, it operates normally with PSAVE mode at light load. The enable pin has a 500kΩ internal pulldown resistor. This resistor is switched in circuit whenever the MODE pin is “Low” or when the part is in undervoltage lockout.
6
EN
Enable pin. When connected to logic high or tied to AVIN pin, the SC185 is on. When connected to logic low,
the device enters shutdown and consumes less than 1µA current (typ.). The enable pin has a 500kΩ internal
pulldown resistor. This resistor is switched in circuit whenever the EN is “Low” or when the part is in undervoltage lockout.
7
PGOOD
Power good indicator. When the output voltage reaches the PGOOD threshold, this pin will be open-drain
(After the PGOOD delay), otherwise, it is pulled low internally.
8
NC
No connection.
9
SS
Soft Start. Connect a soft-start capacitor to program the soft start time. There is a 5µA charging current flowing out of the pin.
10
VOUT
Output voltage sense pin.
11,12,13
PGND
Ground connection for converter power stage.
14,15,16
LX
T
Thermal Pad
5
© 2010 Semtech Corp.
Pin Function
Switching node - connect an inductor between this pin and the output capacitor.
Thermal pad for heatsinking purposes. Recommend to connect it to PGND. It is not connected internally.
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SC185
Block Diagram
AVIN
4
UVLO
PGOOD
PVIN
14
15
16
LX
11
12
13
PGND
5
MODE
7
Faults
Power Good
Logic & Delay
AGND
1
2
0.45V
3
-
+
PGood Comp.
0.575V
-
+
VOUT
10
OVP Comp.
-
+
+
5µA
-PWM
+
Error Amp.
Control Logic
&
MOSFET Drivers
PWM Comp.
SS
9
Faults
0.5V
EN
6
© 2010 Semtech Corp.
Oscillator
& Ramp
BANDGAP
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SC185
Typical Characteristics
Circuit Conditions: CIN= 22uF/6.3V, COUT= 2 x 22uF/6.3V, CSS= 10nF. Unless otherwise noted, L= 1.0uH (TOKO: FDV0530S-1R0).
Efficiency Efficiency
(Forced PWM)
Efficiency (PSAVE Enabled)
100%
100%
95%
95%
90%
90%
Efficiency (%)
Efficiency (%)
VIN=5.0V;VOUT=3.3V
85%
VIN=5.0V;VOUT=1.5V
80%
75%
VIN=3.3V;VOUT=1.5V
Vin = 5V, Vout = 3.3V
Vin = 3.3V, Vout = 1.5V
85%
Vin = 5V, Vout = 1.5V
80%
75%
70%
70%
65%
65%
TA= 25°C
60%
0.0
0.5
1.0
1.5
2.0
2.5
Output Current (A)
3.0
3.5
TA = 25oC
60%
4.0
0.0
0.1
1.0
Output Current (A)
10.0
Load Regulation (Forced PWM)
Total Loss
Total Loss (Forced
PWM)
0.60%
2000
TA=25°C
VIN=3.3V;VOUT=1.5V
0.50%
TA = 25oC
0.40%
1600
Load Regulation
Loss (mW)
0.30%
1200
VIN=5.0V;VOUT=3.3V
800
0.20%
Vin = 5.0V, Vout = 3.3V
0.10%
0.00%
-0.10%
-0.20%
Vin = 3.3V, Vout = 1.5V
-0.30%
400
Vin = 5V, Vout = 1.5V
-0.40%
VIN=5.0V;VOUT=1.5V
-0.50%
-0.60%
0
0.0
0.5
1.0
1.5
2.0
2.5
Output Current (A)
3.0
3.5
0.0
4.0
1.5
2.0
2.5
3.0
3.5
4.0
RDSON
(P & N) Variation
Over Temperature
RDS(ON)
Variation
vs. Temperature
35%
20%
30%
15%
25%
10%
P-Channel
VIN= 5.0V
ILX= ±100mA
5%
Variation
Variation
1.0
Output current (A)
(P & N)vs.
Variation
Line
RDS(ON)RDSON
Variation
Inputover
Voltage
20%
0.5
15%
10%
0%
P-Channel
-5%
5%
-10%
0%
ILX= ±100mA
TA= 25°C
-5%
N-Channel
N-Channel
-15%
-20%
-10%
2.5
3.0
3.5
4.0
4.5
5.0
-40
5.5
© 2010 Semtech Corp.
-15
10
35
60
85
Ambient Temperature (°C)
Input Voltage (V)
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SC185
Typical Waveforms
Circuit Conditions: CIN= 22uF/6.3V, COUT= 2 x 22uF/6.3V, CSS= 10nF. Unless otherwise noted, L= 1.0uH (TOKO: FDV0530S-1R0).
Output Voltage Ripple (VIN=3.3V, VOUT=1.5V, 0A)-Auto
Output Voltage Ripple (VIN=5.0V, VOUT=1.5V, 0A)-Auto
Output Voltage Ripple (VOUT=1.5V), PSAVE Mode
Output Voltage Ripple (VOUT=1.5V),PSAVE Mode
Mode= Low
Mode= Low
Ch3
VOUT
Ch3
VOUT
Ch4
VIN
Ch4
VIN
Ch2
ILX
Ch2
ILX
Ch1
VLX
Ch1
VLX
VIN = 3.3V
IOUT = 0A
VIN = 5.0V
IOUT = 0A
Output Voltage Ripple (VIN=3.3V, VOUT=1.5V, 0A)-PWM
Output Voltage Ripple (VIN=5.0V, VOUT=1.5V, 0A)-PWM
Output Voltage Ripple (VOUT=1.5V), Forced PWM
Output Voltage Ripple (VOUT=1.5V), Forced PWM
Mode= Hi
Mode= Hi
Ch3
VOUT
Ch3
VOUT
Ch4
VIN
Ch4
VIN
Ch2
ILX
Ch2
ILX
Ch1
VLX
Ch1
VLX
VIN = 3.3V
IOUT = 0A
VIN = 5.0V
IOUT = 0A
Output Voltage Ripple (VIN=3.3V, VOUT=1.5V, 4A)-All
Output Voltage Ripple (VIN=5.0V, VOUT=1.5V, 4A)-All
Output Voltage Ripple (VOUT=1.5V) @ Full Load
Output Voltage Ripple (VOUT=1.5V) @ Full Load
Ch3
VOUT
Ch3
VOUT
Ch4
VIN
Ch4
VIN
Ch2
ILX
Ch2
ILX
Ch1
VLX
Ch1
VLX
VIN = 3.3V
IOUT = 4A
© 2010 Semtech Corp.
VIN = 5.0V
IOUT = 4A
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SC185
Typical Waveforms (continued)
Circuit Conditions: CIN= 22uF/6.3V, COUT= 2 x 22uF/6.3V, CSS= 10nF. Unless otherwise noted, L= 1.0uH (TOKO: FDV0530S-1R0).
Output Voltage Ripple (VIN=5.0V, VOUT=3.3V, 0A)-Auto
Output Voltage Ripple (VIN=5.0V, VOUT=3.3V, 0A)-PWM
Output Voltage Ripple (VOUT=3.3V), PSAVE Mode
Output Voltage Ripple (VOUT=3.3V), forced PWM
Mode= Low
Mode= Hi
Ch3
VOUT
Ch3
VOUT
Ch4
VIN
Ch4
VIN
Ch2
ILX
Ch2
ILX
Ch1
VLX
Ch1
VLX
VIN = 5.0V
IOUT = 0A
VIN = 5.0V
IOUT = 0A
Output Voltage Ripple (VIN=5.0V, VOUT=3.3V, 4A)-All
Start-up (VIN=5.0V, VOUT=1.5V, 0A)-Auto
Output Voltage Ripple (VOUT=3.3V) @ Full Load
Start Up (Enable) (VOUT=1.5V), PSAVE Mode
Ch1:VEN
Ch2: IOUT
Ch3: VOUT
Ch4: PGood
Ch3
VOUT
Mode= Low
Ch4
VIN
Ch2
ILX
Ch1
VLX
VIN = 5.0V
IOUT = 4A
VIN = 5.0V
IOUT = 0A
Start-up (VIN=5.0V, VOUT=1.5V, 0A)-PWM
Start-up (VIN=5.0V, VOUT=1.5V, 4A)-All
Start Up (Enable) (VOUT=1.5V), forced PWM
Ch1:VEN
Ch2: IOUT
Ch3: VOUT
Ch4: PGood
Start Up (Enable) (VOUT=1.5V) @Full Load
Ch1:VEN
Ch2: IOUT
Ch3: VOUT
Ch4: PGood
Mode= Hi
VIN = 5.0V
IOUT = 0A
© 2010 Semtech Corp.
VIN = 5.0V
IOUT = 4A
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SC185
Typical Waveforms (continued)
Circuit Conditions: CIN= 22uF/6.3V, COUT= 2 x 22uF/6.3V, CSS= 10nF. Unless otherwise noted, L= 1.0uH (TOKO: FDV0530S-1R0).
Start-up (VIN=5.0V, VOUT=3.3V, 0A)-Auto
Start-up (VIN=5.0V, VOUT=3.3V, 0A)-PWM
Start Up (Enable) (VOUT=3.3V), PSAVE Mode
Ch1:VEN
Ch2: IOUT
Ch3: VOUT
Ch4: PGood
Start Up (Enable) (VOUT=3.3V), forced PWM
Ch1:VEN
Ch2: IOUT
Ch3: VOUT
Ch4: PGood
Mode= Low
VIN = 5.0V
IOUT = 0A
Mode= Hi
VIN = 5.0V
IOUT = 0A
Start-up into Pre-biased Output (VIN=5.0V, VOUT=1.5V, 0A)-PWM
Start-up (VIN=5.0V, VOUT=3.3V, 4A)-All
Start Up (Enable) (VOUT=3.3V) @Full Load
Start Up into Pre-Biased Output (VOUT=1.5V)
Ch1:VEN
Ch2: IOUT
Ch3: VOUT
Ch4: PGood
Ch1:VEN
Ch2: IOUT
Ch3: VOUT
Ch4: PGood
Mode= Hi
540mV
VIN = 5.0V
IOUT = 4A
VIN = 5.0V
IOUT = 0A
Start-up into Pre-biased Output (VIN=5.0V, VOUT=3.3V, 0A)-PWM
Start Up into Pre-Biased Output (VOUT=3.3V)
Ch1:VEN
Ch2: IOUT
Ch3: VOUT
Ch4: PGood
Start-Up into Shorted Output (VIN=5.0V, VOUT=1.5V)-PWM
Start Up (Enable) into Output Short Circuit
Mode= Hi
Mode= Hi
Ch1
VLX
Ch3
VOUT
Ch4
VSS
480mV
Ch2
ILX
VIN = 5.0V
IOUT = 0A
© 2010 Semtech Corp.
VIN = 5.0V
ROUT = 0
10
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SC185
Typical Waveforms (continued)
Circuit Conditions: CIN= 22uF/6.3V, COUT= 2 x 22uF/6.3V, CSS= 10nF. Unless otherwise noted, L= 1.0uH (TOKO: FDV0530S-1R0).
Output Short-Circuit (VIN=5.0V, VOUT=1.5V)-PWM
Recovery from Output Short (VIN=5.0V, VOUT=1.5V)-PWM
Output Short Circuit (VOUT=1.5V)
Recovery from OCP (VOUT=1.5V)
Mode= Hi
Mode= Hi
Ch1
VLX
Ch1
VLX
Ch3
VOUT
Ch3
VOUT
Ch4
PGood
Ch4
VIN
Ch2
ILX
Ch2
ILX
VIN = 5.0V
VIN = 5.0V
IOUT = 0A
Output Short-Circuit (VIN=5.0V, VOUT=3.3V)-PWM
Recovery from Output Short (VIN=5.0V, VOUT=3.3V)-PWM
Output Short Circuit (VOUT=3.3V)
Recovery from OCP (VOUT=3.3V)
Mode= Hi
Mode= Hi
Ch1
VLX
Ch1
VLX
Ch3
VOUT
Ch3
VOUT
Ch4
PGood
Ch4
VIN
Ch2
ILX
Ch2
ILX
VIN = 5.0V
VIN = 5.0V
IOUT = 0A
Transient Response (VIN=5.0V, VOUT=3.3V)-PWM@CCDL
Transient Response (VIN=5.0V, VOUT=1.5V)-PWM@CCDL
Transient Response (VOUT=1.5V, ISTEP=2A)
Transient Response (VOUT=3.3V, ISTEP=2A)
Mode= Hi
Mode= Hi
Ch3
VOUT
Ch3
VOUT
Ch2
IOUT
Ch2
IOUT
VIN = 5.0V
IOUT = 1A to 3A to 1A
VIN = 5.0V
IOUT = 1A to 3A to 1A
© 2010 Semtech Corp.
11
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SC185
Typical Waveforms (continued)
Circuit Conditions: CIN= 22uF/6.3V, COUT= 2 x 22uF/6.3V, CSS= 10nF. Unless otherwise noted, L= 1.0uH (TOKO: FDV0530S-1R0).
Transient Response (VOUT=3.3V, ISTEP=3A)-PSAVE Enabled
Transient Response (VOUT=1.5V, ISTEP=3A)-PSAVE Enabled
© 2010 Semtech Corp.
12
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SC185
Applications Information
frequency being much lower than the PWM mode
frequency.
Detailed Description
The SC185 is a synchronous step-down Pulse Width
Modulated (PWM), DC-DC converter utilizing a 1.5MHz
fixed-frequency voltage mode architecture. The device is
designed to operate in fixed-frequency PWM mode and
has the option to enter power save mode (PSAVE) at light
loads to maximize efficiency. The switching frequency is
chosen to minimize the size of the external inductor and
capacitors while maintaining high efficiency.
If the output load current increases enough to cause
VOUT to decrease below the PSAVE exit threshold (VOUT 4%), the device automatically exits PSAVE and operates
in continuous PWM mode. Note that the PSAVE high
and low threshold levels are both set at or above VOUT to
minimize undershoot when the SC185 exits PSAVE. Figure
1 illustrates the transitions from PWM mode to PSAVE
mode and back to PWM mode.
Operation
During normal operation, the PMOS MOSFET is activated
on each rising edge of the internal oscillator. The period
is set by the onboard oscillator when in PWM mode. The
device has an internal synchronous NMOS rectifier and
does not require a Schottky diode on the LX pin. The
device operates as a buck converter in PWM mode with
a fixed frequency of 1.5MHz at medium to high loads. At
light loads, depending on the MODE pin configuration, the
part will either enter PSAVE mode to maximize efficiency
or stay in forced PWM mode.
Load
Demand
(IOUT)
VV
+2.5%
OUT
OUT +2%
VOUT
BURST
Power Save Mode Operation
Connect the MODE pin to ground to enable the PSAVE
mode. When the load current decreases below the PSAVE
threshold, PWM switching stops and the device automatically enters PSAVE mode. This threshold varies depending
on the input voltage and output voltage setting, optimizing efficiency for all possible load currents - whether in
PWM or PSAVE mode. While in PSAVE mode, output voltage
regulation is controlled by a series of bursts in switching.
During a burst, the inductor current is limited to a peak
value which controls the on-time of the PMOS switch. After
reaching this peak, the PMOS switch is disabled and the
inductor current is forced to near 0mA. Switching bursts
continue until the output voltage climbs to VOUT +2% or
until the PSAVE current limit is reached. Switching is then
stopped to eliminate switching losses, enhancing overall
efficiency. Switching resumes when the output voltage
reaches the lower threshold of VOUT and continues until the
upper threshold again is reached. Note that the output
voltage is regulated hysterically while in PSAVE mode
between VOUT and VOUT + 2%. The period and duty cycle
while in PSAVE mode are solely determined by VIN and VOUT
until PWM mode resumes. This can result in the switching
© 2010 Semtech Corp.
OFF
VVOUT
-2%
OUT - 4%
VLX
PWM Mode at
Medium/High
Load
PSAVE
EXIT
PSAVE Mode at
Light Load
PWM Mode at
Medium/High
Load
Time
Figure 1 — Transitions between PWM and PSAVE
Modes
Protection Features
The SC185 provides the following protection features:
Current Limit
Over-Voltage Protection
Soft-Start Operation
Thermal Shutdown
•
•
•
•
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SC185
Applications Information (continued)
output. When the output is pre-charged by another
supply rail, the SC185 will not discharge the output during
the soft start period.
Current Limit & OCP
The internal PMOS power device in the switching stage is
protected by a current limit feature. If the inductor current
is above the PMOS current limit for 16 consecutive cycles,
the part enters foldback current limit mode and the output
current is limited to the current limit holding current
(ICL_HOLD) which is approximately 900mA. Under this condition, the output voltage will be the product of ICL_HOLD and
the load resistance. When the load presented falls below
the current limit holding level, the output will charge to
the upper PSAVE voltage threshold and return to normal
operation. The SC185 is capable of sustaining an indefinite short circuit without damage. During the soft start, if
current limit has occurred before the SS voltage has
reached 400mV, the part enters foldback current limit
mode. Foldback current limit mode will be disabled during
soft-start after the SS voltage is higher than 400mV.
Shut Down
When the EN pin is low, the SC185 will run in shutdown
mode, drawing less than 1μA (typ.) from the input power
supply. The internal switches and bandgap voltage will be
immediately turned off.
Thermal Shutdown
The device has a thermal shutdown feature to protect
the SC185 if the junction temperature exceeds 160°C.
During thermal shutdown, the on-chip power devices are
disabled, floating the LX output. When the temperature
drops by 10°C, it will initial a soft start cycle to resume
normal operation.
Over-Voltage Protection
Under-Voltage Lockout
In the event of a 15% over-voltage on the output, the
PWM drive is disabled with the LX pin floating. Switching
does not resume until the output voltage falls below the
nominal Vout regulation voltage.
Under-Voltage Lockout (UVLO) is enabled when the
input voltage drops below the UVLO threshold. This
prevents the device from entering an ambiguous state
in which regulation cannot be maintained. Hysteresis of
approximately 300mV is included to prevent chattering
near the threshold. When the AVIN voltage reaches back
to the turn-on threshold and EN is high, the soft-start
mode is resumed.
Soft-Start
The soft-start mode is activated after AVIN reaches it’s
UVLO voltage threshold and EN is set high to enable the
part. A thermal shutdown event will also activate the soft
start sequence. The Soft-start mode controls the slew-rate
of the output voltage during startup thus limiting in-rush
current on the input supply. During start up, the reference
voltage for the error amplifier is clamped by the voltage
on SS pin. The output voltage slew rate during soft start is
determined by the value of the external capacitor connected to the SS pin and the internal 5µA charging current.
The SC185 requires a minimum soft-start time from enable
to final regulation in the order of 200µs, including the
50µs enable delay. As a result the soft start capacitor, Css,
should be higher than 1.5nF. During start up, the chip
operates in forced PWM mode. The value of Css for the
desired soft-start time, tss, can be determined by Equation
1.
t SS = CSS ×
0.5V
5µA
Power Good
The power good (PGOOD) is an open-drain output. When
the output voltage drops below 10% of nominal voltage,
the PGOOD pin is pulled low after a 20μs delay. During
start-up, PGOOD will be asserted 1.8ms (typ.) after the
output voltage reaches 90% of the final regulation
voltage. The faults of over voltage, fold-back current limit
mode and thermal shutdown will force PGOOD low after
a 20µs delay. When recovering from a fault, PGOOD will be
asserted 1.8ms (typ.) after Vout reaches 90% of the final
regulation voltage.
Enable
The EN input is used to enable or disable the device when
the device is not in UVLO. When EN is low (grounded), the
device enters shutdown mode and consumes less than
1μA of current. In shutdown mode, the device tri-states
the LX pin and pulls down the SS pin. The EN pin has a
500kΩ internal pull-down resistor. This resistor is switched
(1)
The SC185 is capable of starting up into a pre-biased
© 2010 Semtech Corp.
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SC185
Applications Information (continued)
value of 1μH to 2.2μH with output ceramic capacitors of
44μF or higher capacitance. Lower inductance should
be considered in applications where faster transient
response is required. More output capacitance will reduce
the output deviation for a particular load transient. When
using low inductance, the maximum peak inductor
current at any condition (normal operation and start up)
can not exceed 5A which is the guaranteed minimum
current limit. The saturation current rating of the inductor
needs to be at least larger than the peak inductor current
which is the maximum output current plus half of inductor
ripple current.
in circuit whenever the EN pin is below its threshold, or
when the device is in under voltage lockout and AVIN
exceeds 0.8V. When the device is enabled, it takes about
50µs for the internal circuitry wake up and begin the softstart up sequence.
Operation Mode Selection
The MODE input is used to select between forced PWM
and automatic PSAVE modes. When the MODE pin is held
high, the device operates in forced continuous PWM mode
regardless of the output load condition. When the MODE
pin is held low (grounded), the device is permitted to
operate in Power Save mode (PSAVE). The MODE pin can
be changed on-the-fly. When the MODE pin is switched
from low to high, the device will transition to forced
continuous PWM mode immediately. When the MODE
pin is switched from high to low, and the load current is
below the PSAVE entry level, the device will transition to
PSAVE mode after 64 clock cycles. The MODE pin has a
500kΩ internal pull-down resistor. This resistor is switched
in circuit whenever the MODE pin is below its threshold,
or when the device is in under voltage lockout but AVIN
exceeds 0.8V.
100% Duty-Cycle Operation
SC185 is capable of operating at 100% duty-cycle. When
the difference between input voltage to output voltage
is less than the minimum dropout voltage, the PMOS
switch is completely on, operating in 100% duty-cycle.
The minimum dropout voltage is the output current
multiplied by the on-resistance of the internal PMOS
switch and the DC-resistance of the inductor when PMOS
switch is on continuously.
Output L-C filter Selection
SC185 has fixed internal loop-gain compensation. It is
optimized for X5R or X7R ceramic output capacitors and
an output L-C filter corner frequency of less than 34KHz.
The output L-C corner frequency can be determined by
Equation 2.
fC =
1
2π L ⋅ COUT
(2)
In general, the inductor is chosen to set the inductor ripple
current to approximately 30% of the maximum output
current. It is recommended to use a typical inductor
© 2010 Semtech Corp.
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SC185
Applications Information (continued)
PCB Layout Considerations
The layout diagram in Figure 2 shows a recommended
top-layer PCB for the SC185 and supporting components.
Figure 3 shows the bottom layer for this PCB. Fundamental
layout rules must be followed since the layout is critical
for achieving the performance specified in the Electrical
Characteristics table. Poor layout can degrade the
performance of the DC-DC converter and can contribute
to EMI problems, ground bounce, and resistive voltage
losses. Poor regulation and instability can result.
The following guidelines are recommended when
developing a PCB layout:
VOUT
L
VIN
1. The input capacitor, CIN should be placed as close to
the PVIN and PGND pins as possible. This capacitor
provides a low impedance loop for the pulsed currents
present at the buck converter’s input. Use short wide
traces to connect as closely to the IC as possible.
This will minimize EMI and input voltage ripple by
localizing the high frequency current pulses.
2. Keep the LX pin traces as short as possible to minimize
pickup of high frequency switching edges to other
parts of the circuit. COUT and L should be connected as
close as possible between the LX and PGND pins, with
a direct return to the PGND pin from COUT.
3. Route the output voltage feedback/sense path away
from the inductor and LX node to minimize noise and
magnetic interference.
4. Use a ground plane referenced to the SC185 PGND
pin. Use several vias to connect to the component
side ground to further reduce noise and interference
on sensitive circuit nodes.
5. If possible, minimize the resistance from the VOUT
and PGND pins to the load. This will reduce the
voltage drop on the ground plane and improve the
load regulation. And it will also improve the overall
efficiency by reducing the copper losses on the output
and ground planes.
GND
COUT
CIN
U1
C1
CSS
GND
d
PG
oo
O
D
M
EN
E
R1
Figure 2 — Recommended PCB Layout (Top Layer)
VIN
VOUT
GND
GND
Figure 3 — Bottom Layer Detail
© 2010 Semtech Corp.
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A
Outline Drawing – 3x3 MLPQ-UT16
A
B
DIM
A
A1
PIN 1
INDICATOR
(LASER MARK)
E
A A2
aaa C
b
D
D1
A1
E
E1
e
L
N
aaa
bbb
A2
A
aaa C
C
SEATING
PLANE
E1
DIM
E
DIMENSIONS
INCHES
MILLIMETERS
MIN NOM MAX MIN NOM MAX
.024
.002
(.006)
.007 .009 .012
.114 .118 .122
.061 .067 .071
.114 .118 D1
.122
.061 .067 .071
.020 BSC
.012 .016 .020
16
.003
.004
.020
.000
- A20.60
0.05
(0.152)
SEATING
0.18 0.23 0.30 PLANE
2.90 3.00 3.10
C
1.55 1.70 1.80
2.90 3.00 3.10
1.55 1.70 1.80
e/2
0.50 BSC
0.30 0.40 0.50
LxN
16
E/2
0.08
0.10
0.50
0.00
A
A1
A2
b
D
D1
E
E1
e
L
N
aaa
bbb
DIMENSIONS
INCHES
MILLIMETERS
MIN NOM MAX MIN NOM MAX
.024
.002
(.006)
.007 .009 .012
.114 .118 .122
.061 .067 .071
.114 .118 .122
.061 .067 .071
.020 BSC
.012 .016 .020
16
.003
.004
.020
.000
0.60
0.05
(0.152)
0.18 0.23 0.30
2.90 3.00 3.10
1.55 1.70 1.80
2.90 3.00 3.10
1.55 1.70 1.80
0.50 BSC
0.30 0.40 0.50
16
0.08
0.10
0.50
0.00
2
1
D1
N
e/2
e
LxN
E/2
E1
SC185
B
PIN 1
INDICATOR
(LASER MARK)
D
A1
D
bxN
D/2
bbb
C A B
NOTES:
2
1
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2.
COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
3.
DAP IS 1.90 x 1.90mm.
N
e
bxN
D/2
bbb
C A B
NOTES:
Land Pattern – 3x3 MLPQ-UT16
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2.
COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
3.
H
DAP IS 1.90 x 1.90mm.
R
DIM
(C)
K
G
C
G
H
K
P
R
X
Y
Z
Z
Y
X
P
DIMENSIONS
INCHES
MILLIMETERS
(.114)
.083
.067
.067
.020
.006
.012
.031
.146
(2.90)
2.10
1.70
1.70
0.50
0.15
0.30
0.80
3.70
NOTES:
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2.
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
3. THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
FUNCTIONAL PERFORMANCE OF THE DEVICE.
© 2010 Semtech Corp.
17
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SC185
© Semtech 2010
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Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805) 498-2111 Fax: (805) 498-3804
www.semtech.com
© 2010 Semtech Corp.
18
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