SC21152
Tiny 1.2A Synchronous Buck
Regulator with Power Good
POWER MANAGEMENT
Features
Description
Input voltage range — 2.3V to 5.5V
1.2A continuous output current
4MHz switching frequency
Adjustable Output Voltage
1% output voltage accuracy
Adaptive On-Time architecture
Up to 100% duty cycle
Up to 96% peak efficiency
Power Save mode for efficient light load operation
>90% efficiency 30mA to 75mA
0.1μA shutdown current
21μA quiescent current
Power Good Indicator
Pre-bias startup protection
Internal soft start
1.5mm x 2mm, 6-Lead MLPD package
WEEE and RoHS compliant and halogen-free
Applications
The SC21152 is an ultra high efficiency 1.2A DC/DC buck
regulator available in a small MLPD package. These features make the SC21152 perfect for small form factor consumer applications where high efficiency is essential.
SC21152 is an Adaptive On-Time 4MHz switching regulator at full load conditions. Under light load conditions it
transitions into PFM Power Save Mode and reduces switching frequency thus saving energy. High switching frequency operation allows the use of a very small 0.47μH
inductor along with small input and output capacitors to
minimize overall circuit size.
The SC21152 is highly configurable with a wide range of
factory options including internal soft start time, fixed or
adjustable output voltage, automatic output discharge,
and switching frequency.
The SC21152 is offered in a small 6-Lead 1.5mm x 2mm
MLPD package.
Tablet PCs
GPS devices
Battery powered equipment
Portable devices
Smart Phones
Wearable Electronics
Typical Application Circuit
Adjustable Output Voltage
Fixed Output Voltage
2.3V-5.5V Supply
Power
Good
EN
Rev 1.0
4.7µF
VIN
LX
SC21152
PG
FB
EN
GND
1.2A
0.47µH Output
10µF
2.3V-5.5V Supply
Power
Good
EN
© 2015 Semtech Corporation
4.7µF
VIN
LX
SC21152
PG
FB
EN
GND
1.2A
0.47µH Output
10µF
SC21152
Ordering Information
Options(3)
Device (1)(2)
SC21152AULTRC
Output(3)
Soft Start(3)
Output Discharge(3)
Adjustable
100μs
Automatic
SC21152EVB
Package
1.5mm x 2mm 6-Lead MLPD
Evaluation Board 1.5mm x 2.0mm 6-Lead MLPD
Notes:
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Lead-free packaging only. Device is WEEE and RoHS compliant and halogen-free.
(3) For other device options not shown contact Semtech Marketing.
The following options can be made available:
VOUT options : adustable and fixed output voltage from 0.8V to 3.6V in 50mV increments.
Soft Start options: internal 100μs, 200μs, 400μs, or 800μs.
Output Discharge options: Automatic Discharge and No Discharge.
Frequency: The default operating frequency is 4MHz with options of 2.2MHz, 3MHz, or 6MHz.
Pin Configuration
Marking Information
1
6
2
5
3
4
T O P V IE W
6 -L e a d M L P D
1 .5 x2 .0 (m m )
RCA
oyw
T O P V IE W
1.5x2 .0 (m m )
6 -L e a d M L P D
Pin Assignments 6-Lead MLPD
Pin #
Name
1
VIN
2
PG
3
EN
4
FB
5
GND
6
LX
PAD
PAD
1.5x2.0 (mm) 6-Lead MLPD Marking Diagram
RCA = Device Marking Code
oyw = Pin 1 Indicator and Date Code
SC21152
Absolute Maximum Ratings
Recommended Operating Conditions
VIN (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +6.0
Input Voltage Range (V) . . . . . . . . . . . . . . . . . . . . . . . . 2.3 to 5.5
EN, FB (V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +(VIN+0.3)
Output Voltage Range, Adjustable VOUT (V) . . . 0.8 to VIN
PG (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to (VIN+0.3)
Output Voltage Range, Fixed VOUT (V) . . . . . . . . . . 0.8 to 3.6
LX (V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +(VIN+0.3)
Maximum Continuous Current (A). . . . . . . . . . . . . . . . . . . . . 1.2
ESD Protection Level (kV)(1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Junction Temperature Range (°C) . . . . . . . . . -40 < TJ < +125
Thermal Information
Thermal Resistance Junction to Ambient(2) (°C/W )
MLPD package. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .120
Maximum Junction Temperature (°C) . . . . . . . . . . . . . . +150
Operating Junction Temperature (°C ). . . . . . . -40 to +125
Storage Temperature Range (°C) . . . . . . . . . . . . -65 to +150
Peak IR Reflow Temperature (10s to 30s) (°C) . . . . . . . +260
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters
specified in the Electrical Characteristics section is not recommended.
NOTES:
(1) Tested according to JEDEC standard JESD22-A114-B.
(2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
Electrical Characteristics
Unless otherwise noted VIN = 5.0V, CIN = 4.7μF, COUT = 10μF, L = 0.47μH, EN = VIN, TJ = -40 to +125°C. Typical values are at TA = 25°C.
Parameter
Symbol
Conditions
Min
Typ
Max
Units
5.5
V
Input Supply
Input Supply Voltage Range
VIN
2.3
Shutdown Current
ISD
EN = 0V
0.1
μA
Quiescent Current
IQ
Operating, No Load, Power Save Option
21
μA
Under Voltage Lock Out Threshold
UVLOrise
VIN rising edge
Under Voltage Lock Out Hysteresis
UVLOhyst
2.05
2.16
2.27
V
85
mV
0.80
V
Output Voltage
Feedback Voltage
Feedback Voltage Tolerance
Feedback Input Leakage Current
Output Voltage Range
VFB
Adjustable VOUT Option only
VFBtol
PWM operation, 500mA load
IFB
Adjustable VOUT Option
Vout
Adjustable Output Voltage
-2
0.8
+/- 1
+2
%
1
25
nA
VIN
V
SC21152
Electrical Characteristics (continued)
Parameter
Symbol
Conditions
Output Voltage Line Regulation
dVline
5.5V > VIN > (Vout +1), PWM operation
0.17
%/V
Output Voltage Load Regulation
dVload
500mA < ILOAD < 1.2A
-0.13
%/A
IOUT
Continuous current
TSS
FB: from rising edge to 95% of final value
100
μs
On Resistance High-side
Rdson_hs
FB = 0V; ILX = 500mA
180
mΩ
On Resistance Low-side
Rdson_ls
FB = VIN; ILX = 500mA
140
mΩ
LX Leakage Current High Side
ILX_hs
VIN = 5.5V; FB = VIN; VLX = 0V
-0.1
μA
LX Leakage Current Low Side
ILX_ls
FB = 0V; VLX = VIN;
Vout Automatic Discharge disabled
0.1
LX Pulldown Current(1)
ILX_pd
EN = 0V; VLX = 1V;
Vout Automatic Discharge enabled
20
Switching Frequency(1)
Fsw
4MHz PWM operation, 500mA load,
adjustable output, VOUT = 2.5V
3.6
4.0
4.4
MHz
High Side Current Limit
ILIMhs
Current from VIN to LX
1.5
2.2
3.0
A
Low Side Current Limit
ILIMls
Current from LX to GND
-0.8
A
Ithzcpfm
Power Save Mode and DC Test condition
0
mA
Over-Temperature Shutdown
Tts_rise
Rising temperature
160
°C
Over-Temperature Hysteresis
Tts_hyst
10
°C
Min
Typ
Max
Units
Output Voltage (continued)
Maximum Output Current
1.2
A
Soft Start(1)
Soft Start Time (1)
LX Switching Node
-1.5
1
μA
mA
Current Sense
Zero Cross Detector Threshold
Thermal Protection
EN Input
EN Input High Threshold
VIHen
1.2
EN Input Low Threshold
VILen
EN Input High Current
IIHen
VEN = VIN
EN Input Low Current
IILen
VEN = 0V
V
0.4
-1
V
μA
1
μA
SC21152
Electrical Characteristics (continued)
Parameter
Symbol
Conditions
Max
Units
PG Low Voltage
VLOpg
IPG = 5mA
0.4
V
PG Open Leakage Current
IOHpg
VPG = VIN
1
μA
PGth
FB rising edge, percentage of FB final value
95
%
PGthhy
percentage of FB final value
(rising to falling edge)
3
%
Power Good Assert Time
PGtdr
from FB = Power Good Threshold
to PG open drain
75
μs
Power Good Deassert Time
PGtdf
from falling edge of FB
to PG pulled to GND
13
μs
Min
Typ
Power Good Output(1)
Power Good Threshold
Power Good Threshold Hysteresis
Notes:
(1) The SC21152 can be configured with a range of options as shown. Contact Semtech Marketing for more information.
VOUT options: adjustable or fixed output voltage.
For fixed VOUT, the voltage can be specified from 0.8V to 3.6V in 50mV increments.
Soft Start options: Fixed 100μs with options of 200μs, 400μs, and 800μs.
Frequency options: Operating frequency is 4MHz with options of 2.2MHz, 3MHz, or 6MHz.
Output Discharge options: VOUT Automatic Discharge or No Discharge.
SC21152
Typical Characteristics
Efficiency vs. Load - 5VIN
VIN = 5.0V, VOUT as shown, Load = 0 to 1.2A
100
95
95
90
90
85
85
Efficiency (%)
Efficiency (%)
100
Efficiency vs. Load - 4.2VIN
80
75
70
65
VIN = 4.2V, VOUT as shown, Load = 0 to 1.2A
80
75
70
65
60
0.8V
1.0V
1.2V
1.5V
60
0.8V
1.0V
1.2V
1.5V
55
1.8V
2.2V
2.5V
3.3V
55
1.8V
2.2V
2.5V
3.3V
50
0.01
0.10
Load (A)
50
0.01
1.00
100
95
95
90
90
85
85
Efficiency (%)
Efficiency (%)
VIN = 3.3V, VOUT as shown, Load = 0 to 1.2A
80
75
70
65
60
0.8V
1.0V
1.2V
55
1.8V
2.2V
2.5V
50
0.01
1.5V
0.10
Load (A)
VIN = 2.3V, VOUT as shown, Load = 0 to 1.2A
80
75
70
65
0.8V
60
50
0.01
1.00
95
90
90
85
85
Efficiency (%)
Efficiency (%)
100
95
80
75
70
65
2.3VIN
3.3VIN
55
4.2VIN
5.0VIN
50
0.01
0.10
Load (A)
1.2V
1.5V
0.10
Load (A)
1.00
Efficiency vs. Load - VOUT 1.8V
VIN as shown, VOUT = 1.0V, Load = 0 to 1.2A
60
1.0V
55
Efficiency vs. Load - VOUT 1.0V
100
1.00
Efficiency vs. Load - 2.3VIN
Efficiency vs. Load - 3.3VIN
100
0.10
Load (A)
1.00
VIN as shown, VOUT = 1.8V, Load = 0 to 1.2A
80
75
70
65
60
2.3VIN
3.3VIN
55
4.2VIN
5.0VIN
50
0.01
0.10
Load (A)
1.00
SC21152
Typical Characteristics
Frequency vs load - 5VIN, PWM operation
4.4
4.3
4.3
4.2
4.2
Frequency (MHz)
Frequency (MHz)
4.4
Frequency vs load - 3.3VIN, PWM operation
VIN = 5.0V, VOUT as shown, Load = 0.5A to 1.2A
4.1
4.0
3.9
3.8
3.7
3.6
0.5
0.6
0.8V
1.0V
1.2V
1.5V
1.8V
2.2V
2.5V
3.3V
0.7
0.8
0.9
Load (A)
1.0
1.1
VIN = 3.3V, VOUT as shown, Load = 0.5A to 1.2A
4.1
4.0
3.9
3.8
0.8V
3.7
3.6
1.2
0.5
VIN = 2.3V, VOUT as shown, Load = 0.5A to 1.2A
30
4.3
Quiescent Current (uA)
Frequency (MHz)
4.0
3.9
3.8
0.8V
3.7
2.3V
0.5
0.6
0.7
1.0V
0.8
0.9
Load (A)
1.0
1.1
1.1
1.2
3.3V
4.2V
5.0V
22
20
18
16
1.2
0.8
1.2
1.6
2.0
2.4
VOUT (V)
2.8
3.2
RDSON vs. Temperature
VIN = 2.3V, VOUT as shown, Load = 0.5A to 1.2A
280
High-side
Low-side
320
VIN = 2.3V, 3.3V,. 4.2V, 5.0V. VOUT as shown., no load, Rbot = 100kΩ
High-side
260
Low-side
240
RDSON (m)
280
RDSON (m)
1.0
24
RDSON vs. Input Voltage
240
200
220
200
180
160
140
160
120
120
80
0.8
0.9
Load (A)
1.8V
26
4.1
360
1.5V
VIN as shown, VOUT as shown, no load, Rbot = 100kΩ
28
4.2
3.6
0.7
1.2V
Quiescent Current vs Output Voltage
Frequency vs load - 2.3VIN, PWM operation
4.4
0.6
1.0V
100
2.3
2.7
3.1
3.5
3.9
VIN (V)
4.3
4.7
5.1
5.5
80
-40
-25
-10
5
20 35 50 65 80
Junction Temperature (°C)
95
110 125
SC21152
Typical Characteristics (continued)
Power Save Operation — no load
Startup - VIN ramp
VIN = 5V, VOUT = 1.5V, 0mA load
VIN = EN = ramp to 2.3V , VOUT = 1.5V, 0mA load
VIN, EN (2V/div)
VOUT (20mV/div)
VOUT (0.5V/div)
PG (2V/div)
LX (5V/div)
LX (5V/div)
Time (4ms/div)
Time (100µs/div)
Power Save Operation — 50mA load
Startup - EN control
VIN = 5V, VOUT = 1.5V, 50mA load
VIN = 5V, VOUT = 1.5V, 0mA load
EN (5V/div)
VOUT (20mV/div)
VOUT (0.5V/div)
PG (5V/div)
LX (5V/div)
LX (5V/div)
Time (2μs/div)
Time (100µs/div)
PWM Operation — 1.2A
Startup - 1.2A load
VIN = 5V, VOUT = 3.3V, 1.2A Load
VIN = 5V, VOUT = 1.5V, 1.2A load
EN (5V/div)
VOUT (20mV/div)
VOUT (2V/div)
PG (5V/div)
LX (5V/div)
LX (5V/div)
Time (200ns/div)
Time (100µs/div)
SC21152
Typical Characteristics (continued)
Load Transient - Power Save PFM to PWM
Load Transient - Power Save PFM to PWM
VIN = 5V, VOUT = 1.0V, Load 0A to 1.2A to 0A
VIN = 5V, VOUT = 3.3V, Load 0A to 1.2A to 0A
VOUT (50mV/div)
VOUT (0.1V/div)
LOAD (1A/div)
LOAD (1A/div)
LX (5V/div)
LX (5V/div)
Time (100μs/div)
Time (100µs/div)
Load Transient - PWM to PWM
Load Transient - PWM to PWM
VIN = 5V, VOUT = 1.0V, Load 0.5A to 1.2A to 0.5A
VOUT (50mV/div)
VIN = 5V, VOUT = 3.3V, Load 0.5A to 1.2A to 0.5A
VOUT (0.1V/div)
LOAD (1A/div)
LOAD (1A/div)
LX (5V/div)
LX (5V/div)
Time (100μs/div)
PWM Minimum Off Operation
Time (100µs/div)
Overcurrent Shutdown and Recovery
VIN = 5V, VOUT = 1.0V, Load 0A to 2.0A to 0A
VIN = 3.6V, VOUT = 3.3V, 500mA Load
VOUT (1V/div)
VOUT (50mV/div)
LOAD (1A/div)
LX (5V/div)
LX (5V/div)
Time (2µs/div)
Time (2ms/div)
SC21152
Detailed Application Circuits
Fixed Output Voltage
2.3V-5.5V Supply
4.7µF
Power
Good
EN
VIN
LX
1.2A
0.47µH Output
SC21152
PG
FB
EN
GND
10µF
Adjustable Output Voltage
2.3V-5.5V Supply
Power
Good
4.7µF
VIN
LX
SC21152
PG
FB
1.2A
0.47µH Output
Ctop
10µF
Rtop
Rbot
EN
VOUT
GND
EN
L
CIN
COUT
0.8V
1.0V
1.2V
1.5V
1.8V
2.2V
0.47uH
Current rating
1.2A minimum
Murata
LQM21PNR47MGH
2x1mm, 40mΩ
4.7uF
0402
X5R
10uF
0402
X5R
Rtop
Ctop
Rbot
zero
n/a
n/a
24.9kΩ
100kΩ
49.9kΩ
100kΩ
88.7kΩ
100kΩ
124kΩ
100kΩ
22pF
24.9kΩ
100kΩ
174Ω
100kΩ
2.8V
249kΩ
100kΩ
3.3V
316kΩ
100kΩ
2.5V
Wurth
74479876147
2x1mm, 40mΩ
10
SC21152
Pin Descriptions - 1.5x2mm 6-lead MLPD
Pin #
Pin Name
Pin Description
1
VIN
Input Supply Voltage for the device and sense point for the input Under Voltage Lock Out.
2
PG
Power Good Output. Open-drain Power Good indicator. An open indicates that the output voltage is
in regulation. The Power Good pin requires an external pullup.
3
EN
Enable input. Connect to logic high or VIN to enable the device. Connect to logic low or GND to disable the device.
4
FB
Feedback input (output voltage sense). For adjustable VOUT options, connect to the midpoint of a
resistor divider between the output capacitor and GND.
For fixed VOUT options connect to the output capacitor.
5
GND
6
LX
PAD
PAD
Ground connection.
Switching node connection to inductor.
Thernal ground for heatsinking. Connect to GND with multiple vias.
Block Diagram
F ixe d /
A d ju sta b le
V o u t S e le ct
EN
G a in A m p
P W M /P o w e r S a ve
C o n tro lle r
D e vice E n a b le
and UVLO
2
G a te
D rive
LX
5
GND
PAD
PAD
IC O M P
Ram p
G en
NMOS
FB
PG
6
IC O M P
PW M Com p
4
3
V IN
PMOS
Ref
FB
1
Power G ood
11
SC21152
Applications Information
Synchronous Buck Converter
The SC21152 is a synchronous buck regulator that employs
an adaptive on-time architecture to provide fast transient
response and very small circuit area. The SC21152 operates over an input voltage range of 2.3 to 5.5V.
Operating Modes
The SC21152 operates in one of four different modes
depending on the status of VIN, VOUT, and load.
PWM Adaptive On-time operation (AOT)
PWM Minimum Off-time operation (Min Off )
PFM (Pulse Frequency Modulation for Power Saving)
Drop-Out operation (LDO mode)
PWM AOT Mode Operation
During PWM AOT operation the controller uses adaptive
on-time control. The FB ripple signal is routed to a gain
stage and then to the internal comparator and reference
which trigger the high-side pulses. The reference includes
a compensation ramp which emulates the inductor
current to provide stable switching.
AOT operation is used when the low-side pulse-width (offtime) is greater than the minimum 60nsec.
A switching cycle in AOT mode is as follows:
•
On-time: the cycle starts with the high-side
mosfet turn-on. The pulse width is set by the
following formula where fsw is 4MHz:
7 21
•
9 287
9 ,10$; u I 6:
Off-time: after the high-side on-time is completed, the low-side mosfet turns on to begin
the off-time. The off-time pulse width is set by
the internal compensation ramp and under
static conditions results in typically 4MHz switching frequency.
Note that the SC21152 enters AOT operation if VIN is less
than VOUT/0.88.
value, typically 60nsec. This indicates that the controller
has reached maximum duty cycle for AOT operation and
longer duty cycles are needed.
In Min Off operation the SC21152 uses constant off-time
control. As in AOT operation, the FB ripple signal is routed
to a gain stage and then to the internal AOT comparator
and reference. In Min Off mode, the FB ripple triggers the
low-side turn-on. Min Off switching cycle operation is as
follows:
•
•
Off-time: The low-side mosfet is on for a fixed
Minimum Off time of 60nsec. This draws current
from the output capacitor through the inductor
via the LX pin and causes the output to fall.
On-time: after the Min Off time of 60nsec has
elapsed, the high-side mosfet turns on. This
helps recharge the output capacitor and bring
the output voltage up. The on-time pulse width
is determined by the internal compensation
ramp.
Drop-out Operation
When operating in Min Off mode, the off-time is constant
and the high-side on-time varies in order to regulate the
output. For cases where VIN is near VOUT, the high-side
will remain on continually with 100% duty cycle. The
output voltage will then be limited by the input voltage,
high-side RDSON and inductor DCR as shown in the following equation:
9287
9,1 ,/2$' u 5'621 '&5
Note that RDSON is a function of input voltage and temperature. Inductor DC resistance also increases with
temperature.
Power Save (PFM) Operation
In PFM operation, the FB ripple signal is routed to a gain
stage and compared to an internal reference. A switching cycle is as follows.
PWM Min Off Operation
The device will enter Minimum Off-time operation (Min
Off ) when VIN, VOUT, and load conditions force the offtime (time between high-side pulses) to the minimum
12
SC21152
Applications Information (continued)
•
•
•
On-time: The high-side mosfet turns on, as triggered by the FB signal falling to the internal reference. The high-side stays on for a
predetermined time based on VIN and VOUT,
then turns off.
Off-time: the low-side mosfet turns on. Inductor
current is monitored via the low-side switch.
When the current falls to zero, the low-side
mosfet turns off.
Dead-time: both mosfets are off. If the FB signal
is higher than the internal reference, both
mosfets remain off. During the dead-time the
load current is supplied by the output capacitor,
causing the output and the FB signal to fall.
When the FB signal falls to the 800mV threshold,
the next high-side pulse begins.
Power Good Indicator
The SC21152 includes a open-drain Power Good (PG)
indicator. The PG output is in a logic high (open) state
when the FB pin is above 95% of the final value (above
760mV). The PG threshold has 3% hysteresis to prevent
chattering.
During a start-up cycle, the PG output is pulled low typically 30usec before the start of mosfet switching. The PG
output stays low until the FB pin exceeds 95% of the final,
which begins the PG Assert Time delay of 75μsec. After
the Assert Time has passed, the PG output will drive high
(open circuit). If the FB signal falls to the falling edge PG
threshold, a Deassert Time delay of 13μsec must elapse
before the PG output will change to logic low.
The PG pin requires a pull-up resistor to provide a logic
high to a microcontroller. If the PG signal is not used at
the system level, the pull-up resistor can be omitted.
As load current increases, the dead-time between pulses
reduces. On any cycle, if the minimum off-time (60nsec)
has elapsed and the FB signal is calling for another highside on-time, the controller exits PFM and enters PWM
operation.
Note that the PG output is high (open) when the EN pin is
low. This prevents power loss in the PG pull-up resistor
when the device is disabled.
Once the controller is in PWM operation, the inductor
current must go below zero (go negative) for 16 switching cycles before the controller will return to PFM
operation.
The SC21152 includes an under-voltage lockout (UVLO)
to inhibit switching until the input voltage exceeds 2.16V.
The VIN UVLO threshold has 85mV hysteresis to prevent
chatter during start-up.
Soft start
The SC21152 contains an internal soft start to reduce
inrush current during start-up. During start-up, the FB
signal is compared to an internal ramp, providing a
gradual ramp on VOUT. The timing to move VOUT from
0V to 95% of final value is typically 100usec. Optional soft
start times of 200usec, 400usec, and 800usec can be
provided.
Pre-bias Start-up
During start-up, the low-side mosfet turns off if the inductor current falls negative (into the LX pin) on any cycle.
This prevents discharge of the output capacitor during
the soft-start cycle. Note that if the output is loaded
during a Pre-Bias start-up, the output voltage may fall
until the internal Soft-start ramp rises above the voltage
seen at the FB pin.
VIN Under-Voltage Lockout
EN Input
The EN input is a logic level input. When EN is set to logic
high, the SC21152 goes through an internal reset to
prepare for operation. The soft-start ramp and mosfet
switching begin typically 50μsec after EN is driven high.
When EN is low (grounded), the SC21152 is off and in its
lowest power state. The power mosfets are off and the PG
output is high (open). The automatic discharge mosfet
on LX is on to discharge the output before the next startup cycle.
Automatic Discharge
When the EN pin is pulled low (grounded), the power
mosfets are tri-stated and the LX pin is connected to GND
through an internal mosfet to discharge the output
capacitors. Typical mosfet RDSON is 40 ohms at 5V and
13
SC21152
Applications Information (continued)
25°C. Note that RDS ON is a function of both VIN and
temperature.
The automatic discharge feature can optionally be disabled from the device.
VOUT
T o F B p in
R to p
C to p
T o F B p in
R bot
Over-current Protection
The SC21152 provides overload protection through pulse
by pulse current limiting. During the high-side on-time,
the high-side switches off if the inductor current exceeds
2.2A typically. The low-side turns on briefly, followed by
the next high-side on-time. Switching continues while
the output overload exists. When the overload is removed,
the SC21152 will resume normal operation. If the overload causes high junction temperature, the SC21152 will
go into Over-temperature protection.
Over-temperature Protection
The SC21152 will shut down if the junction temperature
exceeds typically 160 °C. The device will automatically
restart after the junction temperature has dropped typically 10 °C.
Component Selection
The FB regulation setpoint is 800mV. The output voltage
can be calculated as shown.
9287
§ 5WRS ·
u ¨
¸
© 5ERW ¹
The FB resistor values can be calculated as shown. It is
recommended to use 100kΩ for Rbot to minimize power
loss in the FB network.
5WRS
9287 9 u 5ERW
9
The optimum value for Ctop is 22pF.
The SC21152 is optimized for use with components as
shown in the Detailed Applications Circuit on page 10.
Inductor: the inductor should be 0.47uH and capable of
supporting a minimum current of 1.2A.
Input capacitor: ceramic capacitor, minimum 4.7uF, X5R
or X7R or better dielectric.
Output capacitor: ceramic capacitor, minimum 10uF, X5R
or X7R or better dielectric.
Note when operating in Min Off mode, higher output
capacitance may needed to reduce output ripple when
VIN approaches VOUT.
The FB components provide output voltage sensing and
feedback compensation to regulate the output. The
output voltage is programmed using two resistor along
with a compensation capacitor.
14
SC21152
PCB Layout Guidelines
An optimum layout for the SC21152 is shown below.
When doing the PCB layout the following guidelines
should be used:
The input capacitor must be located as close as possible to
the IC and directly connected to pin 1 (VIN), pin 5 (GND),
and PAD (GND).
The LX trace from pin 6 to the inductor should be as short
as possible, and wide to reduce IR loss and improve heat
dissipation.
The output capacitor should be close to the inductor. The
feedback components (Rtop, Ctop, Rbot, for adjustable
VOUT) should be located as close as possible to the FB pin.
For fixed VOUT, the output capacitor connects directly to
the FB pin.
The FB trace should be short and not routed near noise
signals. Do not route the FB trace under the inductor; the
high impedance of the FB circuit can pick up noise from
the inductor’s magnetic field.
L1
VIN
CIN
LX
COUT
PG
SC21152
EN
GND
VOUT
GND
FB
VOUT sense trace
PCB Layout - Fixed VOUT
L1
VIN
CIN
PG
EN
SC21152
GND
COUT
LX
VOUT
GND
FB Ctop
Rbot Rtop
VOUT sense trace
PCB Layout - Adjustable VOUT
15
SC21152
Outline Drawing — 1.5 x 2mm 6-lead MLPD
16
SC21152
Land Pattern — 1.5x2mm 6-lead MLPD
17
SC21152
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Contact Information
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Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805) 498-2111 Fax: (805) 498-3804
www.semtech.com
18
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