SEMTECH SC4503TSKTRT

SC4503
1.3MHz Step-Up Switching
Regulator with 1.4A Switch
POWER MANAGEMENT
Description
Features
The SC4503 is a 1.3MHz current-mode step-up switching regulator with an integrated 1.4A power transistor.
Its high switching frequency allows the use of tiny surface-mount external passive components. The SC4503
features a combined shutdown and soft-start pin. The
optional soft-start function eliminates high input current
and output overshoot during start-up. The internal compensation network accommodates a wide range of voltage conversion ratios. The internal switch is rated at 34V
making the device suitable for high voltage applications
such as Boost and SEPIC.
‹ Low Saturation Voltage Switch: 260mV at 1.4A
‹ 1.3MHz Constant Switching Frequency
‹ Peak Current-mode Control
‹ Internal Compensation
‹ Programmable Soft-Start
‹ Input Voltage Range From 2.5V to 20V
‹ Output Voltage up to 27V
‹ Uses Small Inductors and Ceramic Capacitors
‹ Low Shutdown Current (< 1μA)
‹ Low Profile 5-Lead TSOT-23 and 8-Lead 2X2mm
MLPD-W packages
‹ Fully WEEE and RohS compliant
The SC4503 is available in low-profile 5-lead TSOT-23 and
8-lead 2X2mm MLPD-W packages. The SC4503’s low
shutdown current (< 1μA), high frequency operation and
small size make it suitable for portable applications.
Applications
‹ Local DC-DC Converters
‹ TFT Bias Supplies
‹ XDSL Power Supplies
‹ Medical Equipment
‹ Digital Cameras
‹ Portable Devices
‹ White LED Drivers
Typical
TypicalApplication
ApplicationCircuit
Circuit
D1
5V
4.7µH
5
C1
1µF
90
C4
15pF
SW
R1
432k
SC4503
OFF ON
4
SHDN/SS
FB
95
12V, 0.5A
10BQ015
1
IN
Efficiency vs Load Current
VOUT
C2
4.7µF
3
GND
2
1.3MHz
85
R2
49.9k
Efficiency (%)
L1
VIN
80
75
70
65
60
VOUT = 12V
55
50
0.001
C1: Murata GRM188R61A105K
C2: Murata GRM21BR61C475K
L1: Sumida CDC5D23B-4R7
0.100
1.000
Load Current (A)
Figure 1(b). Efficiency of the 5V to 12V Boost Converter
Figure 1(a). 5V to 12V Boost Converter
May 4, 2007
0.010
1
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SC4503
POWER MANAGEMENT
Absolute Maximum Ratings
Exceeding the specifications below 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.
Parameter
Symbol
Maximum
Supply Voltage
VIN
-0.3 to 20
SW Voltage
VSW
-0.3 to 34
FB Voltages
VFB
-0.3 to VIN +0.3
VSHDN
-0.3 to VIN +1
Thermal Resistance Junction to Ambient (TSOT - 23)
θ JA
191*
°C/W
Thermal Resistance Junction to Ambient (2X2 mm MLPD-W)
θ JA
78*
°C/W
Maximum Junction Temperature
TJ
150
Storage Temperature Range
TSTG
-65 to +150
Lead Temperature (Soldering)10 sec (TSOT - 23)
TLEAD
260
Peak IR Reflow Temperature (2X2mm MLPD-W)
TIR
260
ESD
2000
SHDN/SS Voltage
ESD Rating (Human Body Model)
Units
V
°C
V
*Calculated from package in still air, mounted to 3” x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad as per JESD51 standards.
Electrical Characteristics
Unless specified: VIN = VSHDN/SS = 3V, -40°C < TA = TJ < 85°C
Parameter
Conditions
Min
Under-Voltage Lockout Threshold
Typ
Max
2.2
2.5
Maximum Operating Voltage
20
Feedback Voltage
Feedback Line Voltage Regulation
1.225
2.5V < VIN < 20V
1.15
V
1.275
0.02
FB Pin Bias Current
Switching Frequency
1.250
Units
%/V
-25
-50
nA
1.30
1.55
MHz
Minimum Duty Cycle
0
%
Maximum Duty Cycle
86
90
Switch Current Limit
1.4
1.9
2.5
A
Switch Saturation Voltage
ISW = 1.4A
260
430
mV
Switch Leakage Current
VSW = 5V
0.01
1
µA
VSHDN/SS = 2V, VFB = 1.5V (not switching)
0.8
1.1
mA
VSHDN/SS = 0
0.01
1
µA
VIN Quiescent Supply Current
VIN Supply Current in Shutdown
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SC4503
POWER MANAGEMENT
Electrical Characteristics (Cont.)
Unless specified: VIN = VSHDN/SS = 3V, -40°C < TA = TJ < 85°C
Parameter
Conditions
SHDN/SS Switching Threshold
Min
VFB = 0V
Typ
Max
Units
1.4
Shutdown Input High Voltage
V
2
V
Shutdown Input Low Voltage
0.4
SHDN/SS Pin Bias Current
VSHDN/SS = 2V
22
50
VSHDN/SS = 1.8V
20
45
VSHDN/SS = 0V
µA
0.1
Thermal Shutdown Temperature
155
Thermal Shutdown Hysteresis
10
°C
Pin Configuration - TSOT - 23
Ordering Information
Top View
SW
1
GND
2
FB
3
5
IN
Device(1,2)
Top Mark
Package
SC4503TSKTRT
BH00
TSOT-23
SC4503EVB
4
Evaluation Board
Notes:
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Available in lead-free package only. Device is WEEE and
RoHS compliant.
SHDN/SS
5-LEAD TSOT-23
Pin Descriptions - TSOT -23
Pin
Pin Name
1
SW
2
GND
3
FB
4
5
Pin Functions
Collector of the internal power transistor. Connect to the boost inductor and the freewheeling
diode. The maximum switching voltage spike at this pin should be limited to 34V.
Ground. Tie to ground plane.
The inverting input of the error amplifier. Tie to an external resistive divider to set the output voltage.
Shutdown and Soft-start Pin. Pulling this pin below 0.4 shuts down the converter. Applying more
than 2V at this pin enables the SC4503. An external resistor and an external capacitor connected to this pin soft-start the switching regulator. The SC4503 will try to pull the SHDN/SS pin
SHDN/SS below its 1.4V switching threshold regardless of the external circuit attached to the pin if VIN
is below the under-voltage lockout threshold. Tie this pin through an optional resistor to IN or
to the output of a controlling logic gate if soft-start is not used. See Applications Information for
more details.
IN
 2007 Semtech Corp.
Power Supply Pin. Bypassed with capacitor close to the pin.
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SC4503
POWER MANAGEMENT
Pin Configuration - 2mm X 2mm MLPD
Ordering Information
Top View
Device(1,2)
SW
1
8
NC
SW
2
7
GND
IN
3
6
GND
SHDN/SS
4
5
FB
SC4503WLTRT
SC4503_MLPD EVB
Top Mark
Package
E00
2mmX2mm
MLPD-W
Evaluation Board
Notes:
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Available in lead-free package only. Device is WEEE and
RoHS compliant.
8-LEAD 2X2mm MLPD-W
Pin Descriptions - 2X2mm MLPD-W
Pin
Pin Name
1,2
SW
3
IN
Pin Functions
Collector of the internal power transistor. Connect to the boost inductor and the freewheeling diode. The maximum switching voltage spike at this pin should be limited to
34V.
Power Supply Pin. Bypassed with capacitor close to the pin.
Shutdown and Soft-start Pin. Pulling this pin below 0.4 shuts down the converter. Applying more than 2V at this pin enables the SC4503. An external resistor and an external
capacitor connected to this pin soft-start the switching regulator. The SC4503 will try
to pull the SHDN/SS pin below its 1.4V switching threshold regardless of the external
circuit attached to the pin if VIN is below the under-voltage lockout threshold. Tie this pin
through an optional resistor to IN or to the output of a controlling logic gate if soft-start is
not used. See Applications Information for more details.
4
SHDN/SS
5
FB
6,7
GND
Ground. Tie to ground plane.
8
N.C.
No Connection.
EDP
 2007 Semtech Corp.
The inverting input of the error amplifier. Tie to an external resistive divider to set the
output voltage.
Solder to the ground plane of the PCB.
4
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SC4503
POWER MANAGEMENT
Block Diagram
IN
5
SW
1
+
Z1
1V
-
REF NOT READY
Q2
SHDN/SS
4
TJ > 155°°C
VOLTAGE
THERMAL
REFERENCE
SHUTDOWN
1.25V
+
EA
-
FB
2
CLK
-
R
+
S
PWM
Q3
Q
D1
ILIM
Q1
+
I-LIMIT
R
SENSE
Σ
OSCILLATOR
+
+
SLOPE COMP
+
ISEN
2
GND
Figure 2. SC4503 Block Diagram
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SC4503
POWER MANAGEMENT
Typical Characteristics
Switching Frequency
vs Temperature
FB Voltage vs Temperature
1.30
1.5
1.4
Frequency (MHz)
FB Voltage (V)
1.25
1.20
1.15
1.2
1.1
1.10
1.0
-50
-25
0
25
50
75
100 125
-50
-25
0
25
50
75
100 125
Temperature (°C)
Temperature (°C)
VIN Under-voltage Lockout
Threshold vs Temperature
Switch Current Limit
vs Temperature
2.6
2.0
2.4
1.8
Current Limit (A)
UVLO Threshold (V)
1.3
2.2
2.0
1.6
1.4
1.8
1.2
1.6
1.0
VSHDN/SS = 3V
-50
-25
0
25
50
75
100 125
-50
-25
0
Temperature (°C)
25
50
75
100 125
Temperature (°C)
VIN Quiescent Current
vs Temperature
Switch Saturation Voltage
vs Switch Current
400
0.80
125°C
0.75
25°C
VIN Current (mA)
VCESAT (mV)
300
200
100
-40°C
0.70
0.65
VFB = 1.5V
0.60
0
0.0
0.5
1.0
1.5
-50
2.0
0
25
50
75
100 125
Temperature (°C)
Switch Current (A)
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-25
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SC4503
POWER MANAGEMENT
Typical Characteristics (Cont.)
Shutdown Pin Current
vs Shutdown Pin Voltage
Shutdown Pin Current
vs Shutdown Pin Voltage
50
-40°C
60
50
Shutdown Pin Current ( µ A)
Shutdown Pin Current ( µ A)
70
25°C
40
30
20
85°C
10
-40°C
0
20
10
85°C
5
10
15
0.0
20
0.5
1.0
1.5
2.0
2.5
Shutdown Pin Voltage (V)
Shutdown Pin Voltage (V)
VIN Quiescent Current
vs Shutdown Pin Voltage
Shutdown Pin
Thresholds vs Temperature
1000
3.0
1.5
VIN = 3V
VFB = 1.5V
600
SHDN Thresholds (V)
800
125°C
25°C
400
200
Switching
1.0
0.5
Shutting Down To IIN < 1µA
-40°C
0.0
0
0.0
0.5
1.0
1.5
-50
2.0
-25
0
25
50
75
100 125
Temperature (°C)
Shutdown Pin Voltage (V)
Switch Current Limit
vs Shutdown Pin Voltage
Switch Current Limit
vs Shutdown Pin Voltage
2.5
2.5
D = 50%
D = 80%
2.0
Current limit (A)
2.0
Current limit (A)
25°C
30
0
0
VIN Current (µ A)
40
-40°C
1.5
85°C
25°C
1.0
-40°C
1.5
1.0
25°C
85°C
0.5
0.5
0.0
0.0
1.2
1.4
1.6
1.8
1.2
2.0
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1.4
1.6
1.8
2.0
Shutdown Pin Voltage (V)
Shutdown Pin Voltage (V)
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SC4503
POWER MANAGEMENT
Applications Information
clamped by D1 and Q1, follows the voltage at the SHDN SS
pin. The input inductor current, which is in turn controlled
by the error amplifier output, also ramps up gradually.
Soft-starting the SC4503 in this manner eliminates high
input current and output overshoot. Under fault condition
(VIN < 2.2V or over-temperature), the soft-start capacitor is
discharged to 1V. When the fault condition disappears, the
converter again undergoes soft-start.
Operation
The SC4503 is a 1.3MHz peak current-mode step-up
switching regulator with an integrated 1.4A (minimum)
power transistor. Referring to the block diagram, Figure
2, the clock CLK resets the latch and blanks the power
transistor Q3 conduction. Q3 is switched on at the trailing
edge of the clock.
Switch current is sensed with an integrated sense resistor.
The sensed current is summed with the slope-compensating ramp and fed into the modulating ramp input of the
PWM comparator. The latch is set and Q3 conduction is
terminated when the modulating ramp intersects the error
amplifier (EA) output. If the switch current exceeds 1.9A (the
typical current-limit), then the current-limit comparator ILIM
will set the latch and turn off Q3. Due to separate pulsewidth modulating and current limiting paths, cycle-by-cycle
current limiting is not affected by slope compensation.
Setting the Output Voltage
An external resistive divider R1 and R2 with its center tap
tied to the FB pin (Figure 3) sets the output voltage.
9
5 = 5 §¨ 287 − ·¸
¹
© 9
(1)
VOUT
SC4503
R1
The current-mode switching regulator is a dual-loop feedback control system. In the inner current loop the EA output
controls the peak inductor current. In the outer loop, the
error amplifier regulates the output voltage. The double
reactive poles of the output LC filter are reduced to a single
real pole by the inner current loop, allowing the internal loop
compensation network to accommodate a wide range of
input and output voltages.
25nA
3
FB
R2
Figure 3. R1- R2 Divider Sets the Output Voltage
The input bias current of the error amplifier will introduce
an error of:
Applying 0.9V at the SHDN SS pin enables the voltage reference. The signal “REF NOT READY” does not go low until
VIN exceeds its under-voltage lockout threshold (typically
2.2V). Assume that an external resistor is placed between
(
)
Q$ • 5°«5 • ∆9287
=−
9287
9
the IN and the SHDN SS pins during startup. The voltage
reference is enabled when the SHDN SS voltage rises to
0.9V. Before VIN reaches 2.2V, “REF NOT READY” is high.
Q2 turns on and the Zener diode Z1 loosely regulates the
(2)
The percentage error of a VOUT = 5V converter with R1 =
100kΩ and R2 = 301kΩ is
(
SHDN SS voltage to 1V (above the reference enabling voltage). The optional external resistor limits the current drawn
during under-voltage lockout.
)
Q$ • N°«N • ∆9287
=−
= −
9287
9
This error is much less than the ratio tolerance resulting
from the use of 1% resistors in the divider string.
When VIN exceeds 2.2V, “REF NOT READY” goes low. Q2 turns
off, releasing SHDN SS. If an external capacitor is connected
from the SHDN SS pin to the ground, the SHDN SS voltage
will ramp up slowly. The error amplifier output, which is
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
where ILIM is the switch current limit.
Duty Cycle
It is worth noting that IOUTMAX is directly proportional to the
9
ratio ,1 and that switching losses are neglected in its
9287
derivation. Equation (4) therefore over-estimates the
maximum output current, however it is a useful first-order
approximation.
The duty cycle D of a boost converter in continuous-conduction mode (CCM) is:
9,1
9287 + 9'
'=
9
− &(6$7
9287 + 9'
−
(3)
Using VCESAT = 0.3V, VD = 0.5V and ILIM =1.4A in (3) and (4),
the maximum output current for three VIN and VOUT combinations are tabulated (Table 1).
where VCESAT is the switch saturation voltage and VD is voltage drop across the rectifying diode.
Maximum Output Current
In a boost switching regulator the inductor is connected
to the input. The inductor DC current is the input current.
When the power switch is turned on, the inductor current
flows into the switch. When the power switch is off, the
inductor current flows through the rectifying diode to the
output. The output current is the average diode current.
The diode current waveform is trapezoidal with pulse width
(1 – D)T (see Figure 4). The output current available from
ON
OFF
ON
Switch Current
OFF
ON
I OUT
ON
OFF
3.3
12
0.754
0.34
3.3
5
0.423
0.80
5
12
0.615
0.53
ON
0
Note: dropout can occur when operating at low input voltages (<3V) and with off times approaching 100ns. Shorten
the PCB trace between the power source and the device
input pin, as line drop may be a significant percentage of
the input voltage. A regulator in dropout may appear as
if it is in current limit. The cycle-by-cycle current limit of
the SC4503 is duty-cycle and input voltage invariant and
should be at least 1.4A. If the converter output is below
its set value and switch current limit is not reached (1.4A),
then the converter is likely in dropout.
Figure 4. Current Waveforms in a Boost Converter
a boost converter therefore depends on the converter operating duty cycle. The power switch current in the SC4503 is
internally limited to at least 1.4A. This is also the maximum
peak inductor or the peak input current. By estimating the
conduction losses in both the switch and the diode, an
expression of the maximum available output current of a
boost converter can be derived:
,2870$;
IOUT (A)
The power transistor in the SC4503 is turned off every
switching period for 80ns. This minimum off time limits the
maximum duty cycle of the regulator. A boost converter with
9
high 287 ratio requires long switch on time and high duty
9,1
cycle. If the required duty cycle is higher than the attainable maximum, then the converter will operate in dropout.
(Dropout is a condition in which the regulator cannot attain
its set output voltage below current limit.)
Diode Current
(1-D)T
D
Maximum Duty-Cycle Limitation
0
DT
VOUT (V)
Table 1. Calculated Maximum Output Currents
IIN
Inductor
Current
VIN (V)
, 9 ª
' 9' − '(9' − 9&(6$7 ) º
= /,0 ,1 « −
−
»
9287 ¬
9,1
¼
Example: Determine the highest attainable output voltage
when boosting from a single Li-ion cell.
(4)
Equation (3) can be re-arranged as:
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
9287 =
9,1 − '9&(6$7
− 9'
−'
lessen jittery tendency but not so steep that large flux swing
decreases efficiency. For continuous-conduction mode
operation, inductor ripple current ΔIL between 0.35A and
0.6A is a good compromise. Setting ΔIL = 0.43A, VD = 0.5V
and f = 1.3MHz in (7),
(5)
Assuming that the voltage of a nearly discharged Li-ion cell
is 2.6V. Using VD=0.5V, VCESAT=0.3V and D=0.86 in (5),
9287
/=
− • <
− = 9
− 9,1
I∆,/
§
·
9,1
9,1 ·
9 §
¨¨ −
¸¸ = ,1 ¨¨ −
¸
9287 + 9' ¹ ©
9287 + ¸¹
©
(8)
where L is in μH.
Transient headroom requirement further reduces the maximum achievable output voltage to below 16V.
Equation (7) shows that for a given VOUT, ΔIL is the highest
(9 + 9' ) . If V varies over a wide range, then
when 9,1 = 287
IN
choose L based on the nominal input voltage.
Minimum Controllable On-Time
The operating duty cycle of a boost converter decreases as
VIN approaches VOUT. Sensed switch current ramp modulates
the pulse width in a current-mode switching regulator. This
current ramp is absent unless the switch is turned on. The
intersection of this ramp with the error amplifier output
determines the switch on-time. The propagation delay
time required to immediately turn off the switch after it is
turned on is the minimum controllable on time. Measured
minimum on time of the SC4503 is load-dependent and
ranges from 180ns to 220ns at room temperature. The
switch in the SC4503 is either not turned on, or, for at least
this minimum. If the regulator requires a switch on-time
less than this controllable minimum, then it will either skip
cycles or start to jitter.
The saturation current of the inductor should be 20-30%
higher than the peak current limit (1.9 A). Low-cost powder
iron cores are not suitable for high-frequency switching
power supplies due to their high core losses. Inductors
with ferrite cores should be used.
Discontinuous-Conduction Mode
9287
in
9,1
continuous-conduction mode is limited by the maximum
duty cycle DMAX:
The output-to-input voltage conversion ratio 0 =
0<
Inductor Selection
The inductor ripple current ΔIL of a boost converter in continuous-conduction mode is
∆,/ =
'(9,1 − 9&(6$7 )
I/
Higher voltage conversion ratios can be achieved by operating the boost converter in full-time discontinuous-conV
duction mode (DCM). Define R = OUT as the equivalent
IOUT
output load resistance. The following inequalities must be
(6)
where f is the switching frequency and L is the inductance.
satisfied for DCM operation:
Substituting (3) into (6) and neglecting VCESAT,
∆,/ =
9,1
9,1 §
¨¨ −
9287 + 9'
I/ ©
·
¸¸
¹
/I 0 − <
5 0
(7)
(9)
and,
In current-mode control, the slope of the modulating
(sensed switch current) ramp should be steep enough to
 2007 Semtech Corp.
=
= − '0$; − ,287 =
10
9287 $
<
5
0
(10)
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
Switch on duty ratio in DCM is given by,
'=
/I
0 0 − 5
When the switch is turned on, the output capacitor supplies
the load current IOUT (Figure 4). The output ripple voltage
due to charging and discharging of the output capacitor
is therefore:
(11)
Higher input current ripples and lower output current are
the drawbacks of DCM operation.
∆9287 =
,287'7
& 287
(13)
Input Capacitor
For most applications, a 10-22µF ceramic capacitor is sufThe input current in a boost converter is the inductor cur- ficient for output filtering. It is worth noting that the output
rent, which is continuous with low RMS current ripples. A ripple voltage due to discharging of a 10µF ceramic capaci2.2-4.7µF ceramic input capacitor is adequate for most tor (13) is higher than that due to its ESR.
applications.
Rectifying Diode
Output Capacitor
For high efficiency, Schottky barrier diodes should be used
Both ceramic and low ESR tantalum capacitors can be as rectifying diodes for the SC4503. These diodes should
used as output filtering capacitors. Multi-layer ceramic have an average forward current rating at least equal to the
capacitors, due to their extremely low ESR (<5mΩ), are output current and a reverse blocking voltage of at least
the best choice. Use ceramic capacitors with stable a few volts higher than the output voltage. For switching
temperature and voltage characteristics. One may be regulators operating at low duty cycles (i.e. low output
tempted to use Z5U and Y5V ceramic capacitors for output voltage to input voltage conversion ratios), it is beneficial
filtering because of their high capacitance density and to use rectifying diodes with somewhat higher average cursmall sizes. However these types of capacitors have high rent ratings (thus lower forward voltages). This is because
temperature and high voltage coefficients. For example, the diode conduction interval is much longer than that of
the capacitance of a Z5U capacitor can drop below 60% the transistor. Converter efficiency will be improved if the
of its room temperature value at –25°C and 90°C. X5R voltage drop across the diode is lower.
ceramic capacitors, which have stable temperature and
voltage coefficients, are the preferred type.
The rectifying diodes should be placed close to the SW
pin of the SC4503 to minimize ringing due to trace inducThe diode current waveform in Figure 4 is discontinuous tance. Surface-mount equivalents of 1N5817 and 1N5818,
with high ripple-content. Unlike a buck converter in which MBRM120, MBR0520L, ZHCS400, 10BQ015 and equivathe inductor ripple current ∆IL determines the output ripple lent are suitable.
voltage. The output ripple voltage of a boost regulator is
much higher and is determined by the absolute inductor Shutdown and Soft-Start
current. Decreasing the inductor ripple current does not
reduce the output ripple voltage appreciably. The current The shutdown ( SHDN SS ) pin is a dual function pin. When
flowing in the output filter capacitor is the difference driven from a logic gate with VOH>2V, the SHDN SS pin
between the diode current and the output current. This functions as an on/off input to the SC4503. When the
shutdown pin is below 2V, it clamps the error amplifier
capacitor current has a RMS value of:
output to 96+'1 66 and reduces the switch current limit.
Connecting RSS and CSS to the SHDN SS pin (Figure 5) slows
9
,287 287 − (12)
the voltage rise at the pin during start-up. This forces the
9,1
peak inductor current (hence the input current) to follow a
If a tantalum capacitor is used, then its ripple current rating slow ramp, thus achieving soft-start.
in addition to its ESR will need to be considered.
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
The minimum SHDN SS voltage for switching is 1.4V. The
graph “Switch Current Limit vs. Shutdown Pin Voltage” in
the “Typical Characteristics” shows that the SHDN SS pin
voltage needs to be at least 2V for the SC4503 to deliver
its rated power. The effect of the SHDN SS voltage on the
SC4503 is analog between 1.4V and 2V. Within this range
the switch current limit is determined not by ILIM but instead by the PWM signal path (see Figure 2). Moreover it
varies with duty cycle and the shutdown pin voltage.
Pulling the SHDN SS pin below 0.4V shuts down the SC4503,
drawing less than 1µA from the input power supply. For
voltages above 2V and below 0.4V, the SHDN SS pin can be
regarded as a digital on/off input. Figure 5 shows several
ways of interfacing the control logic to the shutdown pin. In
Figure 5(a) soft-start is not used and the logic gate drives
the shutdown pin through a small ( ≈ 1kΩ ) optional resistor
RSS. RSS limits the current drawn by the SC4503 internal
VIN
IN
VOH > 2V
VOL < 0.4V
IN
End of Soft-start
VSHDN/SS > 2V
SC4503
SC4503
RSS
RLIM
SHDN/SS
SHDN/SS
CSS
(a)
End of Soft-start
VSHDN/SS > 2V
VOL < 0.4V
(b)
VIN
IN
1.7V < VOH < 2V
VOL ≈ 0
SC4503
RSS
ISHDN/SS
IN
RSS
SHDN/SS
CMDSH-3
CSS
SHDN/SS
ISHDN/SS
CSS
(c)
VIN
End of Soft-start
VSHDN/SS > 2V
SC4503
DSS
(d)
VIN
IN
SC4503
RSS
IN
SC4502
VOH > VIN 1N4148
SHDN/SS
SHDN/SS
RSS
CSS
CSS
(e)
(f)
Figure 5. Methods of Driving the Shutdown Pin and Soft-starting the SC4503
(a) Directly Driven from a Logic Gate. RLIM Limits the Gate Output Current during Fault,
(b) Soft-start Only,
(c) Driven from a Logic Gate with Soft-start,
(d) Driven from a Logic Gate with Soft-start (1.7V < VOH < 2V),
(e) Driven from an Open-collector NPN Transistor with Soft-start and
(f) Driven from a Logic Gate (whose VOH > VIN) with Soft-start.
 2007 Semtech Corp.
12
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
Output filter pole, ωS = −
circuit from the driving logic gate during fault condition.
In Figure 5(f) the shutdown pin is driven from a logic gate
whose VOH is higher than the supply voltage to the SC4503.
The diode clamps the maximum shutdown pin voltage to
one diode voltage above the input power supply.
Compensating zero, ω= = −
IN
OUT
POWER
STAGE
V
OUT
(14)
C4
R1
ESR
R
C2
COMP
Gm
-
FB
+
RC
RO
CC
1.252V
R2
VOLTAGE
REFERENCE
RO is the equivalent output resistance of the error amplifier
9(10,1 − µ$
Figure 6. Simplified Equivalent Model of a Boost
Converter
If the enable signal is less than 2V, then the interfacing
options shown in Figures 5(d) and 5(e) will be preferred. The
methods shown in Figures 5(a) and 5(c) can still be used
however the switch current limit will be reduced. Variations
The poles p1, p2 and the RHP zero z2 all increase phase
shift in the loop response. For stable operation, the overall loop gain should cross 0dB with -20dB/decade slope.
Due to the presence of the RHP zero, the 0dB crossover
ω
frequency should not be more than ] . The internal
compensating zero z1 provides phase boost beyond p2. In
general the converter is more stable with widely spaced
filter pole p2 and the RHP zero z2. The RHP zero moves to
low frequency when either the duty-cycle D or the output
current IOUT increases. It is beneficial to use small inductors
and larger output capacitors especially when operating at
9
high 287 ratios.
9,1
of ,6+'1 66 and switch current limit with SHDN SS pin voltage
and temperature are shown in the “Typical Characteristics”.
Shutdown pin current decreases as temperature increases.
Switch current limit at a given 96+'1 66 also decreases as
temperature rises. Lower shutdown pin current flowing
through RSS at high temperature results in higher shutdown
pin voltage. However reduction in switch current limit (at
a given 96+'1 66 ) at high temperature is the dominant
effect.
A feed-forward capacitor C4 is needed for stability. The value
of C4 can be determined empirically by observing the inductor current and the output voltage during load transient.
µV
µV
and
, C4 is
Starting with a value between
5
5
adjusted until there is no excessive ringing or overshoot in
inductor current and output voltage during load transient.
Sizing the inductor such that its ripple current is about 0.5A
also improves phase margin and transient response.
Feed-Forward Compensation
Figure 6 shows the equivalent circuit of a boost converter.
Important poles and zeros of the overall loop response
are:
Low frequency integrator pole, ωS = −
,
52& &
 2007 Semtech Corp.
5( − ')
.
/
I
V
In order for the SC4503 to achieve its rated switch current,
96+'1 66 must be greater than 2V in steady state. This
puts an upper limit on RSS for a given enable voltage VEN (=
voltage applied to RSS). The maximum specified ,6+'1 66 is
50µA with 96+'1 66 = 9 (see “Electrical Characteristics”).
The largest RSS can be found using (14):
566 <
and
5& & &
Right half plane (RHP) zero, ω= =
During soft-start, CSS is charged by the difference between
the RSS current and the shutdown pin current, ,6+'1 66 . In
steady state, the voltage drop across RSS reduces the shutdown pin voltage according to the following equation:
96+'1 66 = 9(1 − 566 ,6+'1 66
,287
=−
,
9287 & 5& 13
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SC4503
POWER MANAGEMENT
Applications Information (Cont.)
Figures 7(a)-7(c) show the effects of different values of
inductance and feed-forward capacitance on transient responses. In a battery-operated system if C4 is optimized for
the minimum VIN and the maximum load step, the converter
will be stable over the entire input voltage range.
Board Layout Considerations
In a step-up switching regulator, the output filter capacitor,
the main power switch and the rectifying diode carry pulse
currents with high di/dt. For jitter-free operation, the size of
the loop formed by these components should be minimized.
Since the power switch is integrated inside the SC4503,
grounding the output filter capacitor next to the SC4503
ground pin minimizes size of the high di/dt current loop.
The input bypass capacitors should also be placed close to
the input pins. Shortening the trace at the SW node reduces
the parasitic trace inductance. This not only reduces EMI
but also decreases switching voltage spikes.
VOUT
0.5V/div
IL1
0.5A/div
Figure 8 shows how various external components are
placed around the SC4503.
40µs/div
(a) L1 = 5.6µH and C4 = 2.2pF
The large surrounding ground plane acts as a heat sink
for the device.
VOUT
0.5V/div
VOUT
VIN
D1
IL1
0.5A/div
L1
SW
JP
R1
C4
C2
C1
U1
40µs/div
(b) L1 = 5.6µH and C4 = 3.3pF
R2
R3
FB
C3
SHDN/SS
GND
VOUT
0.5V/div
Figure 8. Suggested PCB Layout for the SC4503.
IL1
0.5A/div
40µs/div
(c) L1 = 3.3µH and C4 = 2.7pF
Figure 7. Different inductances and feed-forward capacitances affect the load transient responses of the
3.3V to 12V step-up converter in Figure 10(a).
IOUT is switched between 90mA and 280mA.
 2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Typical Application Circuits
L1
5V
D1
10µH
ZHCS400
+
24V
_
R3
54.9k
5
C1
4.7µF
1
IN
SHDN/SS
220pF
FB
R4
3
301k
GND
C3
56nF
C2
0.22µF
C4
SW
SC4503
4
D2
MM5Z24VT1
C5
22nF
2
R1
63.4
R2
63.4
L1: Murata LQH32C
C1: Murata GRM219R60J475K
Figure 9. Driving Two 6 White LED Strings from 5V. Zener diode D2 protects the converter
from over-voltage damage when both LED strings become open.
 2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Typical Application Circuits
D1
VIN
L1
3.3V
2.7µH
5
R3
15k
12V
10BQ015
1
IN
VOUT
C4
2.2pF
SW
R1
866k
SC4503
C1
2.2µF
4
SHDN/SS
C3
56nF
FB
C2
4.7µF
3
GND
R2
100k
2
L1: Coiltronics LD1
C1: Murata GRM188R61A225K
C2: Murata GRM21BR61C475K
Figure 10(a). 3.3V to 12V Boost Converter with Soft-start
Efficiency vs Load Current
95
90
1.3MHz
Efficiency (%)
85
80
75
70
65
60
VOUT = 12V
55
50
0.001
40µs/div
0.010
0.100
1.000
Upper Trace : Output Voltage, AC Coupled, 0.5V/div
Lower Trace : Input Inductor Current, 0.5A/div
Load Current (A)
Figure 10(b). Efficiency vs Load Current
 2007 Semtech Corp.
Figure 10(c). Load Transient Response of the Circuit
in Figure 10(a). IOUT is switched between
90mA and 280mA
16
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SC4503
POWER MANAGEMENT
Typical Application Circuits
C1
4.7µF
1.5µH
3.3V
ON
5
IN
R3
15k
C4
10pF
SW
C3
56nF
SHDN/SS
FB
85
C2
10µF
3
GND
R2
60.4k
2
90
R1
187k
SC4503
4
95
5V
10BQ015
1
Efficiency vs Load Current
VOUT
Efficiency (%)
OFF
< 0.4V
1-CELL
LI-ION
D1
L1
2.6 - 4.2V
VIN = 4.2V
80
75
VIN = 3.6V
65
60
VOUT = 5V
55
50
0.001
L1: TDK VLF4012AT
C1: Murata GRM188R60J475K
C2: Murata GRM21BR60J106K
VIN = 2.6V
70
1.3MHz
0.010
0.100
1.000
Load Current (A)
Figure 11(a). Single Li-ion Cell to 5V Boost Converter
Figure 11(b). Efficiency of the Li-ion Cell to 5V
Boost Converter
VIN = 2.6V
VIN = 4.2V
40µs/div
40µs/div
Upper Trace : Output Voltage, AC Coupled, 0.2V/div
Lower Trace : Inductor Current, 0.5A/div
Upper Trace : Output Voltage, AC Coupled, 0.2V/div
Lower Trace : Inductor Current, 0.5A/div
Figure 11(c). Load Transient Response. IOUT is switched
between 0.1A and 0.5A
Figure 11(d). Load Transient Response. IOUT is switched
between 0.15A and 0.9A
 2007 Semtech Corp.
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SC4503
POWER MANAGEMENT
Typical Application Circuits
C5
L1
2.6 - 4.2V
3.3µH
5
R3
8.06k
1-CELL
LI-ION
2.2µF
1
IN
10BQ015
L2
3.3µH
SW
VOUT
3.3V, 0.45A
D1
C4
15pF
R1
412k
SC4503
C1
1µF
4
SHDN/SS
C3
0.22µF
FB
C2
10µF
3
GND
R2
249k
2
L1 and L2: Coiltronics DRQ73-3R3
C1: Murata GRM188R61A105K
C2: Murata GRM21BR60J106K
C5: Murata GRM188R61A225K
Figure 12(a). Single Li-ion Cell to 3.3V SEPIC Converter.
Efficiency vs Load Current
85
80
VOUT = 3.3V
VIN = 3.6V
75
Efficiency (%)
70
65
60
55
50
45
VIN = 2.6V
40
VIN = 3.6V
35
VIN = 4.2V
30
0.001
0.010
0.100
40µs/div
1.000
Upper Trace : Output Voltage, AC Coupled, 0.2V/div
Lower Trace : Input Inductor Current, 0.2A/div
Load Current (A)
Figure 12(b). Efficiency vs Load Current
 2007 Semtech Corp.
Figure 12(c). Load Transient Response of the Circuit
in Figure 12(a). IOUT is switched between
100mA and 500mA
18
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SC4503
POWER MANAGEMENT
Typical Application Circuits
D2
D3
D4
D5
C5
0.1µF
C6
0.1µF
C7
0.1µF
D1
L1
3.3V
4.7µH
5
OFF
3.3V
ON
< 0.4V
OUT1
9V (0.3A)
C4
12pF
SW
R1
309k
SC4503
R3
RUN
4
SHDN/SS
17.8k
C1
4.7µF
FB
C2
4.7µF X 2
3
GND
C3
56nF
26V (10mA)
C8
1µF
10BQ015
1
IN
OUT2
R2
49.9k
C9
0.1µF
2
D7
D6
OUT3
-8.5V (10mA)
C10
1µF
D2 - D7 : BAT54S
L1 : Sumida CDC5D23B-4R7M
C2: Murata GRM21BR61C475K
C1: Murata GRM188R61A105K
Figure 13(a). Triple-Output TFT Power Supply with Soft-Start
CH4
CH1
CH2
CH3
40µs/div
400µs/div
CH1 : OUT1 Voltage, 5V/div
CH2 : OUT2 Voltage, 20V/div
CH3 : OUT3 Voltage, 5V/div
CH4 : RUN Voltage, 5V/div
Upper Trace : Output Voltage, AC Coupled, 0.5V/div
Lower Trace : Inductor Current, 0.5A/div
Figure 13(b). TFT Power Supply Start-up Transient as
the RUN Voltage is Stepped from 0 to
3.3V
 2007 Semtech Corp.
Figure 13(c). Load Transient Response. IOUT1
is switched between 50mA and
350mA
19
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SC4503
POWER MANAGEMENT
EVB Schematic
D1
SS13
12VOUT
L1
4.7uH
5VIN
U1
R1
0R
R2
432K
C2
N.P.
C3
10uF
8
7
6
N.C.
SW
GND
SW
GND
VIN
C1
10uF
1
2
3
C4
15pF
5
R5
49.9K
FB
R4
0R
R3
47K
4
SHDN/SS
C5
100nF
SC4503_MLPD
OFF/ON
JP1
L1
4.7uH
D1
SS13
12VOUT
R1
0R
1
R2
432K
C2
N.P.
C3
10uF
2
C4
15pF
3
R4
0R
VIN
FB
20
5VIN
5
R3
47K
GND
SHDN
U1
SC4503
R5
49.9K
 2007 Semtech Corp.
SW
4
C5
100n
C1
10uF
OFF/ON
JP1
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SC4503
POWER MANAGEMENT
Outline Drawing - TSOT-23
DIM
A
e1
A
A1
A2
b
c
D
E1
E
e
e1
L
L1
N
01
aaa
bbb
ccc
D
N
2X E/2
E1
1
E
2
ccc C
2X N/2 TIPS
e
B
D
aaa C
A2
SEATING
PLANE
DIMENSIONS
INCHES
MILLIMETERS
MIN NOM MAX MIN NOM MAX
-
.000
.028
.012
.003
.110
.060
bxN
bbb
.039
.004
.035
.020
.008
.118
.067
.114
.063
.110 BSC
.037 BSC
.075 BSC
.012 .018 .024
(.024)
5
0°
8°
.004
.008
.010
-
0.00
0.70
0.30
0.08
2.80
1.50
-
1.00
0.10
0.90
0.50
0.20
3.00
1.70
2.90
1.60
2.80 BSC
0.95 BSC
1.90 BSC
0.30 0.45 0.60
(0.60)
5
0°
8°
0.10
0.20
0.25
A
H
A1
C
-
C A-B D
c
GAGE
PLANE
0.25
L
01
(L1)
DETAIL
SEE DETAIL
A
A
SIDE VIEW
NOTES:
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS
OR GATE BURRS.
4. REFERENCE JEDEC STD MO-193, VARIATION AB.
Land Pattern - TSOT-23
X
DIM
(C)
G
Z
Y
P
C
G
P
X
Y
Z
DIMENSIONS
INCHES
MILLIMETERS
(.087)
.031
.037
.024
.055
.141
(2.20)
0.80
0.95
0.60
1.40
3.60
NOTES:
1.
 2007 Semtech Corp.
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
21
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SC4503
POWER MANAGEMENT
Outline Drawing - 8 Lead 2X2mm MLPD-W
A
B
D
DIM
E
PIN 1
INDICATOR
(LASER MARK)
A
aaa C
A2
A1
SEATING
PLANE
C
A
A1
A2
b
D
D1
E
E1
e
L
N
aaa
bbb
DIMENSIONS
INCHES
MILLIMETERS
MIN NOM MAX MIN NOM MAX
.028 .030 .031
.000 .001 .002
(.008)
.007 .010 .012
.075 .079 .083
.059 .063 .067
.075 .079 .083
.031 .035 .039
.020 BSC
.008 .012 .016
8
.003
.003
0.70 0.75 0.80
0.00 0.02 0.05
(0.20)
0.18 0.25 0.30
1.90 2.00 2.10
1.50 1.60 1.70
1.90 2.00 2.10
0.80 0.90 1.00
0.50 BSC
0.20 0.30 0.40
8
0.08
0.08
D1
1
E/2
2
LxN
E1
N
bxN
bbb
e
C A B
e/2
D/2
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
Land Pattern - 8 Lead 2X2mm MLPD-W
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
 2007 Semtech Corp.
22
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