SEMTECH SC2618SKTRT

SC2618
Miniature PWM Controller
for Buck Regulator
POWER MANAGEMENT
Description
Features
The SC2618 is a hysteretic mode PWM controller designed for high efficiency, low cost “Point of Load” applications.
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The hysteretic control scheme allows the use of ceramic
output capacitors without the need for compensation
components and difficult calculations.
An internal soft start prevents output voltage from start
up overshoot. A “Hiccup” short circuit protection provides
protection against output short circuits as well as adverse input supply sequencing.
Operating Frequency up to 500kHz
Input supply from 5V to 12V
0.5A gate drive capability
Internal soft start
Internal, trimmed bandgap reference (±1%)
Hiccup mode short circuit protection
Input supply sequence protection
6 lead SOT23 package
Applications
‹ Graphics IC Power supplies
‹ Embedded, low cost, high efficiency converters
Typical Application Circuit
VIN
BOOST
Q1
VCC
VO
5
3
2
BST
DH
VCC
DL
GND
FB
6
Q2
1
4
SC2618
GND
Revision: September 19, 2005
GND
1
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SC2618
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 implied. Exposure to Absolute Maximum rated conditions for extended periods of time may affect device
reliability.
Parameter
Symbol
Maximum
Units
Input Supply Voltage
VCC
15
V
Boost Supply Voltage
VBST
30
V
FB Voltage
VFB
8
V
DL to GND
VDL
-1 to VCC +0.3
V
DH to GND
VDH
-1 to VBST +0.3
V
Operating Ambient Temperature Range
TA
-40 to +85
°C
Operating Junction Temperature Range
TJ
-40 to 125
°C
Storage Temperature Range
TSTG
-65 to 150
°C
Lead Temperature (Soldering) 10s
TLEAD
300
°C
Thermal Resistance Junction to Ambient
θJA
96
Thermal Resistance Junction to Case
θJC
62
°C/W
ESD
2
kV
ESD Rating (Human Body Model)
(1)
°C/W
Note: (1) 1 square inch of FR4, double sided, 1oz, minimum copper weight
Electrical Characteristics
Unless specified: VCC = VIN = 5V, VBST = 12V, VFB = VO; TA = 25°C. Values in bold are over full operating temperature range.
Parameter
Symbol
VC C Supply Voltage
VCC
VC C Qui escent C urrent
IQVCC
VC C Under Voltage Lockout
C onditions
VBST
Boost Qui escent C urrent
IQBST
Soft Start Ti me
Tss
VCC = 5.0V, VREF = 0V, VFB = 100mV
4.0
VFB = 1.200V
1.237
Output Voltage
VO
IO = 20mA; VFB = VO
Max
U nits
14
V
5
10
mA
4.2
4.5
V
29
V
3
mA
100
us
1.250
1.225
IO = 0.2A to 4A
Load Regulati on
Typ
4.75
UVVCC
Boost Supply Voltage
Min
Li ne Regulati on
1.263
1.275
V
1
%
±0.5
%
Short C i rcui t Tri p Voltage
VSCT
1.05
V
Mi ni mum ON/OFF Ti me
tDEL
1
us
FB bi as current
IFB
VFB = 1.25V
-1
uA
Peak D H Si nk/Source C urrent
BST - D H = 4.5V,
D H - GND = 3.5V
D H - GND = 1.5V
0.5
50
A
mA
Peak D L Si nk/Source C urrent
BST - D L = 4.5V,
D L - GND = 3.5V
D L - GND = 1.5V
0.5
50
A
mA
D H, D L Nonoverlappi ngTi me
C LOAD =1000pF; Measured at D H/D L = 2V
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20
50
ns
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SC2618
POWER MANAGEMENT
Pin Configuration
Ordering Information
Part Numbers
P ackag e
SC2618SKTRT(1)(2)
SOT23-6
TOP VIEW
DL
1 6
DH
Notes:
GND
2 5
BST
VCC
3 4
FB
(1) Only available in tape and reel packaging. A reel contains
3000 devices.
(2) SC2618SKTRT is the lead free version. This product is fully
WEEE and RoHS compliant.
(SOT-23 6L)
Pin Descriptions
Pin #
Pin Name
Pin Function
1
DL
2
GND
Analog and Power Ground, connect directly to ground plane, see layout guidelines.
3
VC C
Chip Supply Input Voltage and Low Side FET drive supply.
4
FB
5
BST
Supply voltage for high gate driver.
6
DH
High Side FET drive output.
Low side FET drive output.
Feedback input.
Block Diagram
BST
VCC
UVLO
SSOVER
LEVEL SHIFT AND
HIGH SIDE DRIVE
RESTART
LATCH
SSRESET
Q
S
Q
R
DH
FAULT
LATCH
SSBEGIN
S
Q
R
Q
SHOOT-THRU
CONTROL
SOFT START
VREF
VCC
200mV
DRIVE
LOGIC
FB
SYNCHRONOUS
MOSFET DRIVE
DL
GND
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SC2618
POWER MANAGEMENT
Theory of Operation
The SC2618 is a hysteretic mode PWM controller. It uses
a comparator to generate PWM wave with minimum on
time and off time control. As shown in the block diagram,
the output voltage is fed back to the comparator negative input and compared to a setting voltage. If the output voltage is below its set point, the top gate drive will
turn on and remain on until the minimum on time has
expired AND the output voltage has risen above the set
point. Similarly, if the output voltage is above its set point,
top gate drive will be turned off and bottom gate will turn
on and stay on until the minimum off time has expired
AND the output is below its set point. Because of this
minimum time control scheme, the comparator hysteresis is not required and is internally set to zero.
soft turn on. If, however, one or more supply voltage rails
are late or absent, the output voltage will not rise and
the part will behave as if there was a short circuit at the
output, that is, it will go into hiccup mode. It will remain in
this mode until all the necessary voltage rails are present,
at which time normal operation will start.
If the supply voltage Vcc falls below UVLO threshold during normal operation, the soft start capacitor begins to
discharge. When the voltage reaches the setting level,
the PWM controller control the switching regulator output to ramp down slowly for a soft turn off.
Hiccup Mode Shor
cuit Pr
o t ection
Shortt Cir
Circuit
Pro
Short circuit protection is implemented by comparing the
feedback node with the soft start and reference voltage. If the FB voltage falls more than about 200mV below it’s correct voltage the short circuit latch is set which
immediately disables the top drive. If the short circuit
occurred during soft start, the soft start capacitor continues to charge to it’s full voltage before discharging
towards zero. If the short occurred during steady state
operation, the internal soft start capacitor begins to discharge immediately, at the same rate as it charged, towards zero. Once the soft start capacitor reaches its
minimum voltage, the latches are cleared and a normal
soft start is attempted, if the short circuit condition has
not been cleared, the fault latch will again be set once
the soft start voltage reaches about 200mV and the cycle
will repeat until the short is cleared.
Switching FFreq
req
uency vs. Duty Cy
cle
requency
Cycle
This control scheme will force a buck converter to operate either at minimum on time, or minimum off time, or
both. The SC2618 has minimum time of 1us. Its switching frequency, peaking at 500kHz, can be found in Fig. 1
as long as the voltage ratio of the buck converter has
been decided.
500
F (kHz)
400
300
T
200
=
on
1u
s
To
f
f=
1u
s
It is also possible, with large output capacitor values,
that the output voltage will be unable to rise fast enough
to accurately follow the soft start voltage and a short
circuit trip may result. During soft start the bottom gate
drive is disabled to prevent discharging the output capacitor and so the output capacitor will retain the voltage achieved before the trip and will charge to a higher
voltage during the next hiccup cycle. In this way the output voltage will achieve the set point value but may take
several hiccup cycles to do so.
100
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Duty Cycle
Fig. 1. Switching frequency vs. duty cycle
UVL
O, Star
wn
UVLO,
Startt up and Shut do
down
To initiate the SC2618, supply voltages are applied to
Vcc and BST pins. The top gate (DH) and bottom gate
(DL) are held low until Vcc exceed UVLO threshold, typically 4.5V. At that point, the internal soft start capacitor
begins to charge, the top gate remains low, and the bottom gate is pulled high to turn on the bottom FET. When
the voltage at soft start cap reaches a setting level, the
top and bottom gates of PWM controller begin to switch.
The switching regulator output is slowly ramping up for a
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SC2618
POWER MANAGEMENT
Component Selection
General design guideline of switching power supplies can
be applied to the component selection for SC2618.
Induct
or and MOSFET
Inductor
MOSFETss
The selection of inductor and MOSFETs should meet thermal requirement because they are power loss dominant
components. Pick an inductor with as high inductance
as possible if without adding extra cost and size. Higher
inductance, lower ripple current, smaller core loss and
higher efficiency. However, too high inductance slows
down output transient response. It is recommended to
choose the inductance that gives the inductor ripple current to be approximate 20% of maximum load current.
So choose inductor value from:
Output Filter Components
The purpose of soft start is to control inductor current
during start up, this in turn controls the amount of excess charge the output capacitor will receive and therefore the output voltage overshoot that will occur at the
end of soft start.
VIN
Q1
L=
L
VO
The MOSFETs are selected from their Rdson, gate charge,
and package. The SC2618 provides 0.5A gate drive current. To drive a 25nC gate charge MOSFET gives 25nC/
0.5A=50ns switching time. The switching time ts contributes to the top MOSFET switching loss:
Q2
Co
GND
V
5
⋅ VO ⋅ (1 − O )
I O ⋅ f osc
VIN
GND
2618-02
Fig. 2. Output filter components
PS = I O ⋅VIN ⋅ t S ⋅ f OSC
VO
VP
There is no switching loss for the bottom MOSFET because of its zero voltage switching. The conduction losses
of the top and bottom MOSFETs are given by:
VREF
vC
0
PC _ TOP = I O2 ⋅ Rdson ⋅ D
IL
PC _ BOT = I O2 ⋅ Rdson ⋅ (1 − D )
iL
If the requirement of total power losses for each MOSFET
is given, the above equations can be used to calculate
the values of Rdson and gate charge can be calculated
using above equations, then the devices can be determined accordingly. The solution should ensure the
MOSFET is within its maximum junction temperature at
highest ambient temperature.
0
t
tSS
tP
Fig. 3. Soft start, inductor current and overshoot
Normally, controllers with an external soft start capacitor, have soft start set so long that overshoot is undetectable. Since the soft start for the SC2618 is internal,
it is necessarily limited and the maximum values of L and
Co should meet the constraints of:
L ⋅ CO =
2
t SS
⋅ XS
50
where tss is soft start time, 100us typically, and Xs is the
percentage overshoot allowable.
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Output Capacitor(s)
The output capacitors should be selected to meet both
output ripple and transient response criteria. The output
capacitor ESR causes output ripple VRIPPLE during the
inductor ripple current flowing in. To meet output ripple
criteria, the ESR value should be:
RESR <
5
L ⋅ f OSC ⋅ VRIPPLE
V
VO ⋅ (1 − O )
VIN
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SC2618
POWER MANAGEMENT
Component Selection (Cont.)
The output capacitor ESR also causes output voltage transient VT during a transient load current IT flowing in. To
meet output transient criteria, the ESR value should be:
RESR <
Divider Ratio
The top resistor of the voltage divider can be chosen
from 5k to 15k. Then the bottom resistor is found from
VT
IT
Rbot =
To meet both criteria, the smaller one of above two ESRs
is required.
1.25V
⋅ Rtop
VO −1.25V
where 1.25V is the internal reference voltage of the
SC2618.
The output capacitor value also contributes to load transient response. Based on a worst case where the inductor energy 100% dumps to the output capacitor during
the load transient, the capacitance then can be calculated by:
I T2
C > L⋅ 2
VT
Input Capacitor
The input capacitor should be chosen to handle the RMS
ripple current of a synchronous buck converter. This value
is given by:
2
I RMS = (1 − D ) ⋅ I IN
+ D ⋅ ( I o − I IN )2
where Io is the load current, IIN is the input average current, and D is the duty cycle. Choosing low ESR input
capacitors will help maximize ripple rating for a given size.
Bootstrap Circuit
The SC2618 uses an external bootstrap circuit to provide a voltage at BST pin for the top MOSFET drive. This
voltage is held up by a bootstrap capacitor. Typically, it is
recommended to use a 1uF ceramic capacitor with 25V
rating and a commonly available diode IN4148 for the
bootstrap circuit.
Filter for Vcc Supply Power
For the pin of Vcc, it is recommended to use a 1uF/25V
ceramic capacitor for decoupling. In addition, place a
small resistor (10 ohm) in between Vcc pin and the supply power for noise reduction.
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SC2618
POWER MANAGEMENT
Layout Guidelines
Careful attention to layout requirements are necessary
for successful implementation of the SC2618 controller.
High currents switching at up to 500kHz are present in
the application and their effect on ground plane voltage
differentials must be understood and minimized.
grounds for the rest of the system and c) minimize source
ringing, resulting in more reliable gate switching signals.
Cin should consist of the bulk input capacitors and must
be supplemented with one or more ceramic decoupling
capacitors as close as possible to the FETs
1). The high power parts of the circuit should be laid out
first. A ground plane should be used, the number and
position of ground plane interruptions should be such as
to not unnecessarily compromise ground plane integrity.
Isolated or semi-isolated areas of the ground plane may
be deliberately introduced to constrain ground currents
to particular areas, for example the input capacitor and
bottom FET ground.
3). The connection between the junction of Q1, Q2 and
the output inductor should be a wide trace or copper
region. It should be as short as practical. Since this connection has fast voltage transitions, keeping this connection short will minimize EMI. The connection between
the output inductor and the output capacitors should be
a wide trace or copper area, there are no fast voltage or
current transitions in this connection and length is not
so important, however adding unnecessary impedance
will reduce efficiency.
2). The loop formed by the Input Capacitor(s) (Cin), the
Top FET (Q1) and the Bottom FET (Q2) must be kept as
small as possible. This loop contains all the high current,
fast transition switching. Connections should be as wide
and as short as possible to minimize loop inductance.
Minimizing this loop area will a) reduce EMI, b) lower
ground injection currents, resulting in electrically “cleaner”
VCC IN
VPWR IN
VBST IN
10uF
10
U1
5
3
2
Q1
BST
DH
VCC
DL
GND
FB
6
1
Vout
Cin
4
L
Q2
Cout
SC2618
0.1uF
GND
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SC2618
POWER MANAGEMENT
Layout Guidelines (Cont.)
Currents in Power Section
Vin
+
Vout
+
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SC2618
POWER MANAGEMENT
Outline Drawing - SOT23-6
DIM
A
e1
2X E/2
A
A1
A2
b
c
D
E1
E
e
e1
L
L1
N
01
aaa
bbb
ccc
D
N
EI
1
E
2
ccc C
2X N/2 TIPS
e
B
D
aaa C
A2
SEATING
PLANE
bxN
bbb
.057
.035
.000
.006
.035 .045 .051
.020
.010
.003
.009
.110 .114 .118
.060 .063 .069
.110 BSC
.037 BSC
.075 BSC
.012 .018 .024
(.024)
6
0°
10°
.004
.008
.008
1.45
0.90
0.00
0.15
.90 1.15 1.30
0.25
0.50
0.08
0.22
2.80 2.90 3.00
1.50 1.60 1.75
2.80 BSC
0.95 BSC
1.90 BSC
0.30 0.45 0.60
(0.60)
6
0°
10°
0.10
0.20
0.20
A
H
A1
C
DIMENSIONS
INCHES
MILLIMETERS
MIN NOM MAX MIN NOM MAX
H
c
GAGE
PLANE
C A-B D
0.25
L
01
(L1)
SEE DETAIL
A
DETAIL
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.
Minimum Land Pattern - SOT23-6
DIMENSIONS
INCHES
MILLIMETERS
X
DIM
(C)
G
Z
Y
P
(.098)
.055
.037
.024
.043
.141
C
G
P
X
Y
Z
(2.50)
1.40
0.95
0.60
1.10
3.60
NOTES:
1.
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805)498-2111 FAX (805)498-3804
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