SC4508A
Buck or Buck-Boost (Inverting)
Current Mode Controller
POWER MANAGEMENT
Description
Features
The SC4508A is a low voltage current mode switching
regulator controller that drives a P-channel power
MOSFET with programmable switching frequency. It can
be configured in either buck or buck boost (inverting) converters. The converters can be operated from 2.7V to
15V input voltage range. The typical operating supply
current is 3mA and a shutdown pin allows the user to
turn the controller off reducing it to less than 200µA.
The output voltage can adjusted by external resistor divider. The switching frequency is programmable up to
1.5MHz, allowing small inductor and capacitor values to
minimize PCB space. The operating current level is programmable via an external sense resistor. Accessible reference voltage allows users to make output voltage as
low as they want.
Wide input voltage range 2.7V to 15V
Programmable output voltage
Programmable switching frequency up to 1.5MHz
Buck or buck boost (inverting) configuration
Current mode control with slope compensation
Very low quiescent current in shutdown mode
Accessible reference voltage
Hiccup mode after 32 cycle-by-cycle OCP
4mm x 4mm MLPQ-12 lead free package. This
product is fully WEEE and RoHS compliant
Applications
Low power point of use converters
Single or multiple output low power converters
Positive and/or negative output voltage
DSL cards
Graphic cards
I/O cards
Negative bias supplies
Typical Application Circuits
Vin
Vin
V+
V+
VREF
VREF
FB+
SC4508A
FB-
CS
FB+
OUT
COMP
GND
Vout
OSC
SS/EN
Revision: July 25, 2007
CS
OUT
COMP
Vout
SS/EN
SC4508A
FB-
1
GND
OSC
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SC4508A
POWER MANAGEMENT
Absolute Maximum Rating
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.
Parameter
Symbol
Maximum
Units
-0.3 to 16
V
3.2
V
FB+, FB-, COMP, OSC to GND
5
V
VREF Current
1
mA
VDD to GND
SS/EN to GND(1)
Thermal Resistance, Junction to Ambient
θJ A
48
°C/W
Thermal Resistance, Junction to Case
θJ C
3
°C/W
Storage Temperature Range
TSTG
-60 to +150
°C
Junction Temperature Range
TJ
-40 to +150
°C
TPKG
260
°C
Peak IR Reflow Temperature 10 - 40s
Note: (1) Voltage from internal circuitry could be higher than 3.2V. See Application Information, Soft-Start section.
Electrical Characteristics
Unless specified: VDD = 12V, VFB+ = VREF, VFB- = 0, OUT = open, CVDD = 1uF, CSS/EN = 0.1uF, COSC = 330pF. TA = TJ = -40°C to 125°C
Parameter
Test Conditions
Min
Typ
Max
Unit
15
V
500
µA
Pow er Supply
Input Voltage Range
2.7
Quiescent Current
SS/EN = low
200
Operating Current
SS/EN = high, No load
3
mA
Undervoltage Lockout
Start Threshold
VDD rising
2.35
UVLO Hysteresis
2.5
2.55
100
V
mV
Oscillator
Frequency Range
100
Frequency
450
Charge Current
500
1500
KHz
550
KHz
100
µA
Error Amplifier
Feedback Input Voltage
-0.2
Offset Voltage
0.7
2
V
mV
Input Bias Current
100
Transconductance
5
mS
100
µA
Output Source or Sink Current
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50
2
300
nA
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SC4508A
POWER MANAGEMENT
Electrical Characteristics (Cont.)
Unless specified: VDD = 12V, VFB+ = VREF, VFB- = 0, OUT = open, CVDD = 1uF, CSS/EN = 0.1uF, COSC = 330pF. TA = TJ = -40°C to 125°C
Parameter
Test Conditions
Min
Typ
Max
Unit
PWM Comparator
Maximum Duty Cycle
C = 1.5nF, 100kHz
97
%
C = 100pF, 1.5MHz
95
%
Minimum On Time
200
nS
Slope Compensation
63
mV/Ts
Delay to Output
50
ns
VREF Reference
Output Voltage
Output Voltage
TA = 25°C, VDD = 5V
0.4925
0.5
0.5075
V
0.496
0.5
0.504
V
1
mA
Output Current
Line Regulation
VDD = 2.7 to 15V, IVREF = 1mA
5
10
mV
Load Regulation
VDD = 5V, IVREF = 0 to 1mA
2
4
mV
VSS/EN > 0.9V
20
VSS/EN < 0.9V
10
Soft Start/Enable/Shutdow n
Charge Current
Discharge Current
µA
12
Enable Logic Voltage
mA
2
V
SHDN Logic Voltage
0.35
V
130
mV
Current Limit
Cycle by Cycle Threshold
V D D = 5V
90
110
Consecutive Overcurrent
Clock Cycles before Auto
Restart Shutdown
32
Delay to Output
50
nS
Gate Drive On-Resistance(H)
8
Ohm
Gate Drive On-Resistance(L)
8
Ohm
Output
Gate Drive On-Resistance(H)
V D D = 5V
15
Ohm
Gate Drive On-Resistance(L)
V D D = 5V
15
Ohm
Rise Time
COUT = 1000pF
20
nS
Fall Time
COUT = 1000pF
20
nS
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SC4508A
POWER MANAGEMENT
Pin Configuration
Ordering Information
Part Number
TOP VIEW
VDD
OSC
COMP
12
11
10
OUT
1
9
FB+
CS
2
8
FB-
SS/EN
3
7
PGND
4
5
6
VREF
NC
AGND
P ackag e
(1)
SC4508AMLTRT(2)
MLPQ -12
SC4508ABUCKEVB
Evaluation Board
SC4508ABUCK-BOOSTEVB
Evaluation Board
Notes:
(1) Only available in tape and reel packaging. A reel
contains 3000 devices.
(2) Lead free product. This product is fully WEEE and
RoHS compliant.
(MLPQ - 12, 4mm x 4mm)
Pin Descriptions
Pin #
Pin Name
1
OUT
2
CS
3
SS/EN
Soft start pin. Connects an external capacitor between this pin and AGND. The ramp up
time is defined by the capacitor. The device goes into shutdown when VSS/EN is pulled
below 0.25V.
4
VREF
0.5V reference output. VREF can source up to 1mA. Bypass with a 0.1uF ceramic
capacitor from VREF to AGND.
5
NC
No connection.
6
AGND
Analog ground.
7
PGND
Power ground.
8
FB -
Error amplifier inverting input.
9
FB +
Error amplifier non-inverting input.
10
COMP
Compensation pin for internal transconductance error amplifier. Connect loop compensation
network from COMP to AGND.
11
OSC
Oscillator frequency set input. Connect a ceramic capacitor from OSC to AGND to set the
internal oscillator frequency from 100KHz to 1.5MHz. Use equation
to set the
100µA
f=
oscillator frequency. C is the capacitor from OSC to AGND.
C • 0.65
12
VD D
Supply voltage. Bypass a 1uF ceramic capacitor from VDD to PGND.
THERMAL
PAD
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Pin Function
Gate driver output for external P-MOSFET. OUT swings from VDD to PGND.
Current sense input pin. Connect a current sense resistor between VDD and CS.
Pad for heatsinking purposes. Connect to ground plane using multiple vias. Not connected
internally.
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SC4508A
POWER MANAGEMENT
Block Diagram
Marking Information
Top View
yyww = Date Code (Example: 0012)
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SC4508A
POWER MANAGEMENT
Application Information
operating up to 1.5 MHz. It is necessary to consider the
operating duty-ratio before deciding the switching
frequency.
The SC4508A is designed to control buck (step down) or
buck-boost (inverting) converter with P-channel MOSFET
as a switch using current mode, programmable switching
frequency architecture. During steady state operation,
the switch is turned on each cycle and turned off when
the voltage across current sense resistor exceeds the
voltage level at COMP pin set by voltage loop error
amplifier. A fixed 0.5V artificial ramp is added internally
to the amplified current signal for operations when dutycycle is larger than 50%. In over load or output shortage
condition, if the sensed current signal reaches typical
100mV, the switch is turned off immediately in the same
cycle. If the sensed current signal continues up to 32
cycles, not only the switch is turned off but also the soft
start capacitor is discharged by a internal MOSFET to
ground then charging back to threshold 1.4V during which
the switch is held off. With the “hiccup” mode over
current protection, the thermal stress is reduced in the
faulty conditions.
Minimum Switch On Time Consideration
In the SC4508A the falling edge of the clock turns on
the MOSFET. The inductor current and the sensed voltage
ramp up. After the sensed voltage crosses a threshold
determined by the error amplifier output, the MOSFET is
turned off. The propagation delay time from the turn-on
of the controlling MOSFET to its turn-off is the minimum
switch on time. The SC4508A has a minimum on time of
about 180ns at room temperature. This is the shortest
on interval of the controlling PFET. The controller either
does not turn on the MOSFET at all or turns it on for at
least 80ns.
For a buck converter, the operating duty cycle is VO/VIN.
So the required on time for the MOSFET is VO/(VINfs). If
the frequency is set such that the required pulse width is
less than 180ns, then the converter will start skipping
cycles. Due to minimum on time limitation, simultaneously
operating at very high switching frequency and very short
duty cycle is not practical. If the voltage conversion ratio
VO/VIN and hence the required duty cycle is higher, the
switching frequency can be increased to reduce the sizes
of passive components.
There will not be enough modulation headroom if the on
time is simply made equal to the minimum on time of the
SC4508A. For ease of control, we recommend the
required pulse width to be at least 1.5 times the minimum
on time.
Frequency Setting
The switching frequency in the SC4508A is userprogrammable. The advantages of using constant
frequency operation are simple passive component
selection and ease of feedback compensation. Before
setting the operating frequency, the following trade-offs
should be considered.
1)
2)
3)
4)
5)
Passive component size
Circuitry efficiency
EMI condition
Minimum switch on time and
Maximum duty ratio
Current Sense and Current Limit
The SC4508A senses peak inductor current by a current
sense resistor. The sensed voltage is referenced to VDD
and the typical current limit threshold is 100mV. The
current sense resistor can be calculated by the following
equation assuming the current limit is 20% above peak
incuctor current:
For a given output power, the sizes of the passive
components are inversely proportional to the switching
frequency, whereas MOSFET’s/Diodes switching losses
are proportional to the operating frequency. Other issues
such as heat dissipation, packaging and the cost issues
are also to be considered. The frequency bands for signal
transmission should be avoided because of EM
interference.
100mV
Rs = 120% • I (pk )
L
The free-running frequency of the internal oscillator can
be programmed with an external capacitor from the OSC
pin to the ground. The SC4508A controller is capable of
2007 Semtech Corp.
1V −V
V +V
IN
O
O
D
IL (pk) = IO + 2 f • L ( V + V ) for Buck
s
IN
D
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SC4508A
POWER MANAGEMENT
Application Information (Cont.)
IL (pk ) = IO
VIN + VO + VD
VIN
+
undergoes soft-start. If overload persists, the SC4508A
will undergo repetitive auto-shutdown-restart hiccup
mode.
VO + VD
1 VIN
(
)
2 fs • L VIN + VO + VD
In normal operation, the VREF voltage follows the SS/
EN voltage from 0 to 0.5V as the SS/EN voltage ramps
up from 1.4V to 1.9V. After the SS/EN voltage rises
above 1.9V, it settles at final value depending upon VDD.
If VDD higher than 7V, it is clamped around 6.8V. In the
worst case, the clamped voltage is about 10V. Therefore,
the external soft-start capacitor should be rated at least
16V.
for Buck − Boost
IO - full load current
VO - output voltage
VIN - input voltage
VD - diode forward voltage drop
fS - switching frequency
L - inductor
The SS/EN pin can also be used as the enable input. The
MOSFET will be turned off if the SS/EN pin is pulled below
0.5V.
Error Amplifier
The error amplifier in the SC4508A is a transconductance
error amplifier, which is easily configured as a type two
compensator by connecting compensation network from
the COMP pin to AGND. The output voltage of the error
amplifier is compared to the sensed current signal
(amplified by gain 8) plus internal 500mV ramp to generate
duty cycle.
Converter Specifications
Buck or buck-boost converter design includes the
following specifications:
Input voltage range: Vin ∈ [ Vin,min , Vin,max ]
Input voltage ripple (peak-to-peak): DVin
Output voltage: Vo
Output voltage accuracy: e
Output voltage ripple (peak-to-peak): DVo
Nominal output (load) current: Io
Maximum output current limit: Io,max
Output (load) current transient slew rate: dIo (A/s)
Circuit efficiency: h
Both the non-inverting and the inverting inputs of the
error amplifier are brought out as device pins so that a
converter can be configured as either buck or buck-boost
converter.
Soft-Start and Overload Protection
The undervoltage lockout circuit discharges the SS/EN
capacitors. After V DD rises above 2.5V, the SS/EN
capacitors are slowly charged by internal 10uA current
source. As the SS/EN capacitor continues to be charged,
the VREF and the COMP voltage follows. The converter
gradually delivers increasing power to the output. The
inductor current follows the COMP voltage envelope until
the output goes into regulation.
Selection criteria and design procedures for the
following are described.
1) output inductor (L) type and value
2) output capacitor (Co) type and value
3) input capacitor (Cin) type and value
4) power MOSFETs
5) current sensing and limiting circuit
6) voltage sensing circuit
7) loop compensation network
After the SS/EN capacitor is charged above 1.4V (high
enough for the error amplifier to provide full load current),
the overload detection circuit is activated. If the CS pin
senses 32 consective switching cycles of over current,
the SC4508A will shut down and hold off the MOSFET
while discharging the soft-start capacitor. The SS/EN
capacitor is discharged with an internal 12mA current
sink. The overload latch is reset when the SS/EN capacitor
is discharged below 0.5V. The SS/EN capacitor is then
recharged with the 10µA current source and the converter
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Inductor (L) and Ripple Current
The output inductor selection/design is based on the
output DC and transient requirements. Both output
current and voltage ripples are reduced with larger
inductors but it takes longer to change the inductor
current during load transients. Conversely smaller
inductors results in lower DC copper losses but the AC
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SC4508A
POWER MANAGEMENT
Application Information (Cont.)
core losses (flux swing) and the winding AC resistance
losses are higher. A compromise is to choose the
inductance such that peak-to-peak inductor ripplecurrent is 20% to 30% of the rated output load current
or the inductor DC current. Assuming that the inductor
current ripple (peak-to-peak) value is ∆IL = δ *Idc, the
inductance value will then be:
L=
Co
Lesl
Resr
VIN − VO VO + VD
(
) for Buck
fs • ∆IL VIN + VD
Figure 1. An equivalent circuit of Co
If the current through the branch is ib(t), the voltage
across the terminals will then be:
VO + VD
VIN
L=
(
) for Buck-Boost
fs • ∆IL VIN + VO + VD
t
v o ( t ) = Vo +
The peak current in the inductor becomes (1+δ/2)*Idc
and the RMS current is:
IL,rms = Idc 1 +
∫
This basic equation illustrates the effect of ESR, ESL
and Co on the output voltage.
δ2
12
The first term is the DC voltage across Co at time t=0.
The second term is the voltage variation caused by the
charge balance between the load and the converter
output. The third term is voltage ripple due to ESL and
the fourth term is the voltage ripple due to ESR. The
total output voltage ripple is then a vector sum of the
last three terms.
The Idc is the inductor average or DC current.
The following are to be considered when choosing
inductors:
a) Inductor core material: For high efficiency applications
above 350KHz, ferrite, Kool-Mu and polypermalloy
materials should be used. Low-cost powdered iron cores
can be used for cost sensitive-applications below 350KHz
but with attendant higher core losses.
b) Select inductance value: Sometimes the calculated
inductance value is not available off-the-shelf. The
designer can choose the adjacent (larger) standard
inductance value. The inductance varies with
temperature and DC current. It is a good engineering
practice to re-evaluate the resultant current ripple at
the rated DC output current.
c) Current rating: The saturation current of the inductor
should be at least 1.5 times of the peak inductor current
under all conditions.
Since the inductor current is a triangular waveform in
buck configuration with peak-to-peak value δ*Io, the
ripple-voltage caused by inductor current ripples is:
∆v C ≈
δIo
8Co fs
the ripple-voltage due to ESL is:
∆v ESL = L esl fs
δIo
D
and the ESR ripple-voltage is:
∆v ESR = R esr δIo
Output Capacitor (Co) and Vout Ripple
Aluminum capacitors (e.g. electrolytic, solid OS-CON,
POSCAP, tantalum) have high capacitances and low
ESL’s. The ESR has the dominant effect on the output
ripple voltage. It is therefore very important to minimize
the ESR.
The output capacitor provides output current filtering in
steady state and serves as a reservoir during load
transient. The output capacitor can be modeled as an
ideal capacitor in series with its parasitic ESR (Resr) and
ESL (Lesl) (Figure 1).
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di ( t )
ib ( t )dt + L esl b + Resr ib ( t )
Co 0
dt
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SC4508A
POWER MANAGEMENT
Application Information (Cont.)
the following remarks are made to clarify some practical
issues.
When determining the ESR value, both the steady state
ripple-voltage and the dynamic load transient need to be
considered. To keep the steady state output ripple-voltage
< ∆Vo, the ESR should satisfy:
R esr1 <
Remark 1: High frequency ceramic capacitors may not
carry most of the ripple current. It also depends on the
capacitor value. Only when the capacitor value is set
properly, the effect of ceramic capacitor low ESR starts
to be significant.
For example, if a 10µF, 4mΩ ceramic capacitor is
connected in parallel with 2x1500µF, 90mΩ electrolytic
capacitors, the ripple current in the ceramic capacitor is
only about 42% of the current in the electrolytic
capacitors at the ripple frequency. If a 100µF, 2mΩ
ceramic capacitor is used, the ripple current in the
ceramic capacitor will be about 4.2 times of that in the
electrolytic capacitors. When two 100µF, 2mΩ ceramic
capacitors are used, the current ratio increases to 8.3.
In this case most of the ripple current flows in the
ceramic decoupling capacitor. The ESR of the ceramic
capacitors will then determine the output ripple-voltage.
∆Vo
δIo
To limit the dynamic output voltage overshoot/
undershoot within α (say 3%) of the steady state output
voltage) from no load to full load, the ESR value should
satisfy:
R esr 2 <
αVo
Io
Then, the required ESR value of the output capacitors
should be:
Resr = min{Resr1,Resr2 }
The voltage rating of aluminum capacitors should be at
least 1.5Vo. The RMS current ripple rating should also
be greater than:
Remark 2: The total equivalent capacitance of the filter
bank is not simply the sum of all the paralleled capacitors.
The total equivalent ESR is not simply the parallel
combination of all the individual ESRs either. Instead they
should be calculated using the following formulae.
δIo
2 3
Usually it is necessary to have several capacitors of the
same type in parallel to satisfy the ESR requirement.
The voltage ripple cause by the capacitor charge/
discharge should be an order of magnitude smaller than
the voltage ripple caused by the ESR. To guarantee this,
the capacitance should satisfy:
Co >
2
C eq (ω) :=
10
2πfsR esr
2
2
(R1a C1a + R1b C1b )ω2 C1a C1b + (C1a + C1b )
2
R eq (ω) :=
Buck-boost converter has higher ripple current than buck
in output. The RMS value is the most important factor to
consider. It has to be less than the output capacitor ripple
current rating.
The buck-boost output capacitor RMS current is:
IRMS _ CAP ≈ IO
2
(R1a + R1b )2 ω2C1a C1b + (C1a + C1b )2
2
2
2
R1aR1b (R1a + R1b )ω2C1a C1b + (R1b C1b + R1a C1a )
2
2
(R1a + R1b )2 ω2 C1a C1b + (C1a + C1b )2
where R 1a and C 1a are the ESR and capacitance of
electrolytic capacitors, and R1b and C1b are the ESR and
capacitance of the ceramic capacitors respectively.
(Figure 2)
VO + Vd
VIN
∆v ESR = R esr ⋅ Id
C1a
where, Vd and Id are rectifier forward voltage and current.
R1a
In many applications, several low ESR ceramic capacitors
are added in parallel with the aluminum capacitors in
order to further reduce ESR and improve high frequency
decoupling. Because the values of capacitance and ESR
are usually different in ceramic and aluminum capacitors,
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C1b
R1b
Ceq
Req
Figure 2. Equivalent RC branch.
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SC4508A
POWER MANAGEMENT
Application Information (Cont.)
Req and Ceq are both functions of frequency. For rigorous
design, the equivalent ESR should be evaluated at the
ripple frequency for voltage ripple calculation when both
ceramic and electrolytic capacitors are used. If R1a = R1b
= R1 and C1a = C1b = C1, then Req and Ceq will be frequencyindependent and
Req = 1/2 R1 and Ceq = 2C1
Input Capacitor (Cin)
The input supply to the converter usually comes from a
pre-regulator. Since the input supply is not ideal, input
capacitors are needed to filter the current pulses at the
switching frequency. A simple buck converter is shown in
Figure 3.
It can be seen that the current in the input capacitor
pulses with high di/dt. Capacitors with low ESL should be
used. It is also important to place the input capacitor
close to the MOSFETs on the PC board to reduce trace
inductances around the pulse current loop.
The RMS value of the capacitor current is approximately
ICin = Io D[(1 +
δ2
D
D
)(1 − )2 + 2 (1 − D) ]
η
η
12
The power dissipated in the input capacitors is then:
PCin = ICin2Resr
For reliable operation, the maximum power dissipation
in the capacitors should not result in more than 10oC of
temperature rise. Many manufacturers specify the
maximum allowable ripple current (RMS) rating of the
capacitor at a given ripple frequency and ambient
temperature. The input capacitance should be high
enough to handle the ripple current. For higher power
applications, multiple capacitors are placed in parallel to
increase the ripple current handling capability.
Sometimes meeting tight input voltage ripple
specifications may require the use of larger input
capacitance. At full load, the peak-to-peak input voltage
ripple due to the ESR is:
Figure 3. A simple model for the converter input
In Figure 3 the DC input voltage source has an internal
impedance Rin and the input capacitor Cin has an ESR of
Resr. MOSFET and input capacitor current waveforms, ESR
voltage ripple and input voltage ripple are shown in Figure
4.
δ
∆v ESR = R esr (1 + )Idc
2
The peak-to-peak input voltage ripple due to the capacitor
is:
∆v C ≈
DIdc
Cin fs
From these two expressions, CIN can be found to meet
the input voltage ripple specification.
Power MOSFETs Selection
Main considerations in selecting the MOSFETs are power
dissipation, cost and packaging. Switching losses and
conduction losses of the MOSFETs are directly related to
the total gate charge (Cg) and channel on-resistance
(Rds(on)). In order to judge the performance of MOSFETs,
the product of the total gate charge and on-resistance is
used as a figure of merit (FOM). Transistors with the same
FOM follow the same curve in Figure 5.
Figure 4. Typical waveforms at converter input.
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SC4508A
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Application Information (Cont.)
manufacturers’ data sheet. From the FDS6675
datasheet, Rds(on) is less than 14mΩ when Vgs is greater
than 10V. However R ds(on) increases by 30% as the
junction temperature increases from 25oC to 110oC.
Gate Charge (nC)
50
40
Cg( 100 , Rds)
Cg( 200 , Rds)
Cg( 500 , Rds)
1
The switching losses can be estimated using the simple
formula:
20
Pts = 21 ( t r + t f )(1 + 2δ )Idc Vin fs
0
0
5
15
20
1
Rds
On-resistance (mOhm)
10
20
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
Figure 5. Figure of Merit curves.
where tr is the rise time and tf is the fall time of the
switching process. Different manufacturers have
different definitions and test conditions for t and t . To
r
f
clarify these, we sketch the typical MOSFET switching
characteristics under clamped inductive mode in Figure
6.
The closer the curve is to the origin, the lower is the FOM.
This means lower switching loss or lower conduction loss
or both. It may be difficult to find MOSFETs with both low
Cg and low Rds(on. Usually a trade-off between Rds(on and
Cg has to be made.
MOSFET selection also depends on applications. In many
applications, either switching loss or conduction loss
dominates for a particular MOSFET. For buck and buckboost converters with high input to output voltage ratios,
the MOSFET is hard switched but conducts with very low
duty cycle. For such applications, MOSFETs with low Cg
should be used.
MOSFET power dissipation consists of:
a) conduction loss due to the channel resistance Rds(on),
b) switching loss due to the switch rise time tr and fall
time tf, and
c) the gate loss due to the gate resistance RG.
The RMS value of the MOSFET switch current is calculated
as:
Figure 6. MOSFET switching characteristics
In Figure 6,
Qgs1 is the gate charge needed to bring the gate-to-source
voltage Vgs to the threshold voltage Vgs_th,
Qgs2 is the additional gate charge required for the switch
current to reach its full-scale value Ids and
.
Qgd is the charge needed to charge gate-to-drain (Miller)
capacitance when Vds is falling.
Switching losses occur during the time interval [t1, t3].
Defining tr = t3-t1 and tr can be approximated as:
2
tr =
δ
IQrms = Idc D(1 + 12
)
The conduction losses are then
Ptc = IQrms2 Rds(on)
Idc is average inductor current. In buck converter, it is
also load current. In buck-boost, it is load current divided
by 1-D.
Rds(on) varies with temperature and gate-source voltage.
Curves showing R ds(on) variations can be found in
2007 Semtech Corp.
(Qgs 2 + Qgd )Rgt
Vcc − Vgsp
where Rgt is the total resistance from the driver supply
rail to the gate of the MOSFET. It includes the gate driver
internal impedance Rgi, external resistance Rge and the
gate resistance Rg within the MOSFET i.e.
Rgt = Rgi+Rge+Rg
Vgsp is the Miller plateau voltage shown in Figure 11.
Similarly an approximate expression for tf is:
11
www.semtech.com
SC4508A
POWER MANAGEMENT
Application Information (Cont.)
tf =
(Tj,max, usually 125oC) is not exceeded under the worstcase condition. The equivalent thermal impedance from
junction to ambient (θja) should satisfy:
(Qgs2 + Qgd )R gt
Vgsp
θ ja ≤
Only a portion of the total losses (Pg = QgVccfs) is dissipated
in the MOSFET package. Here Qg is the total gate charge
specified in the datasheet. The power dissipated within
the MOSFET package is:
Ptg =
Rg
Rgt
Qg Vcc fs
The total power loss of the top switch is then:
Pt = Ptc+Pts+Ptg
If the input supply of the power converter varies over a
wide range, then it will be necessary to weigh the relative
importance of conduction and switching losses. This is
because conduction losses are inversely proportional to
the input voltage. Switching loss however increases with
the input voltage. The total power loss of MOSFET should
be calculated and compared for high-line and low-line
cases. The worst case is then used for thermal design.
Freewheeling Diode Selection
The Schottky diode is recommended as freewheeling
diode in the both Buck and Buck-Boost applications. The
diode conducts during the off-time. The diode voltage
and current ratings are selected based on the peak
reverse voltage, the peak current and average power
dissipation. The following could be used to determine the
diode reversed voltage:
VD(REV ) = VIN , ID(PEAK ) = IO +
∆IL
V − VO
,ID( AVG ) = IO IN
for Buck
2
VIN + VD
VD(REV ) = VIN + VO , ID(PEAK ) = IO (
VIN + VO + VD
VIN
)+
∆IL
,ID( AVG ) = IO
2
for Buck − Boost
The most stressful condition for the diode occurs when
the output is shorted. Under this condition, due to the
VOUT = 0, the diode conducts at close to 100% duty cycle.
Therefore, attention should be paid to the thermal
condition when laying out a board.
Once the power losses (P loss ) for the MOSFET and
freewheeling diode are known, thermal and package
design at component and system level should be done
to verify that the maximum die junction temperature
2007 Semtech Corp.
Tj,max − Ta,max
Ploss
θja depends on the die to substrate bonding, packaging
material, the thermal contact surface, thermal compound
property, the available effective heat sink area and the
air flow condition (free or forced convection). Actual
temperature measurement of the prototype should be
carried out to verify the thermal design.
Overload Protection and Hiccup
During start-up, the capacitor from the SS/EN pin to
ground functions as a soft-start capacitor. After the
converter starts and enters regulation, the same
capacitor operates as an overload shutoff timing
capacitor. As the load current increases, the cycle-bycycle current-limit comparator will first limit the inductor
current. If the over-current persists for more than 32
consecutive switching cycles, the controller will shut off
the MOSFETs. Meanwhile an internal 12mA current
source discharges the soft-start capacitor CSS connected
to the SS/EN pin.
When the capacitor is discharged to 0.5V, a 10µA current
source recharges the SS/EN capacitor to 0.9V and driver
stage is enabled. Then a 20uA current source continues
to charge the soft-start capacitor. As the soft-start
capacitor reaches 1.4V, VREF will start to follow the softstart capacitor voltage until VREF=0.5V. If overload
persists, the controller will shut down the converter when
the soft-start capacitor voltage exceeds 1.4V. The
converter will repeatedly start and shut off until it is no
longer overloaded. This hiccup mode of overload
protection is a form of foldback current limiting. The
following calculations estimate the average inductor
current when the converter output is shorted to the
ground.
a) The time taken to charge the capacitor from 0.5V to
0.9V
t ssr1 = CSS
( 0.9 − 0.5 ) V
10uA
If CSS = 0.1µF, tssr1 is calculated as 4ms.
b) The time to charge the capacitor from 0.9V to 1.4V
(driver is enabled but output duty cycle is 0)
12
www.semtech.com
SC4508A
POWER MANAGEMENT
Application Information (Cont.)
(1.4 − 0.9)V
20µA
t ssr 2 = CSS
When CSS = 0.1µF, tssr2 is calculated as 2.5ms. Note that
during this period, the converter does not start switching
until SS/EN reaches 1.4V.
c) The effective start-up time is:
t sse =
32
200KHz
Assuming inductor current hitting current limit for 32
cycles after SS/EN reaches 1.4V and fs=200KHz. The
average inductor current is then:
ILeff = ILIM
t sse
t ssr1 + t ssr 2
ILeff ≈ 0.025 ILIM and is independent of the soft start
capacitor value. The converter will not overheat in hiccup.
Vo (V)
0.6
0.9
1.2
1.5
1.8
2.5
3.3
(1- h)/h
0.2
0.8
1.4
2
2.6
4
5.6
Ro1 (Ohm) 200
806
1.4K
2K
2.61K 4.02K 5.62K
Ro2 (Ohm) 1K
1K
1K
1K
1K
R o2 =
h
R o1
1− h
Once either R o1 or R o2 is chosen, the other can be
calculated for the desired output voltage Vo. Since the
number of standard resistance values is limited, the
calculated resistance may not be available as a standard
value resistor. As a result, there will be a set error in the
converter output voltage. This non-random error is
caused by the feedback voltage divider ratio. It cannot
be corrected by the feedback loop.
The following table lists a few standard resistor
combinations for realizing some commonly used output
voltages.
2007 Semtech Corp.
1K
Only the voltages in boldface can be precisely set with
standard 1% resistors.
From this table, one may also observe that when the
value
1 − h Vo − 0.5
=
h
0.5
and its multiples fall into the standard resistor value
chart (1%, 5% or so), it is possible to use standard value
resistors to exactly set up the required output voltage
value.
In buck-boost converter, output voltage is set by
R o2 =
Setting the Output Voltage
The non-inverting input of the error amplifier is brought
out as a device pin (Pin 9) to which the user can connect
Pin 4 or an external voltage reference. A simple voltage
divider (Ro1 at top and Ro2 at bottom) sets the converter
output voltage. In buck converter, the voltage feedback
gain (h=0.5/Vo ) is related to the divider resistors value
as:
1K
0.5
Ro1
VO
The input bias current of the error amplifier also causes
an error in setting the output voltage. The inverting input
bias currents of error amplifiers is -100nA. Since the
non-inverting input is biased to 0.5V in buck converter,
the percentage error in the second output voltage will be
–100% · (0.1µA) · R R /[0.5 · (R +R ) ]. To keep
o1 o2
o1 o2
this error below 0.2%, R //R
< 10kΩ.
o1
o2
Loop Compensation
SC4508A is a current-mode controller. Current-mode
control is a dual-loop control system in which the inductor
peak current is loosely controlled by the inner currentloop. The higher gain outer loop regulates the output
voltage. Since the current loop makes the inductor appear
as a current source, the complex high-Q poles of the
output LC networks is split into a dominant pole
determined by the output capacitor and the load
resistance and a high frequency pole. This pole-splitting
property of current-mode control greatly simplifies loop
compensation.
13
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SC4508A
POWER MANAGEMENT
Application Information (Cont.)
The inner current-loop is unstable (sub-harmonic
oscillation) unless the inductor current up-slope is steeper
than the inductor current down-slope. For stable
operation above 50% duty-cycle, a compensation ramp
is added to the sensed-current. In the SC4508A the
compensation ramp is made switching frequency
dependent. The slope of the compensation ramp is:
h=
0.5
Vo
For the rated output current Io, the current sensing gain
k is fixed as:
k=
1
8 ⋅ RS
S e = 500 * fsmV
From Figure 7, the transfer function from the voltage
error amplifier output (vc ) to the converter output (vo )is:
The slope of the internal compensation ramp is above
the minimal slope requirement for current loop stability
and is sufficient for all the applications. With the inner
current loop stable, the output voltage is then regulated
with the outer voltage feedback loop. A simplified
equivalent circuit model of the buck converter with
current mode control is shown in Figure 7.
s
Vo (s)
s z1
:= Gvc (s) = kR o
s
Vc (s)
1+
sp1
1+
where, the single dominant pole is:
sp1 =
1
(R o + R oesr )Co
and the zero due to the output capacitor ESR is:
s z1 =
1
R oesr Co
The dominant pole moves as output load varies.
The controller transfer function (from the converter
output (vo ) to the voltage error amplifier output (vc) is:
s
sz2
gm
Gc ( s ) =
s(C 2 + C3 ) 1 + s
sp 2
k
1+
where
sz2 =
Figure 7. A simple model of buck converter with current
mode control.
The transconductance error amplifier (in the SC4508A)
has a gain (g m ) of 100µA/V. The target of the
compensation design is to select the compensation
network consisting of C 2, C 3 and R 2, along with the
feedback resistors Ro1, Ro2 and the current sensing gain,
such that the converter output voltage is regulated with
satisfactory dynamic performance.
With the output voltage Vo known, the feedback gain h
and the feedback resistor values are determined using
the equations given in the “Output Voltage Setting”
section with:
2007 Semtech Corp.
1
R 2C 2
and
sp 2 =
1
C 2C 3
R2
C 2 + C3
The loop transfer function is then:
T(s)=Gvc(s)GC(s)h
To simplify design, we assume that C3<<C2, Roesr<<Ro,
selects S p1 =S z2 and specifies the loop crossover
frequency (fc). It is noted that the crossover frequency
determines the converter dynamic bandwidth. With these
14
www.semtech.com
SC4508A
POWER MANAGEMENT
Application Information (Cont.)
assumptions, the controller parameters are determined
as follows:
C2 =
100
gmkR o
h
2πfc
R2 =
100
76.67
20 ⋅ log( T ( f )
R oCo
C2
)
53.33
30
0
and
6.67
C3 =
R oesr Co
R2
16.67
− 40
40
1
For example, if Vo=3.3V, Io=2A, fs=300kHz, Co=100uF,
Roesr=10mΩ, Rs=35mΩ, one can calculate that::
Ro =
h=
Vo
= 1.65Ω
Io
− 85
and
π
1
= 3.57
8 ⋅ RS
g m kR o
h ≈ 23 . 6 nF
2 π fc
3
4
5
6
100 1 .10 1 .10 1 .10 1 .10
5
f
10 ⋅ 10
85
89
arg( T ( f ) )
91
93
− 95 95
If the converter crossover frequency is set around 1/10
of the switching frequency, fc = 30kHz, the controller
parameters then can be calculated as:
C2 =
10
87
0 .5
= 0.152
Vo
180
k=
1
1
1
10
3
4
5
6
100 1 .10 1 .10 1 .10 1 .10
5
f
10 ×10
Figure 8. The loop transfer function Bode plot of the
buck example.
where, gm is the error amplifier transconductance gain
(100 µΩ−1).
It is clear that the resulted crossover frequency is about
30kHz with phase margin 91o.
If we use C2 = 22 nF,
For buck-boost converter, a simplified equivalent circuit
model of the buck converter with current mode control
is shown in Figure 9 and the transfer function from the
voltage error amplifier output (vc ) to the converter output
(vo ) is:
R2 =
R o Co
= 7.5kΩ
C2
use R2 = 7.5kΩ.
It is further calculated that:
Vo (s)
1− D
:= Gvc (s) = k
Ro
Vc (s)
1+ D
R C
C3 = oesr o ≈ 134pF
R2
use C 3 = 120pF. The Bode plot of the loop transfer
function (magnitude and phase) is shown in Figure 8.
2007 Semtech Corp.
15
(1 −
s
s zRHP
1+
)(1 +
s
sp1
s
)
s z1
www.semtech.com
SC4508A
POWER MANAGEMENT
Application Information (Cont.)
sp 2 =
where, the single dominant pole is:
sp1 =
1+ D
R o Co
1
C 2C 3
R2
C 2 + C3
The loop transfer function is then
T(s)=Gvc(s)GC(s)h
To simplify design, we assume that C3<<C2, Roesr<<Ro,
With these assumptions, the controller zero is placed at
the converter dominant pole, the controller second pole
is placed at the converter ESR zero or RHP zero
depending on whichever is lower. The DC gain is finally
adjusted for desired phase margin. The controller
parameters are determined as following:
Assuming a DC gain ω I,
C2 =
R2 =
gm
ωI
1
C 2 ⋅ s p1
and
Figure 9. A simple model of buck-boost converter with
current mode control.
and the zero due to the output capacitor ESR is:
s z1 =
1
R oesr Co
C3 =
For example, if V in=12V, V o=-12V, I o=1A, f s=300kHz,
D=0.51, C o=100uF, R oesr=35mΩ, R s=35mΩ, one can
calculate that::
and the RHP zero associated to the topoly is:
s zRHP
Ro =
(1 − D)2 ⋅ R o
=
D ⋅L
The dominant pole moves as input voltage and output
load varies.
The controller transfer function (from the converter
output (vo ) to the voltage error amplifier output (vc) is:
s
sz2
gm
Gc ( s ) =
s(C 2 + C3 ) 1 + s
sp 2
1+
1
1
C3 =
or
R 2 ⋅ s z1
R2 ⋅ s zRHP
h=
Vo
= 12Ω
Io
0.5
= 0.04
Vo + 0.5
Set ω I = 500, the controller parameters then can be
calculated as:
C2 =
gm
h ≈ 400nF
ωI
where
sz2 =
and
2007 Semtech Corp.
1
R 2C 2
where, gm is the error amplifier transconductance gain
(100 µΩ−1).
If we use C2 = 390 nF,
16
www.semtech.com
SC4508A
POWER MANAGEMENT
Application Information (Cont.)
bandwidth is reduced in order to reject some high
frequency noises. In the final working circuit, the loop
transfer function should be measured using network
analyzer and compared with the design to ensure circuit
stability under different line and load conditions. The load
transient response behavior is further tested and
measured to meet the specification.
1
R2 =
≈ 2.03kΩ
sp1C2
use R2 = 2kΩ.
Since SzRHP<sz1, it is further calculated that::
C3 =
1
≈ 2.92nF
R 2 ⋅ s zRHP
use C 3 = 3.3nF. The Bode plot of the loop transfer
function (magnitude and phase) is shown in Figure 10.
60
60
43.33
20⋅ log( T ( f )
26.67
)
Layout Guidelines
In order to achieve optimal electrical, thermal and noise
performance for high frequency converters, attention
must be paid to the PCB layouts. The goal of layout
optimization is to place components properly and identify
the high di/dt loops to minimize them. The following
guideline should be used to ensure proper functions of
the converters.
10
0
6.67
23.33
− 40
40
1
− 80
1
10
3
4
5
6
100 1 .10 1 .10 1 .10 1 .10
5
f
10⋅ 10
80
100
180
π
120
arg( T ( f ) )
140
160
− 180 180
1
1
10
3
4
100 1 .10 1 .10
f
5
6
1 .10 1 .10
5
10×10
Figure 10. The loop transfer function Bode plot of the
buck-boost example.
It is clear that the resulted crossover frequency is about
1kHz with phase margin 90o.
In some initial prototypes, if the circuit noise makes the
control loop jitter, it is suggested to use a bigger C3 value
than the calculated one here. Effectively, the converter
2007 Semtech Corp.
1. A ground plane is recommended to minimize noises
and copper losses, and maximize heat dissipation.
2. Start the PCB layout by placing the power components
first. Arrange the power circuit to achieve a clean
power flow route. Put all the connections on one side
of the PCB with wide copper filled areas if possible.
3. The VDD bypass capacitors should be placed next to
the VDD and PGND, AGND pins respectively.
4. Separate the power ground from the signal ground.
In SC4508A, the power ground PGND connection
should make PFET driving current loop as small as
possible. The signal ground AGND should be tied to
the negative terminal of the output capacitor.
5. The trace connecting the feedback resistors to the
output should be short, direct and far away from the
noise sources such as switching node and switching
components. Minimize the traces between OUT and
the gates of the PFETs to reduce their impedance to
drive the MOSFET.
7. Minimize the loop including input capacitors, top/
bottom MOSFETs. This loop passes high di/dt current.
Make sure the trace width is wide enough to reduce
copper losses in this loop.
8. Maximize the trace width of the loop connecting the
inductor, PFET and the output capacitors.
9. Connect the ground of the feedback divider and the
compensation components directly to the AGND pin
of the SC4508A by using a separate ground trace.
Then connect this pin to the ground of the output
capacitor as close as possible.
17
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SC4508A
POWER MANAGEMENT
Evaluation Board Schematic, Buck
12V
GND
U1
SC4508A
C4
1uF
12
3
C5
0.1uF
11
C6
330pF
f s=470KHz
5
6
7
VDD
SS/EN
OSC
U1
CS
OUT
VREF
NC
FB+
AGND
FB-
PGND
COMP
R7
10K
C3
1nF
R1
0.09
R2 10
2
4
C7 TP2
0.1uF
9
C2
47uF/16V
TP1
1
4
C1
47uF/16V
8
U2 FDFS2P102A
5
3
6
2
7
1
8
TP3
L1
10uH
5V/1A
TP4
10
R4
20K
C11
2.7nF
C8
470uF/6.3V
C9
470uF/6.3V
GND
C10
2nF
R5
18.0K
TP5
TP6
TP7
R6
2.0K
TP8
Bill of Materials
Item
Quantity
Reference
1
2
C1,C2
2
1
C3
1nF
3
1
C4
1uF
4
2
C5,C7
0.1uF
5
1
C6
330pF
6
2
C8,C9
7
1
C 10
2nF
8
1
C11
2.7nF
9
1
L1
10uH
10
1
R1
0.09
11
1
R2
10
12
1
R4
20K
13
1
R5
18.0K
14
1
R6
2.0K
15
1
R7
10K
16
1
U1
S C 4508A
17
1
U2
F D F S 2P 102A
2007 Semtech Corp.
Part
47uF/16V
470uF/6.3V
18
Manufacturer
Sanyo P/N: 16TPB47M
Sanyo P/N: 16TPB470M
Semtech Corp.
Fairchild P/N: FDFS2P102A
www.semtech.com
SC4508A
POWER MANAGEMENT
Evaluation Board Schematic, Buck-Boost
TP9
12V
GND
U1
SC4508A
C4
1uF
12
3
C5
0.1uF
11
C6
680pF
f s=250KHz
5
6
7
VDD
CS
SS/EN
OSC
U1
OUT
VREF
NC
FB+
AGND
FB-
PGND
COMP
R7
10K
C3
1nF
TP10
2
R1
0.06
R2 100
1
C7
0.1uF
8
C2
47uF/16V
TP1
U2 Si4831DY
4
5
4
9
C1
47uF/16V
3
6
2
7
1
8
TP3
TP4
10
R4
20K
C11
2.7nF
C10
100pF
C8
47uF/16V
C9
100uF/6.3V
L1
33uH
-5V/0.5A
R5
4.99K
GND
R6 499
TP5
TP6
TP7
TP8
Bill of Materials
Item
Quantity
Reference
1
3
C1,C2,C8
2
1
C3
1nF
3
1
C4
1uF
4
2
C 5, C 7
0.1uF
5
1
C6
680pF
6
1
C9
100uF/6.3V
7
1
C 10
100pF
8
1
C11
2.7nF
10
1
L1
33uH
11
1
R1
0.06
12
1
R2
100
13
1
R4
20K
14
1
R5
4.99K
15
1
R6
499
16
1
R7
10K
19
1
U1
S C 4508A
Semtech Corp.
20
1
U2
S i 4831D Y
Vishay
2007 Semtech Corp.
Part
47uF/16V
19
Manufacturer
Sanyo P/N: 16TPB47M
www.semtech.com
SC4508A
POWER MANAGEMENT
Typical Characteristics
Buck-Boost Converter Startup
Buck Converter Sartup
5mS/DIV
a: OUTPUT VOLTAGE, 5V/DIV
b: SS/EN PIN VOLTAGE, 5V/DIV
C: INDUCTOR CURRENT, 2A/DIV
D: VREF PIN VOLTAGE, 0.5V/DIV
50mS/DIV
a: OUTPUT VOLTAGE, 5V/DIV
b: SS/EN PIN VOLTAGE, 5V/DIV
C: INDUCTOR CURRENT, 2A/DIV
D: VREF PIN VOLTAGE, 0.5V/DIV
Buck CCM Operation
Buck-Boost DCM Operation
500nS/DIV
a: OUT PIN VOLTAGE, 10V/DIV
b: PHASE NODE VOLTAGE, 10V/DIV
c: INDUCTOR CURRENT, 1A/DIV
500nS/DIV
a: OUT PIN VOLTAGE, 10V/DIV
b: PHASE NODE VOLTAGE, 10V/DIV
c: INDUCTOR CURRENT, 200mA/DIV
Buck-Boost Overcurrent Protection
Buck Overcurrent Protection
10mS/DIV
a: OUT PIN VOLTAGE, 10V/DIV
b: SS/EN PIN VOLTAGE, 1V/DIV
c: INDUCTOR CURRENT, 2A/DIV
10mS/DIV
a: OUT PIN VOLTAGE, 10V/DIV
b: SS/EN PIN VOLTAGE, 1V/DIV
c: INDUCTOR CURRENT, 2A/DIV
2007 Semtech Corp.
20
www.semtech.com
SC4508A
POWER MANAGEMENT
Outline Drawing - MLPQ-12, 4 x 4
A
D
DIMENSIONS
INCHES
MILLIMETERS
DIM
MIN NOM MAX MIN NOM MAX
B
PIN 1
INDICATOR
(LASER MARK)
A
A1
A2
b
D
D1
E
E1
e
L
N
aaa
bbb
E
A2
A
.031
.040
.002
.000
(.008)
.010 .012 .014
.153 .157 .161
.074 .085 .089
.153 .157 .161
.074 .085 .089
.031 BSC
.018 .022 .026
12
.003
.004
1.00
0.80
0.05
0.00
(0.20)
0.25 0.30 0.35
3.90 4.00 4.10
1.90 2.15 2.25
3.90 4.00 4.10
1.90 2.15 2.25
0.80 BSC
0.45 0.55 0.65
12
0.08
0.10
SEATING
PLANE
aaa C
C
A1
D1
LxN
E/2
2
E1
1
N
bxN
bbb
e
C A B
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 - MLPQ-12, 4 x 4
K
DIM
2x (C)
H
2x G
Y
X
2x Z
C
G
H
K
P
X
Y
Z
DIMENSIONS
INCHES
MILLIMETERS
(.148)
.106
.091
.091
.031
.016
.041
.189
(3.75)
2.70
2.30
2.30
0.80
0.40
1.05
4.80
P
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
2007 Semtech Corp.
21
www.semtech.com