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LTC3542EDC#TRMPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC3542
7
3542fa
The LTC3542 uses a constant frequency, current mode,
step-down architecture. The operating frequency is set at
2.25MHz and can be synchronized to an external oscillator.
To suit a variety of applications, the selectable MODE/SYNC
pin allows the user to trade-off noise for effi ciency.
The output voltage is set by an external divider returned
to the V
FB
pin. An error amplifi er compares the divided
output voltage with a reference voltage of 0.6V and adjusts
the peak inductor current accordingly.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the V
FB
voltage is below the reference voltage. The
current fl ows into the inductor and the load increases until
the current limit is reached. The switch turns off and energy
stored in the inductor fl ows through the bottom switch
(N-channel MOSFET) into the load until the next clock cycle.
The peak inductor current is controlled by the internally
compensated output of the error amplifi er. When the load
current increases, the V
FB
voltage decreases slightly below
the reference. This decrease causes the error amplifi er to
increase its output voltage until the average inductor cur-
rent matches the new load current. The main control loop
is shut down by pulling the RUN pin to ground.
Low Load Current Operation
By selecting MODE/SYNC pin, two modes are available to
control the operation of the LTC3542 at low load currents.
Both modes automatically switch from continuous opera-
tion to the selected mode when the load current is low.
To optimize effi ciency, the Burst Mode operation can be
selected. When the converter is in Burst Mode operation,
the peak current of the inductor is set to approximately
60mA regardless of the output load. Each burst event can
last from a few cycles at light loads to almost continuously
cycling with short sleep intervals at moderate loads. In
between these burst events, the power MOSFETs and any
unneeded circuitry are turned off, reducing the quiescent
current to 26μA. In this sleep state, the load current is
being supplied solely from the output capacitor. As the
output voltage drops, the EA amplifi er’s output rises above
the sleep threshold and turns the top MOSFET on. This
process repeats at a rate that is dependent on the load
demand. By running cycles periodically, the switching
losses which are dominated by the gate charge losses of
the power MOSFETs are minimized.
For lower ripple noise at low load currents, the pulse skip
mode can be used. In this mode, the regulator continues
to switch at a constant frequency down to very low load
currents, where it will begin skipping pulses.
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases to 100%, which is the
dropout condition. In dropout, the PMOS switch is turned
on continuously with the output voltage being equal to the
input voltage minus the voltage drops across the internal
P-channel MOSFET and the inductor. An important design
consideration is that the R
DS(ON)
of the P-channel switch
increases with decreasing input supply voltage (See Typical
Performance Characteristics). Therefore, the user should
calculate the power dissipation when the LTC3542 is used
at 100% duty cycle with low input voltage (See Thermal
Considerations in the Applications Information Section).
Low Supply Operation
To prevent unstable operation, the LTC3542 incorporates
an undervoltage lockout circuit which shuts down the part
when the input voltage drops below about 2V.
Internal Soft-Start
At start-up when the RUN pin is brought high, the internal
reference is linearly ramped from 0V to 0.6V in about 1ms.
The regulated feedback voltage follows this ramp resulting
in the output voltage ramping from 0% to 100% in 1ms.
The current in the inductor during soft-start is defi ned
by the combination of the current needed to charge the
output capacitance and the current provided to the load
as the output voltage ramps up. The start-up waveform,
shown in the Typical Performance Characteristics, shows
the output voltage start-up from 0V to 1.8V with a 500mA
load and V
IN
= 3.6V (refer to Figure 3a).
OPERATION
LTC3542
8
3542fa
A general LTC3542 application circuit is shown in Figure1.
External component selection is driven by the load require-
ment and begins with the selection of the inductor L. Once
the inductor is chosen, C
IN
and C
OUT
can be selected.
the burst clamp. Lower inductor values result in higher
ripple current which causes the transition to occur at lower
load currents. This causes a dip in effi ciency in the upper
range of low current operation. In Burst Mode operation,
lower inductance values cause the burst frequency to
increase.
Inductor Core Selection
Different core materials and shapes change the size/current
and price/current relationships of an inductor. Toroid or
shielded pot cores in ferrite or permalloy materials are small
and don’t radiate much energy, but generally cost more
than powdered iron core inductors with similar electrical
characteristics. The choice of which style inductor to use
often depends more on the price vs size requirements
and any radiated fi eld/EMI requirements than on what the
LTC3542 requires to operate. Table 1 shows some typi-
cal surface mount inductors that work well in LTC3542
applications.
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V
OUT
/V
IN
.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
II
VVV
V
RMS MAX
OUT IN OUT
IN
≈
()
–
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– ΔI
L
/2. This formula has a maximum at
V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case
is commonly used to design because even signifi cant
deviations do not offer much relief. Note that capacitor
manufacturer’s ripple current ratings are often based on
only 2000 hours life time. This makes it advisable to further
derate the capacitor, or choose a capacitor rated at a higher
temperature than required. Several capacitors may also be
paralleled to meet the size or height requirements of the
V
IN
LTC3542
RUN
R1
3542 F01
R2
C
F
L
C
IN
C
OUT
SW
V
IN
2.7V TO 5.5V
V
OUT
V
FB
MODE/SYNC
GND
Figure 1. LTC3542 General Schematic
Inductor Selection
The inductor value has a direct effect on ripple current ΔI
L
,
which decreases with higher inductance and increases with
higher V
IN
or V
OUT
, as shown in following equation:
ΔI
V
L
V
V
L
OUT
O
OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
ƒ •
–1
where f
O
is the switching frequency. A reasonable starting
point for setting ripple current is ΔI
L
= 0.4 • I
OUT(MAX)
,
where I
OUT(MAX)
is 500mA. The largest ripple current ΔI
L
occurs at the maximum input voltage. To guarantee that
the ripple current stays below a specifi ed maximum, the
inductor value should be chosen according to the follow-
ing equation:
L
V
I
V
V
OUT
OL
OUT
IN MAX
=
⎛
⎝
⎜
⎞
⎠
⎟
ƒ •
–
()
Δ
1
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 600mA rated
inductor should be enough for most applications (500mA
+ 100mA). For better effi ciency, chose a low DC-resistance
inductor.
The inductor value will also have an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor’s peak current falls below a level set by
APPLICATIONS INFORMATION
LTC3542
9
3542fa
design. An additional 0.1μF to 1μF ceramic capacitor is
also recommended on V
IN
for high frequency decoupling,
when not using an all ceramic capacitor solution.
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfi ed, the RMS current
rating generally far exceeds the I
RIPPLE(P-P)
requirement,
except for an all ceramic solution. The output ripple (ΔV
OUT
)
is determined by:
ΔΔV I ESR
C
OUT L
OUT
≈+
⎛
⎝
⎜
⎞
⎠
⎟
1
8• •ƒ
O
where f
O
is the switching frequency, C
OUT
is the output
capacitance and ΔI
L
is the inductor ripple current. For a fi xed
output voltage, the output ripple is highest at maximum
input voltage since ΔI
L
increases with input voltage.
If tantalum capacitors are used, it is critical that the capaci-
tors are surge tested for use in switching power supplies.
An excellent choice is the AVX TPS series of surface mount
tantalums, available in case heights ranging from 2mm to
4mm. These are specially constructed and tested for low
ESR so they give the lowest ESR for a given volume. Other
capacitor types include Sanyo POSCAP, Kemet T510 and
T495 series, and Sprague 593D and 595D series. Consult
the manufacturer for other specifi c recommendations.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high
ripple current rating, high voltage rating and low ESR
are tempting for switching regulator use. However, the
ESR is so low that it can cause loop stability problems.
Since the LTC3542’s control loop does not depend on
the output capacitor’s ESR for stable operation, ceramic
capacitors can be used to achieve very low output ripple
and small circuit size. X5R or X7R ceramic capacitors are
recommended because these dielectrics have the best
temperature and voltage characteristics of all the ceramics
for a given value and size.
Great care must be taken when using only ceramic input
and output capacitors. When a ceramic capacitor is used
at the input and the power is being supplied through long
wires, such as from a wall adapter, a load step at the output
can induce ringing at the V
IN
pin. At best, this ringing can
couple to the output and be mistaken as loop instability.
At worst, the ringing at the input can be large enough to
damage the part. For more information, see Application
Note 88. The recommended capacitance value to use is
10μF for both input and output capacitors.
Table 1. Representative Surface Mount Inductors
MANUFACTURER
PART NUMBER VALUE
(μH)
MAX DC
CURRENT
(A)
DCR
(Ω) SIZE (mm
3
)
Sumida CDRH2D11-2RM 2.2 0.780 0.098 3.2 × 3.2 × 1.2
CDRH3D16 2.2 1.2 0.075 3.8 × 3.8 × 1.8
CMD4D11 2.2 0.95 0.116 4.4 × 5.8 × 1.2
CDH2D09B 3.3 0.85 0.15 2.8 × 3 × 1
CLS4D09 4.7 0.75 0.15 4.9 × 4.9 × 1
Murata LQH32CN 2.2 0.79 0.097 2.5 × 3.2 × 1.55
LQH43CN 4.7 0.75 0.15 4.5 × 3.2 × 2.6
TDK IVLC453232 2.2 0.85 0.18 4.8 × 3.4 × 3.4
VLF3010AT-
2R2M1R0
2.2 1.0 0.12 2.8 × 2.6 × 1
APPLICATIONS INFORMATION
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LTC3542EDC#TRMPBF

Mfr. #:
Buy LTC3542EDC#TRMPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 500mA, 2.25MHz Sync Buck DC/DC Conv
Lifecycle:
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