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LTC3530EMS#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC3530
7
3530fb
The LTC3530 provides high effi ciency, low noise power
for a wide variety of handheld electronic devices. The
LTC proprietary topology allows input voltages above,
below or equal to the output voltage by properly phasing
the output switches. The error amp output voltage on V
C
determines the output duty cycle of the switches. Since
V
C
is a fi ltered signal, it provides rejection of frequencies
from well below the switching frequency. The low R
DS(ON)
,
low gate charge synchronous switches provide high fre-
quency pulse width modulation control at high effi ciency.
High effi ciency is achieved at light loads when Burst Mode
operation is entered and the LTC3530’s quiescent current
drops to a low 40μA.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation is programmed by an external
resistor from R
T
to ground, according to the following
equation:
f
(kHz)
=
33,170
R
T(k)
Error Amp
The error amplifi er is a voltage mode amplifi er. The loop
compensation components are confi gured around the
amplifi er (from FB to V
C
) to obtain stability of the converter.
For improved bandwidth, an additional R-C feed-forward
network can be placed across the upper feedback divider
resistor. The voltage on SHDN/SS clamps the error amp
output, V
C
, to provide a soft-start function.
Internal Current Limit
There are two different current limit circuits in the LTC3530.
Each has internally fi xed thresholds which vary inversely
with V
IN
.
The fi rst circuit is a high speed peak current limit compara-
tor that will shut off switch A once the current exceeds
2.5A typical. The delay to output of this comparator is
typically 50ns.
A second amplifi er will source current out of FB to drop
the output voltage once the peak input current exceeds
2A typical. This method provides a closed loop means of
clamping the input current. During conditions where V
OUT
is near ground, such as during a short-circuit or during
startup, this threshold is cut to 670mA (typ), providing a
foldback feature. For this current limit feature to be most
effective, the Thevenin resistance from FB to ground should
be greater than 100kΩ.
Reverse Current Limit
During fi xed frequency operation, the LTC3530 operates
in forced continuous conduction mode. The reverse cur-
rent limit amplifi er monitors the inductor current from
the output through switch D. Once the negative inductor
current exceeds 640mA typical, the LTC3530 will shut off
switch D.
Four-Switch Control
Figure 1 shows a simplifi ed diagram of how the four in-
ternal switches are connected to the inductor, V
IN
, V
OUT
and GND. Figure 2 shows the regions of operation for the
LTC3530 as a function of the internal control voltage, V
CI
.
3
SW1
4
SW2
PMOS A
NMOS B
7
V
IN
PMOS D
NMOS C
3530 F01
6
V
OUT
85%
D
MAX
BOOST
D
MIN
BOOST
D
MAX
BUCK
DUTY
CYCLE
0%
V4 (≈1.5V)
V3 (≈1.15V)
BOOST REGION
BUCK REGION
BUCK/BOOST REGION
V2 (≈1V)
V1 (≈0.7V)
3530 F02
A ON, B OFF
PWM CD SWITCHES
D ON, C OFF
PWM AB SWITCHES
FOUR SWITCH PWM
INTERNAL
CONTROL
VOLTAGE, V
CI
Figure 1. Simplifi ed Diagram of Output Switches Figure 2. Switch Control vs Internal Control Voltage, V
CI
OPERATION
LTC3530
8
3530fb
Depending on the control voltage, the IC will operate in
either buck, buck/boost or boost mode. The V
CI
voltage
is a level shifted voltage from the output of the error amp
(V
C
). The four power switches are properly phased so the
transfer between operating modes is continuous, smooth
and transparent to the user. When V
IN
approaches V
OUT
the buck/boost region is reached where the conduction
time of the four switch region is typically 150ns. Referring
to Figures 1 and 2, the various regions of operation will
now be described.
Buck Region (V
IN
> V
OUT
)
Switch D is always on and switch C is always off during
this mode. When the internal control voltage, V
CI
, is above
voltage V1, output A begins to switch. During the off-time of
switch A, synchronous switch B turns on for the remainder
of the time. Switches A and B will alternate similar to a
typical synchronous buck regulator. As the control volt-
age increases, the duty cycle of switch A increases until
the maximum duty cycle of the converter in buck mode
reaches D
MAX_BUCK
, given by:
D
MAX_BUCK
= 100 – D4
SW
%
where D4
SW
= duty cycle % of the four switch range.
D4
SW
= (150ns • f) • 100 %
where f = operating frequency, Hz.
Beyond this point the “four switch,” or buck/boost region
is reached.
Buck/Boost or Four Switch (V
IN
≈ V
OUT
)
When the internal control voltage, V
CI
, is above voltage
V2, switch pair AD remain on for duty cycle D
MAX_BUCK
,
and the switch pair AC begins to phase in. As switch pair
AC phases in, switch pair BD phases out accordingly.
When the V
CI
voltage reaches the edge of the buck/boost
range, at voltage V3, the AC switch pair completely phase
out the BD pair, and the boost phase begins at duty cycle
D4
SW
. The input voltage, V
IN
, where the four switch region
begins is given by:
V
IN
=
V
OUT
1– (150ns • f)
The point at which the four switch region ends is given
by:
V
IN
= V
OUT
(1 – D) = V
OUT
(1 – 150ns • f) V
Boost Region (V
IN
< V
OUT
)
Switch A is always on and switch B is always off during
this mode. When the internal control voltage, V
CI
, is above
voltage V3, switch pair CD will alternately switch to provide
a boosted output voltage. This operation is typical of a
synchronous boost regulator. The maximum duty cycle
of the converter is limited to 90% typical and is reached
when V
CI
is above V4.
BURST MODE OPERATION
Burst mode reduces the LTC3530’s quiescent current
consumption at light loads and improves overall conver-
sion effi ciency, increasing battery life. During Burst Mode
operation the LTC3530 delivers energy to the output until
it is regulated and then goes into sleep mode where the
outputs are off and quiescent current drops to 40μA (typ).
In this mode the output ripple has a variable frequency
component that depends upon load current, and will
typically be about 2% peak-to-peak. Burst Mode opera-
tion ripple can be reduced slightly by using more output
capacitance (47μF or greater). Another method of reducing
Burst Mode operation ripple is to place a small feed-forward
capacitor across the upper resistor in the V
OUT
feedback
divider network (as in Type III compensation).
During the period where the device is delivering energy to
the output, the peak switch current will be equal to 450mA
typical and the inductor current will terminate at zero
current for each cycle. In this mode the typical maximum
average output current is given by:
I
MAX(BURST)BUCK
450mA
2
;V
OUT
< V
IN
I
MAX(BURST)BOOST
450mA
2
•
V
IN
V
OUT
;V
OUT
> V
IN
I
MAX(BURST)
Buck-Boost ≈ 350mA; V
OUT
≈ V
IN
.
Since the input and output are connected together for
most of the cycle.
OPERATION
LTC3530
9
3530fb
The effi ciency below 1mA becomes dominated primarily
by the quiescent current. The Burst Mode operation ef-
fi ciency is given by:
EFFICIENCY
•I
LOAD
40μA + I
LOAD
where η is typically 90% during Burst Mode operation.
Automatic Burst Mode Operation Control
Burst Mode operation can be automatic or manually con-
trolled with a single pin. In automatic mode, the IC will
enter Burst Mode operation at light load and return to fi xed
frequency operation at heavier loads. The load current at
which the mode transition occurs is programmed using a
single external resistor from BURST to ground, according
to the following equations:
Enter Burst Mode: I
BURST
=
8.8
R
BURST
Leave Burst Mode: I
BURST
=
11.2
R
BURST
where R
BURST
is in kΩ and I
BURST
is the load transition
current in Amps. Do not use values of R
BURST
greater
than 500kΩ.
For automatic operation, a fi lter capacitor must also be
connected from BURST to ground. The equation for the
minimum capacitor value is:
C
BURST(MIN)
C
OUT
•V
OUT
60,000
where C
BURST(MIN)
and C
OUT
are in μF
In the event that a load transient causes FB to drop by more
than 4% from the regulation value while in Burst Mode
operation, the IC will immediately switch to fi xed frequency
mode and an internal pull-up will be momentarily applied
to BURST, rapidly charging C
BURST
. This prevents the IC
from immediately re-entering Burst Mode operation once
the output achieves regulation.
Manual Burst Mode Operation
For manual control of Burst Mode operation, the RC
network connected to BURST can be eliminated. To force
fi xed frequency mode, BURST should be connected to
V
IN
. To force Burst Mode operation, BURST should be
grounded. When commanding Burst Mode operation
manually, the circuit connected to BURST should be able
to sink up to 2mA.
For optimum transient response with large dynamic loads,
the operating mode should be controlled manually by the
host. By commanding fi xed frequency operation prior to
a sudden increase in load, output voltage droop can be
minimized. Note that if the load current applied during
forced Burst Mode operation (BURST pin is grounded)
exceeds the current that can be supplied, the output voltage
will start to droop and the IC will automatically come out
of Burst Mode operation and enter fi xed frequency mode,
raising V
OUT
. Once regulation is achieved, the IC will then
enter Burst Mode operation once again, and the cycle will
repeat, resulting in about 4% output ripple.
Burst Mode Operation to Fixed Frequency Transient
Response
In Burst Mode operation, the compensation network is
not used and V
C
is disconnected from the error amplifi er.
During long periods of Burst Mode operation, leakage
currents in the external components or on the PC board
could cause the compensation capacitor to charge (or
discharge), which could result in a large output transient
when returning to fi xed frequency mode of operation, even
at the same load current. To prevent this, the LTC3530
incorporates an active clamp circuit that holds the voltage
on V
C
at an optimal voltage during Burst Mode operation.
This minimizes any output transient when returning to
fi xed frequency mode operation. For optimum transient
response, Type 3 compensation is also recommended to
broad band the control loop and roll off past the two pole
response of the output LC fi lter. See Closing the Feedback
Loop under Applications Information.
OPERATION
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LTC3530EMS#PBF

Mfr. #:
Buy LTC3530EMS#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 1.8V to 5.5Vin, 600mA, 2MHz Buck-Boost Converter
Lifecycle:
New from this manufacturer.
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