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LTC3863IDE#PBF

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LTC3863
19
3863fa
For more information www.linear.com/3863
Figure 5a. Tw o Different Modes of Output Tracking
Figure 5b. Setup for Ratiometric and Coincident Tracking
TIME
Coincident Tracking
EXTERNAL
SUPPLY
EXTERNAL
SUPPLY
–V
OUT
(V
OUT
< 0V)
–V
OUT
(V
OUT
< 0V)
VOLTAGE
TIME
3863 F05a
Ratiometric Tracking
VOLTAGE
R1 = R
FB1
– R
FB2
|V
OUT
| > 0.8V
EXT. V
R2 = R
FB2
Coincident Tracking Setup
TO SS
R
FB1
V
OUT
TO V
FBN
TO FB TO FB
R
FB2
R1
EXT. V
R2
R1+ R2
≥
R2
TO SS
R
FB1
V
OUT
TO V
FBN
R
FB2
3863 F05b
Ratiometric Tracking Setup
0.8V
EXT. V
applicaTions inForMaTion
Discontinuous and Continuous Operation
The LTC3863 operates in discontinuous conduction (DCM)
until the load current is high enough for the inductor
current to be positive at the end of the switching cycle.
The output load current at the continuous/discontinuous
boundary, I
OUT(CDB)
, is given by the following equation:
I
OUT(CDB)
=
V
IN(MAX)
2
• |V
OUT
|+V
D
( )
2•L • f • V
IN(MAX)
+|V
OUT
|+V
D
( )
2
The continuous/discontinuous boundary is inversely
proportional to the inductor value. Therefore, if required,
I
OUT(CDB)
can be reduced by increasing the inductor value.
External Soft-Start and Output Tracking
Start-up characteristics are controlled by the voltage on
the SS pin. When the voltage on the SS pin is less than
the internal 0.8V reference, the LTC3863 regulates the
V
FB
pin voltage to the voltage on the SS pin. When the SS
pin is greater than the internal 0.8V reference, the V
FB
pin
voltage regulates to the 0.8V internal reference. The SS
pin can be used to program an external soft-start function
or to allow V
OUT
to track another supply during start-up.
Soft-start is enabled by connecting a capacitor from
the SS pin to ground. An internal 10µA current source
charges the capacitor, providing a linear ramping voltage
at the SS pin that causes V
OUT
to rise smoothly from 0V
to its final regulated value. The total soft-start time will
be approximately:
t
SS
= C
SS
•
0.8V
10µA
When the LTC3863 is configured to track another supply,
a voltage divider can be used from the tracking supply to
the SS pin to scale the ramp rate appropriately. Tw o com
-
mon implementations of tracking as shown in Figure5a
are coincident and ratiometric. For coincident tracking,
LTC3863
20
3863fa
For more information www.linear.com/3863
applicaTions inForMaTion
choose the divider ratio for the external supply as shown
in Figure 5b. Ratiometric tracking could be achieved by
using a different ratio than the feedback (Figure 5b).
Note that the soft-start capacitor charging current is always
flowing, producing a small offset error. To minimize this
error, select the tracking resistive divider values to be small
enough to make this offset error negligible.
Short-Circuit Faults: Current Limit and Foldback
The inductor current limit is inherently set in a current mode
controller by the maximum sense voltage and R
SENSE
. In
the LTC3863, the maximum sense voltage is 95mV, mea-
sured across
the inductor sense resistor, R
SENSE
, placed
across the V
IN
and SENSE pins. The output current limit
is approximately:
I
LIMIT(MIN)
=
95mV
R
SENSE
–
∆I
L
2
•
V
IN(MIN)
V
IN(MIN)
+|V
OUT
|+V
D
( )
The current limit must be chosen to ensure that I
LIMIT(MIN)
> I
OUT(MAX)
under all operating conditions. The inductor
current limit should be greater than the inductor current
required to produce maximum output power at worst-case
efficiency. For the LTC3863, both minimum and maximum
V
IN
cases should be checked to determine the worst-case
efficiency.
Short-circuit fault protection is assured by the combination
of current limit and frequency foldback. When the output
feedback voltage, V
FB
, drops below 0.4V, the operating
frequency, f, will fold back to a minimum value of 0.18•f
when V
FB
reaches 0V. Both current limit and frequency
foldback are active in all modes of operation. In a short-
circuit fault condition, the output current is first limited
by current limit and then further reduced by folding back
the operating frequency as the short becomes more se
-
vere. The
worst-case fault condition occurs when V
OUT
is shorted to ground.
Short-Circuit Recovery and Internal Soft-Start
An internal soft-start feature guarantees a maximum posi
-
tive output voltage slew rate in all operational cases. In a
short-cir
cuit recovery condition for example, the output
recovery rate is limited by the internal
soft-start so that
output
voltage overshoot and excessive inductor current
buildup is prevented.
The internal soft-start voltage and the external SS pin
operate independently. The output will track the lower of
the two voltages. The slew rate of the internal soft-start
voltage is roughly 1.2V/ms, which translates to a total
soft-start time of 650µs. If the slew rate of the SS pin is
greater than 1.2V/ms the output will track the internal soft-
start ramp. To assure robust fault recovery, the internal
soft-start feature is active in all operational cases. If a
short-circuit condition occurs which causes the output to
drop significantly, the internal soft-start will assure a soft
recovery when the fault condition is removed.
The internal soft-start assures a clean soft ramp-up from
any fault condition that causes the output to droop, guar
-
anteeing a maximum ramp rate in soft-start, short-circuit
fault
release. Figure 6 illustrates how internal soft-start
controls the output ramp-up rate under varying scenarios.
Figure 6. Internal Soft-Start (6a) Allows Soft-Start without an
External Soft-Start Capacitor and Allows Soft Recovery from (6b)
a Short-Circuit
TIME~650µs
(6a)
––V
OUT
V
IN
VOLTAGE
3863 F06
INTERNAL SOFT-START INDUCED START-UP
(NO EXTERNAL SOFT-START CAPACITOR)
TIME
SHORT-CIRCUIT
(6b)
–V
OUT
VOLTAGE
INTERNAL SOFT-START
INDUCED RECOVERY
LTC3863
21
3863fa
For more information www.linear.com/3863
applicaTions inForMaTion
V
IN
Undervoltage Lockout (UVLO)
The LTC3863 is designed to accommodate applications
requiring widely varying power input voltages from
3.5V to 60V. To accommodate the cases where V
IN
drops significantly once in regulation, the LTC3863 is
guaranteed to operate down to a V
IN
of 3.5V over the
full temperature range.
The implications of both the UVLO rising and UVLO falling
specifications must be carefully considered for low V
IN
operation. The UVLO threshold with V
IN
rising is typi-
cally 3.5V (with a
maximum of 3.8V) and UVLO falling is
typically 3.25V (with a maximum of 3.5V). The operating
input voltage range of the LTC3863 is guaranteed to be
3.5V to 60V over temperature, but the initial V
IN
ramp
must exceed 3.8V to guarantee start-up.
Minimum On-Time Considerations
The minimum on-time, t
ON(MIN)
, is the smallest time
duration that the LTC3863 is capable of turning on the
power MOSFET, and is typically 220ns. It is determined
by internal timing delays and the gate charge required to
turn on the MOSFET. Low duty cycle applications may
approach this minimum on-time limit, so care should be
taken to ensure that:
t
ON(MIN)
<
|V
OUT
|+V
D
( )
f • V
IN(MAX)
+|V
OUT
|+V
D
( )
If the duty cycle falls below what can be accommodated
by the minimum on-time, the controller will skip cycles.
However, the output voltage will continue to regulate.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
the dominant contributors and therefore where efficiency
improvements can be made. Percent efficiency can be
expressed as:
% Efficiency = 100% - (L1+L2+L3+…)
where L1, L2, L3, etc., are the individual losses as a per
-
centage of input power.
Although
all dissipative elements in the circuit produce
losses, four main sources account for most of the losses
in LTC3863 application circuits.
1. Conduction Loss: Conduction losses result from the
P-channel MOSFET R
DS(ON)
, inductor resistance DCR,
the current sense resistor R
SENSE
, and input and output
capacitor ESR. The current through DCR is continuous.
The currents through both the P-channel MOSFET and
Schottky diode are discontinuous. The following equa
-
tion may be used to determine the total conduction loss
(P
COND
) in continuous conduction mode:
P
COND
≈
I
OUT
2
1–D
( )
2
+
∆I
L
2
12
•
R
DCR
+D• R
DS(ON)
+R
SENSE
+R
ESR(CIN)
( )
+ 1–D
( )
•R
ESR(COUT)
2. Transition Loss: Transition loss of the P-channel
MOSFET becomes significant
only when operating
at high input voltages (typically 20V or greater.) The
P-channel transition losses (P
PMOSTRL
) can be deter-
mined from the following equation:
P
PMOSTRL
=
f • C
MILLER
• V
IN
+|V
OUT
|+V
D
( )
2
2
•
I
OUT
1–D
•
R
DN
V
IN
– V
CAP
( )
– V
MILLER
+
R
UP
V
MILLER
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LTC3863IDE#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 60V Low IQ Inverting Buck-Boost DC/DC PMOS Controller
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