OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT3645EMSE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3645
13
3645f
APPLICATIONS INFORMATION
boost circuit. If the input voltage is ramped slowly, or if
the LT3645 is turned on with the EN/UVLO pin when the
output is already in regulation, then the boost capacitor
might not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load generally
goes to zero once the circuit has started. The worst case
situation is when V
IN
is ramping very slowly. Figure 4a
shows the minimum input voltage needed to start a 5V
application versus output current. Figure 4b shows the
minimum input voltage needed to start a 3.3V application
versus output current.
Soft-Start
The LT3645 includes a 500µs internal soft-start for the
buck converter and a 500µs soft-start for the LDO regula-
tor. Both soft-starts are reset if the EN/UVLO pin is low, if
V
IN
drops below 3.4V (undervoltage), if V
IN
exceeds 36V
(overvoltage), or when the die temperature exceeds 160°C
Figure 4.
(4a) Typical Minimum Input Voltage,
V
OUT
= 5V
(4b) Typical Minimum Input Voltage,
V
OUT
= 3.3V
(thermal shutdown). The soft-start for the LDO can also be
reset by pulling the EN2 pin low. The soft-start functions
act to reduce the maximum input current during startup.
Soft-start can not be disabled in the LT3645.
Reversed Input Protection
In some systems, the output will be held high when the
input to the LT3645 is absent. This may occur in bat-
tery charging applications or in battery backup systems
where a battery or some other supply is diode OR’d with
the LT3645’s output. If the V
IN
pin is allowed to fl oat and
the EN/UVLO pin is held high (either by a logic signal
or because it is tied to V
IN
), then the LT3645’s internal
circuitry will draw its quiescent current through its SW
pin. This is fi ne if the system can tolerate a few mA in this
state. You can reduce this current by grounding the EN/
UVLO pin, then the SW pin current will drop to essentially
zero. However, if the V
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT3645 can
OUTPUT CURRENT (mA)
1
5.0
INPUT VOLTAGE (V)
5.5
6.5
6.0
7.5
7.0
8.0
10
100
3645 F04a
1000
V
IN
TO RUN
V
IN
TO START
OUTPUT CURRENT (mA)
1
3.0
INPUT VOLTAGE (V)
4.0
3.5
5.0
4.5
6.0
5.5
6.5
10
100
3645 F04b
1000
V
IN
TO RUN
V
IN
TO START
LT3645
14
3645f
APPLICATIONS INFORMATION
pull large currents from the output through the SW pin
and the V
IN
pin. Figure 5 shows a circuit that will run only
when the input voltage is present and that protects against
a shorted or reversed input.
Frequency Compensation (Buck)
The LT3645 uses current mode control to regulate the
loop. This simplifi es loop compensation. In particular, the
LT3645 does not require the ESR of the output capacitor
for stability, allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size. A low
ESR output capacitor will typically provide for a greater
margin of circuit stability than an otherwise equivalent
capacitor with higher ESR, although the higher ESR will
tend to provide a faster loop response. Figure 6 shows an
equivalent circuit for the LT3645 control loop.
Figure 5. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output; It Also Protects the Circuit from
a Reversed Input. The LT3645 Runs Only When the Input Is Present
Figure 6. Model for Loop Response
The error amplifi er (g
m
) is a transconductance type with
fi nite output impedance. The power section, consisting
of the modulator, power switch, and inductor, is modeled
as a transconductance amplifi er (G) generating an output
current proportional to the voltage at the V
C
node. Note
that the output capacitor integrates this current, and that
the capacitor on the V
C
node (C
C
) integrates the error
amplifi er output current, resulting in two poles in the
loop. R
C
provides a zero. With the recommended output
capacitor, the loop crossover occurs above the R
C
C
C
zero.
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. With a larger
ceramic capacitor that will have lower ESR, crossover may
be lower and a phase lead capacitor connected across
R1 in the feedback divider may improve the transient
response. Large electrolytic capacitors may have an ESR
3645 F05
V
IN
D4
GND
SW
DA
V
CC2
NPG
FB
OUT2
V
IN
EN/UVLO
EN2
FB2
BACKUP
BOOST
LT3645
G
R
C
R1
R2
g
m
= 100µA/V
G = 1A/V
R
C
= 150k
C
C
= 60pF
OUT
C
OUT
CERAMIC
C
C
1M
0.8V
g
m
C
PL
ELECTROLYTIC
ESR
+
3645 F06
LT3645
15
3645f
large enough to create an additional zero, and the phase
lead might not be necessary. If the output capacitor is
different than the recommended capacitor, stability should
be checked across all operating conditions, including input
voltage and temperature.
Figure 7 shows the transient response of the LT3645 with a
few output capacitor choices. The output is 3.3V. The load
current is stepped from 0.25A to 0.5A and back to 0.25A,
and the oscilloscope traces show the output voltage. The
upper photo shows the recommended value. The second
photo shows the improved response (faster recovery)
resulting from a phase lead capacitor.
APPLICATIONS INFORMATION
Frequency Compensation (LDO)
The LT3645 LDO requires an output capacitor for stability.
It is designed to be stable with most low ESR capacitors
(typically ceramic, tantalum or low ESR electrolytic). A
minimum output capacitor of 2.2F with an ESR of 0.5
or less is recommended to prevent oscillations. Larger
values of output capacitance decrease peak deviations
and provide improved transient response for larger load
current changes. Bypass capacitors, used to decouple
individual components powered by the LT3645, increase
the effective output capacitor value. For improvement in
transient performance, place a capacitor across the OUT2
Figure 7.
With Phase Lead Capacitor
No Phase Lead Capacitor
pin and the FB2 pin. Capacitors up to 1nF can be used. This
bypass capacitor reduces system noise as well.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics used are specifi ed with EIA temperature char-
acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but they tend to have strong voltage
and temperature coeffi cients as shown in Figures 8 and 9.
When used with a 5V regulator, a 16V 10F Y5V capaci-
tor can exhibit an effective value as low as 1F to 2F
for the DC bias voltage applied and over the operating
Figure 8. Ceramic Capacitor DC Bias Characteristics
Figure 9. Ceramic Capacitor Temperature Characteristics
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3645 F08
20
0
–20
–40
–60
–80
–100
0
4
8
10
26
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
3645 F09
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24

LT3645EMSE#TRPBF

Mfr. #:
Buy LT3645EMSE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 36Vin (55V Trans) 500mA Buck plus 200mA LDO
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet