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PDF LTC3704 Data sheet ( Hoja de datos )
| Número de pieza | LTC3704 | |
| Descripción | Wide Input Range / No RSENSE Positive-to-Negative DC/DC Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LTC3704 (archivo pdf) en la parte inferior de esta página. Total 28 Páginas | ||
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.com LTC3704TM
U Wide Input Range, No RSENSE
FEATUREtSaSheet4 Positive-DtoES-NCReIgPaTItOive DC/DC Controllers High Efficiency Operation (No Sense
aResistor Required)
.Ds Wide Input Voltage Range: 2.5V to 36V
ws Current Mode Control Provides Excellent Transient
wResponse
ws High Maximum Duty Cycle (Typ 92%)
ms ±1% Internal Voltage Reference
os ±2% RUN Pin Threshold with 100mV Hysteresis
.cs Micropower Shutdown: IQ = 10µA
s Programmable Switching Frequency
(50kHz to 1MHz) with One External Resistor
Us Synchronizable to an External Clock Up to 1.3 × fOSC
t4s User-Controlled Pulse Skip or Burst Mode® Operation
s Internal 5.2V Low Dropout Voltage Regulator
s Capable of Operating with a Sense Resistor for High
eOutput Voltage Applications (VDS >36V)
es Small 10-Lead MSOP Package
APPLICATIOUS Shs SLIC Power Supplies
tas Telecom Power Supplies
s Portable Electronic Equipment
as Cable and DSL Modems
s Router Supplies
The LTC®3704 is a wide input range, current mode,
positive-to-negative DC/DC controller that drives an
N-channel power MOSFET and requires very few external
components. Intended for low to high power applications,
it eliminates the need for a current sense resistor by
utilizing the power MOSFET’s on-resistance, thereby maxi-
mizing efficiency.
The IC’s operating frequency can be set with an external
resistor over a 50kHz to 1MHz range, and can be synchro-
nized to an external clock using the MODE/SYNC pin.
Burst Mode operation at light loads, a low minimum
operating supply voltage of 2.5V and a low shutdown
quiescent current of 10µA make the LTC3704 ideally
suited for battery-operated systems.
For applications requiring constant frequency operation,
the Burst Mode operation feature can be defeated using
the MODE/SYNC pin. Higher than 36V switch voltage
applications are possible with the LTC3704 by connecting
the SENSE pin to a resistor in the source of the power
MOSFET.
The LTC3704 is available in the 10-lead MSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
No RSENSE is a trademark of Linear Technology Corporation.
TYPICAL APPwLICwATIwO .D .comRC
U3k
et4CC1
taShe4.7nF
R1
1M
1
RUN
2
ITH
10
SENSE
9
VIN
LTC3704
38
NFB INTVCC
47
FREQ
GATE
56
MODE/SYNC GND
RT
80.6k
1%
RFB1
1.21k
1%
RFB2
3.65k
1%
CVCC
4.7µF
•
L1*
•
CDC
47µF
L2*
M1
D1
CIN
47µF
VIN
5V to 15V
VOUT
–5.0V
3A to 5A
COUT
100µF
(X2)
GND
3704 TA01
aCIN, CDC : TDK C5750X5R1C476M
COUT: TDK C5750X5R0J107M
.DCVCC: TAIYO YUDEN LMK316BJ475ML
D1: MBRD835L (ON SEMICONDUCTOR)
L1, L2: BH ELECTRONICS BH510-1009
M1: Si4884 (SILICONIX/VISHAY)
www 1Figure 1. High Efficiency Positive to Negative Supply
Conversion Efficiency
100
90
80 VIN = 5V
70 VIN = 15V
60
VIN = 10V
50
40
30
20
0.001
0.01 0.1
1.0
OUTPUT CURRENT (A)
10
3704 TA01b
3704f
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
RUN Thresholds vs VIN
1.5
RUN Thresholds vs Temperature
1.40
LTC3704
RT vs Frequency
1000
1.35
1.4
1.30
1.3
1.25
100
1.2
0
10 20 30
VIN (V)
40
3704 G10
Frequency vs Temperature
325
320
315
310
305
300
295
290
285
280
275
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
3704 G13
INTVCC Load Regulation
TA = 25°C
5.2
5.1
5.0
0
10 20 30 40 50 60 70 80
INTVCC LOAD (mA)
3704 G16
1.20
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
3704 G11
Maximum Sense Threshold
vs Temperature
160
155
150
145
140
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
3704 G14
INTVCC Line Regulation
5.4
TA = 25°C
5.3
5.2
5.1
0
5 10 15 20 25 30 35 40
VIN (V)
3704 G17
10
0 100 200 300 400 500 600 700 800 900 1000
FREQUENCY (kHz)
3704 G12
SENSE Pin Current vs Temperature
45
GATE HIGH
VSENSE = 0V
40
35
–50 –25 0 25 50 75 100 125 150
TEMPERATURE (°C)
INTVCC Dropout Voltage
vs Current, Temperature
500
3704 G15
450
150°C
400
125°C
350
75°C
300
25°C
250
200 0°C
150
–50°C
100
50
0
0 5 10 15 20
INTVCC LOAD (mA)
3704 G18
3704f
5
5 Page 
LTC3704
APPLICATIO S I FOR ATIO
cause the LTC3704 to exceed its maximum junction
temperature rating. The junction temperature can be
estimated using the following equations:
IQ(TOT) ≈ IQ + f • QG
PIC = VIN • (IQ + f • QG)
TJ = TA + PIC • RTH(JA)
The total quiescent current IQ(TOT) consists of the static
supply current (IQ) and the current required to charge and
discharge the gate of the power MOSFET. The 10-pin
MSOP package has a thermal resistance of RTH(JA) =
120°C/W.
As an example, consider a power supply with VIN = 5V and
VSW(MAX) = 12V. The switching frequency is 500kHz, and
the maximum ambient temperature is 70°C. The power
MOSFET chosen is the IRF7805, which has a maximum
RDS(ON) of 11mΩ (at room temperature) and a maximum
total gate charge of 37nC (the temperature coefficient of
the gate charge is low).
IQ(TOT) = 600µA + 37nC • 500kHz = 19.1mA
PIC = 5V • 19.1mA = 95mW
TJ = 70°C + 120°C/W • 95mW = 81.4°C
This demonstrates how significant the gate charge current
can be when compared to the static quiescent current in
the IC.
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in a continuous mode at high VIN. A tradeoff
between the operating frequency and the size of the power
MOSFET may need to be made in order to maintain a
reliable IC junction temperature. Prior to lowering the
operating frequency, however, be sure to check with
power MOSFET manufacturers for their latest-and-great-
est low QG, low RDS(ON) devices. Power MOSFET manu-
facturing technologies are continually improving, with
newer and better performance devices being introduced
almost yearly.
Output Voltage Programming
The output voltage is set by a resistor divider according to
the following formula:
VO = VREF • 1+ RR21 + INFB •R2
where VREF = –1.230V, and INFB is the current which flows
out of the NFB pin (INFB = –7.5µA). In order to properly
dimension R2, including the effect of the NFB pin current,
the following formula can be used:
R2
=
VOUT
VREF
R1
− VREF
+ INFB
The nominal 7.5µA current which flows out of the NFB pin
has a production tolerance of approximately ±2.5µA, so an
output divider current of 500µA (R1 = 2.49k) results in a
0.5% uncertainty in the output voltage. For low power
applications where the output voltage tolerance is less
important, efficiency can be increased by increasing the
value of R1.
Programming Turn-On and Turn-Off Thresholds
with the RUN Pin
The LTC3704 contains an independent, micropower volt-
age reference and comparator detection circuit that re-
mains active even when the device is shut down, as shown
in Figure 8. This allows users to accurately program an
input voltage at which the converter will turn on and off.
The falling threshold voltage on the RUN pin is equal to the
internal reference voltage of 1.248V. The comparator has
100mV of hysteresis to increase noise immunity.
The turn-on and turn-off input voltage thresholds are
programmed using a resistor divider according to the
following formulas:
VIN(OFF) = 1.248V • 1+ RR21
VIN(ON) = 1.348V • 1+ RR21
3704f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3704.PDF ] | |
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