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PDF LTC3836 Data sheet ( Hoja de datos )
| Número de pieza | LTC3836 | |
| Descripción | No RSENSETM Low VIN Synchronous Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
n No Current Sense Resistors Required
n Out-of-Phase Controllers Reduce Required Input
Capacitance
n All N-Channel Synchronous Drive
n VIN Range: 2.75V to 4.5V
n Constant-Frequency Current Mode Operation
n 0.6V ±1.5% Voltage Reference
n Low Dropout Operation: 97% Duty Cycle
n True PLL for Frequency Locking or Adjustment
n Selectable Pulse-Skipping/Continuous Operation
n Tracking Function
n Internal Soft-Start Circuitry
n Power Good Output Voltage Monitor
n Output Overvoltage Protection
n Micropower Shutdown: IQ = 6.5μA
n Tiny Low Profile (4mm × 5mm) QFN and Narrow
SSOP Packages
APPLICATIONS
n General Purpose 3.3V to 1.X Supplies
n Single Lithium-Ion Powered Devices
n Distributed DC Power Systems
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. No RSENSE
is a trademark of Linear Technology Corporation. All other trademarks are the property of their
respective owners. Protected by U.S. Patents including 5481178, 5929620, 6144194, 6304066,
6498466, 6580258, 6611131.
LTC3836
Dual 2-Phase,
No RSENSETM Low VIN
Synchronous Controller
DESCRIPTION
The LTC®3836 is a 2-phase dual output synchronous
step-down switching regulator controller with tracking
that drives external N-channel power MOSFETs using few
external components. The constant-frequency current
mode architecture with MOSFET VDS sensing eliminates
the need for sense resistors and improves efficiency.
The power loss and noise due to the ESR of the input
capacitance are minimized by operating the two control-
lers out-of-phase. Pulse-skipping operation provides high
efficiency at light loads. The 97% duty cycle capability
provides low dropout operation, extending operating time
in battery-powered systems.
The operating frequency is selectable from 300kHz to
750kHz, allowing the use of small surface mount induc-
tors and capacitors. For noise sensitive applications, the
LTC3836 operating frequency can be externally synchro-
nized from 250kHz to 850kHz.
The LTC3836 features an internal 1ms soft-start that can
be extended with an external capacitor. A tracking input al-
lows the second output to track the first during start-up.
The LTC3836 is available in the tiny thermally enhanced
(4mm × 5mm) QFN and 28-lead narrow SSOP packages.
TYPICAL APPLICATION
High Efficiency, 2-Phase, Dual Synchronous DC/DC Step-Down Converter
VOUT1
1.8V AT
15A
VIN
SBEONOSSET11+ SBEONOSSET22+
TG1 TG2
0.47μH
SW1 SW2
LTC3836
BG1 BG2
118k
100μF
×2
PGND
820pF
VFB1
VFB2
ITH1 ITH2
SGND
59k 15k
820pF
15k
22μF
×3
VIN
3.3V
0.47μH
59k
59k
VOUT2
1.2V AT
15A
100μF
×2
3836 TA01
Efficiency/Power Loss vs Load Current
100 3.3V-1.8V
90 EFFICIENCY
10000
80 3.3V-1.2V
EFFICIENCY
70
1000
60
50
3.3V-1.2V
40 POWER LOSS
30 3.3V-1.8V
20 POWER LOSS
100
10
10
0
10
CIRCUIT OF FIGURE 15
1
100 1000 10000 100000
LOAD CURRENT (mA)
3836 TA01b
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1 page  
LTC3836
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted.
Maximum Current Sense Voltage
vs ITH Pin Voltage
www.dat1a0s0heet4u.com
FORCED CONTINUOUS
MODE
80 PULSE-SKIPPING
MODE
60
40
20
0
–20
0.5
1 1.5
ITH VOLTAGE (V)
2
3836 G09
RUN/SS Pull-Up Current
vs Temperature
1.0
0.9
0.8
0.7
0.6
0.5
0.4
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3836 G13
Undervoltage Lockout Threshold
vs Temperature
2.50
2.45
VIN RISING
2.40
2.35
2.30
VIN FALLING
2.25
2.20
2.15
2.10
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3836 G16
Regulated Feedback Voltage
vs Temperature
0.606
0.605
0.604
0.603
0.602
0.601
0.600
0.599
0.598
0.597
0.596
0.595
0.594
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3836 G11
Maximum Current Sense
Threshold vs Temperature
135 IPRG = FLOAT
130
125
120
115
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3836 G14
Shutdown Quiescent Current
vs Input Voltage
18
16
14
12
10
8
6
4
2
0
2.5 3.0 3.5 4.0 4.5
INPUT VOLTAGE (V)
3836 G17
Shutdown (RUN/SS) Threshold
vs Temperature
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
–60 –40 –20 0 20 40 60 80 100
TEMPERATURE (°C)
3836 G12
Oscillator Frequency
vs Temperature
10
8
6
4
2
0
–2
–4
–6
–8
–10
–60 –40 –20 0 20 40 60
TEMPERATURE (°C)
80 100
3836 G15
RUN/SS Start-Up Current
vs Input Voltage
0.9
RUN/SS = 0V
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2.5 3.0 3.5 4.0
INPUT VOLTAGE (V)
4.5
3836 G18
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5 Page 
LTC3836
OPERATION (Refer to Functional Diagram)
When the SYNC/FCB pin is tied to a DC voltage above
0.6V or when it is clocked by an external clock source
wwwt.doatuassheeetht4eu.cpohmase-locked loop (see Frequency Selection
and Phase-Locked Loop), the LTC3836 operates in PWM
pulse-skipping mode at light loads. In this mode, the
current comparator ICMP may remain tripped for several
cycles and force the main N-channel MOSFET to stay off
for the same number of cycles. The inductor current is
not allowed to reverse, though (discontinuous operation).
This mode, like forced continuous operation, exhibits low
output ripple as well as low audio noise and reduced RF
interference. However, it provides low current efficiency
higher than forced continuous mode. During start-up or
a short-circuit condition (VFB1 or VFB2 ≤ 0.54V), the
LTC3836 operates in pulse-skipping mode (no cur-
rent reversal allowed), regardless of the state of the
SYNC/FCB pin.
Short-Circuit Protection
When an output is shorted to ground (VFB < 0.12V), the
switching frequency of that controller is reduced to one-
fifth of the normal operating frequency. The other controller
maintains regulation in pulse-skipping mode.
The short-circuit threshold on VFB2 is based on the smaller
of 0.12V and a fraction of the voltage on the TRACK/SS2
pin. This also allows VOUT2 to start up and track VOUT1
more easily. Note that if VOUT1 is truly short-circuited
(VOUT1 = VFB1 = 0V), then the LTC3836 will try to regulate
VOUT2 to 0V if a resistor divider on VOUT1 is connected to
the TRACK/SS pin.
Output Overvoltage Protection
As a further protection, the overvoltage comparator (OV)
guards against transient overshoots, as well as other more
serious conditions that may overvoltage the output. When
the feedback voltage on the VFB pin has risen 13.33% above
the reference voltage of 0.6V, the main N-channel MOSFET
is turned off and the synchronous N-channel MOSFET is
turned on until the overvoltage is cleared.
Frequency Selection and Phase-Locked Loop (PLLLPF
and SYNC/FCB Pins)
The selection of switching frequency is a tradeoff between
efficiency and component size. Low frequency opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
The switching frequency of the LTC3836’s controllers can
be selected using the PLLLPF pin.
If the SYNC/FCB is not being driven by an external clock
source, the PLLLPF can be floated, tied to VIN or tied to
SGND to select 550kHz, 750kHz or 300kHz respectively.
A phase-locked loop (PLL) is available on the LTC3836
to synchronize the internal oscillator to an external clock
source that is connected to the SYNC/FCB pin. In this case,
a series RC should be connected between the PLLLPF pin
and SGND to serve as the PLL’s loop filter. The LTC3836
phase detector adjusts the voltage on the PLLLPF pin to
align the turn-on of controller 1’s top MOSFET to the ris-
ing edge of the synchronizing signal. Thus, the turn-on
of controller 2’s top MOSFET is 180 degrees out-of-phase
with the rising edge of the external clock source.
The typical capture range of the LTC3836’s phase-locked
loop is from approximately 200kHz to 1MHz, and is guar-
anteed over temperature between 250kHz and 850kHz.
In other words, the LTC3836’s PLL is guaranteed to lock
to an external clock source whose frequency is between
250kHz and 850kHz.
Dropout Operation
Each top MOSFET driver is biased from the floating boot-
strap capacitor CB, which normally recharges during each
off cycle through an external diode when the top MOSFET
turns off. If the input voltage VIN decreases to a voltage
close to VOUT, the loop may enter dropout and attempt to
turn on the top MOSFET continuously. The dropout detec-
tor detects this and forces the top MOSFET off for about
200ns every fourth cycle to allow CB to recharge.
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11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC3836.PDF ] | |
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