LINEAR TECHNOLOGY LT3570 Technical data

L DESIGN FEATURES
V
IN1
SHDN1 SHDN2 SHDN3
SHDN1 SHDN2 SHDN3
SW1
FB1
100nF
1nF
100nF
10µF
1µF
2.5V 40mA
31.6k
3.3V 1A
22k
143k
12V
275mA
5V
22k
10.2k
10.2k
21.5k
10.2k
22.1k
10µF
10µF
1nF
100nF
3.3µH
4.7µH
D2
D3
D1
BOOST
SW2
FB2
SS2
NPN_DRV
FB3
V
C2
SS1 V
C1
V
IN2VIN3
BIAS
RTSYNC
LT3570
GND
Q1
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Buck, Boost and LDO Regulators Combined in a 4mm × 4mm QFN
Introduction
The LT3570 simp lifies compl ex multi-rail power supply designs by integrating three DC/DC regulators into a single package: a current mode buck regulator, a current mode boost regulator, and an LDO controller.
The buck and boost regulators each have a current limit of 1.5A. The LDO controller has an output current capability of 10mA and combines with an external NPN transistor to create a linear regulator. The frequency of the switching regulators can be set from 500kHz to 3MHz by an external resistor or synchronized to an exter­nal oscillator. The independent input voltages for each regulator offers a wide operating range from 2.5V up to 40V. Each regulator also has its own shutdown circuitry and the buck and boost regulators have their own soft­start circuitry.
The typical application shown in Figure 1 generates 3.3V at 1A from the buck regulator, 2.5V at 40mA from the LDO controller and 12V at 275mA
from the boost regulator, all from a 5V input supply voltage and with an overall efficiency around 85%.
Features
Available in either a 24-lead 4mm × 4mm QFN or a 20-pin TSSOP package, the LT3570 is a constant frequency current mode regulator. If all SHDN pins are held low, zero quiescent current is drawn from the input supplies and the part is turned
The LT3570 simplifies
complex multi-rail
power supply designs by
integrating three DC/DC
regulators into a single
package: a current mode
buck regulator, a current
mode boost regulator, and
an LDO controller.
by Chris Falvey
off. Any SHDN pin voltage exceeding
1.5V will turn on the corresponding regulator. A precise shutdown pin threshold allows for easy integration of input supply undervoltage lockout. All three regulators share the same internal 800mV reference voltage. For each regulator, an external resistor divider programs the output voltage to any value above the part’s reference voltage. The switching frequency is set with an external resistor from the RT pin to GND. This allows a trade off between minimizing component size (by using higher switching frequen­cies) and maximizing efficiency (by using lower switching frequencies). Additionally, running at a low switch­ing frequency allows for applications that require larger VIN-to-V The adjustable and synchronizable switching frequency also allows the user to keep the switching noise out of critical wireless and audio bands.
Both the buck and boost regulators
control the slew rate of the output
ratios.
OUT
28
Figure 1. A typical 5V input to 3.3V, 2.5V and 12V application
Linear Technology Magazine • June 2008
V
IN1
SHDN1 SHDN2 SHDN3
SHDN1 SHDN2 SHDN3
SW1
FB1
C8 100nF
C7 1nF
C5 10nF
C2
10µF
C3 1µF
V
OUT3
3.3V 500mA
R3 340k
V
OUT2
5V
R8 25k
R1 100k
V
OUT1
8V
250mA
V
IN
8V TO 30V
R7 25k
R2 11k
R4
64.9k
R5 205k
R6
64.9k
R9
24.9k
C9 10µF
C1
10µF
C6 1nF
10nF
L2
10µH
10µH
D2
D3
D1
BOOST
SW2
FB2 SS2
NPN_DRV
FB3
V
C2
SS1 V
C1
V
IN2VIN3
BIAS
RTSYNC
LT3570
GND
Q1
300µs PROPAGATION DELAY
– 2mV DC STEP
I
OUT1
200mA/
DIV
V
OUT1
100mV/
DIV
V
OUT2
100mV/
DIV
100µs/DIV
300µs PROPAGATION DELAY
– 2mV DC STEP
V
OUT1
20V/
DIV
V
IN
5V/
DIV
V
OUT2
5V/
DIV
1ms/DIV
Figure 2. Dying gasp system keeps power even when battery is disconnected.
V
IN1
SHDN1 SHDN2 SHDN3
SW1
FB1
C8 100nF
C7 1nF
C5 10nF
10µF
C3 1µF
V
OUT3
2.5V 100mA
R3 205k
V
OUT2
3.3V 200mA
R8 25k
R1
1.10M
V
OUT1
36V
V
IN
12V
R7 25k
R2
23.7k
R4
64.9k
R5 137k
R6
64.9k
R9
24.9k
C9 10µF
C1
10µF
C6 1nF
C4
10nF
L2
10µH
L1
10µH
D2
D3
D1
BOOST
SW2
FB2
SS2
NPN_DRV
FB3
V
C2
SS1 V
C1
V
IN2VIN3
BIAS
RTSYNC
LT3570
GND
Q1
voltage during start-up. A controlled ramp reduces inrush current on the input supply and minimizes output overshoot. An external capacitor con­nected between the SS pin and ground programs the slew rate. The voltage on the SS pin overrides the internal reference voltage to the error amplifier and is charged by a 4.5µA internal current source.
DESIGN FEATURES L
The BIAS pin allows the internal circuitry to draw current from a lower voltage supply than the input, reduc­ing power dissipation and increasing efficiency. Normally, the quiescent current is supplied from V the voltage on the BIAS pin exceeds
2.5V the current is supplied from the BIAS pin. The BIAS pin is only available on the 24-lead QFN package.
, but when
IN2
Figure 3. Output waveforms when power is removed from the circuit in Figure 2
Applications
“Dying Gasp” Application
The LT3570 provides an ideal solu­tion for any “dying gasp” system. Figure 2 shows a typical application powering an airbag controller. In an automobile accident, the battery may get disconnected from the shock sen­sors yet the airbag must still fire. In this application, the battery supplies power to the boost regulator. V set to 36V and drives V
and V
IN2
the inputs to the buck regulator and the LDO controller, respectively. Even after the input supply is removed, the buck regulator and the LDO continue to function properly for more than 3ms, as the energy continues to be supplied from the output capacitor of the boost regulator. The buck regulator turns off when V
approaches the
IN2
input undervoltage lockout of 2.3V (see Figure 3).
continued on page 41
OUT1
is
IN3,
Figure 4. DSL modem application
Linear Technology Magazine • June 2008
Figure 5. Step response of Figure 4 with boost current stepped from 200mA to 400mA
29
POWER LOSS (mW)
EFFICIENCY (%)
LOAD CURRENT (A)
100.1
100
0
10k
100
1
1k
10
20
30
40
50
60
70
80
90
VIN = 24V
V
OUT3
= 12V
2+1 MODE
EFFICIENCY
POWER LOSS
CONTINUOUS MODE
PULSE-SKIPPING MODE
BURST MODE OPERATION
V
OUT1,2
200mV/DIV
I
L2
2A/DIV
I
L1
2A/DIV
25µs/DIV
VIN = 14V V
OUT1,2
(NOM) = 1.8V
LOAD STEP ON V
OUT1,2
= 1A TO 6A
Figure 2. Post-package trimming of the LTC3853’s current sense comparators provides excellent current sharing between channels 1 and 2, even during a transient.
sync with frequencies between 250kHz and 750kHz.
The LTC3853 can be set to operate in one of three modes under light load conditions. Burst Mode operation of­fers the highest light load efficiency by switching in a “burst” of one to several pulses replenishing the charge stored in the output capacitors, followed by a long sleep period when the load current is supplied by the output capacitors. Forced continuous mode offers fixed frequency operation from no load to full load, providing the lowest output voltage ripple at the cost of light load efficiency. Pulse-skipping mode oper­ates by preventing inductor current reversal by turning off the synchro­nous switch as needed. This mode is a compromise between the other two modes, offering lower ripple than Burst Mode operation and better light load efficiency than forced continuous mode. Regardless of the mode selected, the LTC3853 operates in constant fre­quency mode at higher load currents. Figure 3 shows the efficiency in each of the three modes.
Each of the LTC3853’s channels can be enabled with its own RUN pin, or slewed up or down with its own TRACK/SS pin. Tracking holds the feedback voltage to the lesser of the internal reference voltage or the voltage on TRACK/SS, which can be brought up with an external ramp or with its own 1.2µA internal current source. With all of the TRACK/SS pins held low and any output enabled through its RUN pin, the 5V INTVCC is still available for ancillary keep-alive circuits.
Pulse-skipping mode is always enabled at start-up to prevent sink­ing current from a pre-biased output voltage. When the output reaches 80% of the set value, the part switches over to forced continuous mode until the output has entered the POWER GOOD window, at which point it switches over to the selected mode of opera­tion. Forced continuous mode reduces the output ripple as the power good threshold is crossed, to ensure that the POWER GOOD indicators make just one low to high transition.
Three different max current com­parator sense thresholds can be set via the ILIM pin. The current is sensed using a high speed rail-to-rail differ­ential current sense comparator. The circuit of Figure 1 uses accurate sense resistors between the inductors and the outputs. For reduced power loss at high load currents, the LTC3853 can also monitor the parasitic resistance of the inductor (DCR sensing). Peak inductor current is limited on a cycle­by-cycle basis and is independent of duty cycle. If load current is high enough to cause the feedback voltage
DESIGN IDEAS L
Figure 3. Efficiency for channel 3 in Figure 1— in each of the three modes of operation
to drop, current limit fold back protects the power components by reducing the current limit. For predictable tracking, current limit fold back is disabled during start-up. Input undervoltage lockout, output overvoltage shutdown and thermal shutdown also protect the power components and the IC from damage.
Conclusion
The LTC3853’s small footprint belies its versatility and extensive feature set. From inputs up to 24V it can regulate three separate outputs, or it can be configured for higher currents by tying channels 1 and 2 together. Either way, the phase relationship between channels is automatically optimized to reduce ripple currents. At low duty cycles, the short minimum on-time ensures constant frequency operation, and peak current limit remains constant even as duty cycle changes. The cost-effective LTC3853 incorporates these features, and more, into a 40-pin 6mm × 6mm QFN package.
L
LT3570, continued from page 29
DSL Modem
Figure 4 shows an application for a DSL modem or set-top box. The sup­ply voltage for V adapter that can range from 8V to 30V.
IN2
comes from a wall
This voltage is stepped down to 5V at 100mA for V the power to drive both the boost regulator and LDO controller. V is set to 8V at 200mA and V
Linear Technology Magazine • June 2008
, which then supplies
OUT2
OUT3
is set
OUT1
to 3.3V at 500mA. Figure 5 shows the load step response of V with a 200mA load step on V
OUT1
and V
OUT1
Conclusion
The LT3570 is a monolithic dual output switching regulator (buck and boost) with a NPN LDO controller and is ideal for a broad variety of applica­tions. Because the LT3570 offers a high
OUT2
.
level of system integration, it greatly simplifies board design for complex applications that need multiple volt­age supply rails. With the flexibility of independent supply inputs and adjust­able frequency, the user can set a wide array of custom output voltages. The LT3570 is a feature rich solution that satisfies the needs for multiple output voltages in a compact solution.
L
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