DN255 - LT1619: Tiny Boost Controller Provides Efficient Solutions for Low Voltage Inputs

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Tiny Boost Controller Provides Efficient Solutions
for Low Voltage Inputs – Design Note 255
Keith Szolusha
The LT1619 provides a complete solution for low input
voltage applications that require low side MOSFET drive.
It is a 300kHz, current mode PWM controller capable
of operating from inputs ranging from 1.9V to 18V. The
rail-to-rail, 1A MOSFET driver is capable of driving an
external MOSFET gate to within 350mV of the supply
rail and to within 100mV of ground. Bootstrapping the
driver supply pin (DRV) to the output enables the power
supply to operate from input voltages as low as 1.9V yet
still drive the MOSFET gate voltage high enough for full
enhancement. The 53mV low side current limit threshold
R2
12.4k
R1
37.4k
1
2
3
220pF
75k 4
S/S
VIN
FB
DRV
LT1619
VC
GND
GATE
SENSE
15nF
8
7
6
5
+
0.1μF
C1
22μF
improves efficiency by reducing the sense resistor’s
power dissipation. At light loads, the controller automatically switches to Burst Mode® operation to conserve
power. In shutdown, the LT1619 requires only 15μA of
quiescent current.
3.3V to 5V Converters
Figure 1 shows a 3.3V to 5V/2.2A boost supply using
the LT1619. Low parts count, small size and high efficiency (greater than 90%) make it a perfect solution
when a moderate amount of 5V power is required in a
predominately 3.3V system. The output voltage can be
returned to the DRV pin, further enhancing M1.
Low current-sense voltage, although more efficient, can
be more susceptible to switching noise. However, the
internal current sense amplifier is blanked for 280ns
to prevent spurious switching spikes (and therefore
PWM jitter) across the sense resistor caused by the
gate charging current at switch turn-on. Although this
blanking sets a minimum switch on-time, the controller
is capable of skipping cycles at light load with Burst
Mode operation disabled.
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
VIN
3.3V
L1
5.6μH
5A
D1
0.1μF
M1
Si9804
+
95
COUT
440μF
RSENSE
0.01Ω
DN257 F01a
C1: PANASONIC EEFCDOK220R
COUT: KEMET T495X227K010AS (w2)
D1: MBRD835L
L1: COILCRAFT DO5022P-562
M1: VISHAY Si9804
RSENSE: IRC LRC-LRF1206-01-R010-F
Figure 1a. 3.3V to 5V Step-Up Converter Rated for 2.2A
04/01/255_conv
VIN = 3.3V
VOUT = 5V
90
VOUT
5V
2.2A
EFFICIENCY (%)
Introduction
The expanding world of low voltage portable and microprocessor electronics has created the need for small,
highly efficient, low cost, low voltage boost controllers with high current capabilities. The new LT®1619,
available in a tiny MS8 package, is a high current, low
input voltage boost controller with a powerful rail-torail MOSFET driver and ultralow current-sense voltage
(53mV). These features, used in the common boost or
versatile SEPIC topologies, provide low cost, efficient
and tiny DC/DC solutions for many applications, including 3.3V to 5V converters and automotive-range 12V
to 5V converters.
85
80
75
70
1
10
100
LOAD CURRENT (mA)
1000
DN257 F01b
Figure 1b. Efficiency of Figure 1
Choosing the MOSFET
The LT1619 is designed to drive an N-channel MOSFET
with up to 60nC of total gate charge (QG). Recently,
significant advances have been made in low voltage
(<30V) power MOSFETs. 10mΩ, low voltage, low
threshold FETs with less than 60nC of gate charge are
readily available. MOSFETs with less than 60nC of gate
charge can be driven directly by the LT1619, resulting
in a simple, low cost design.
Automotive Supply
Figure 2 shows a 5V, 1A SEPIC (single-ended primary
inductance converter) designed to operate from a 12V
battery. The bias supply on VIN and DRV limits the
voltage to about 4V maximum at start-up, limiting
the amount of quiescent power lost and maintaining a
high efficiency. The LT1619 is powered from the output
through D2 after regulation is achieved, increasing
efficiency.
The low input voltage threshold of the LT1619 allows
the battery voltage to drop to 3V or less during normal
operation (cold-crank support). With a sublogic-level
MOSFET, this converter can still be started at voltage
levels as low as 3V without having to add extra components. The power supply will also work well with a
battery voltage as high as 18V, which provides margin
for battery charging voltage (15V) plus several volts of
inductive spiking.
SEPIC capacitor C2 provides a path for continuous input
current and directs T1’s leakage energy to the output.
Using C2 increases efficiency and reduces input capacitor ripple current requirements. The LT1619’s 300kHz
operating frequency allows for smaller magnetics (0.45"
× 0.45" × 0.25” h) and smaller capacitors than required
by lower frequency controllers.
Conclusion
The LT1619 solves many of the problems associated
with low input voltage source DC/DC converters. Its
numerous features make it an ideal choice for a wide
range of applications requiring low side MOSFET power
transistors, high efficiency, low cost and high currents
in very little board space.
T1
CTX10-4
VIN
4V TO 18V
3.2k
t
D2
Q1
UPS120
FMMT3904
VOUT
8
7
VIN DRV OUT
SENSE
C1
220μF
1μF
4.7V
ZENER
LT1619
VFB
47Ω
5
4
+VOUT
C2
10μF
25V
37.4k
CERAMIC
L2
DT1608C-222
VOUT
5V
0A-1A
1μF
CERAMIC
C6
100μF
C7
100μF
10nF
3
220pF
R5
75k
C5
15nF
GND
D1
UPS120
2
SS GND VC
1
Q2
6
t
RSENSE
0.012Ω
12.4k
1000pF
GND
DN257 F02
C1: SANYO 25CV226AX 220μF
C6, C7: CORNELL DUBILRER ESRE101M08R
Q2: FAIRCHILD FDC655AN
RSENSE: IRC LRC-LRF1206-01-R012-F
Figure 2. SEPIC Conversion Delivers 5V at 1A from a 12V Battery
Data Sheet Download
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dn255f_conv LT/TP 0401 345K • PRINTED IN THE USA
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