DN493 - Bootstrap Biasing of High Input Voltage Step-Down Controller Increases Converter Efficiency

Bootstrap Biasing of High Input Voltage Step-Down Controller
Increases Converter Efficiency
Design Note 493
Goran Perica and Victor Khasiev
Introduction
High voltage buck DC/DC controllers such as the LTC3890
(dual output) and LTC3891 (single output) are popular in
automotive applications due to their extremely wide 4V to
60V input voltage range, eliminating the need for a snubber and voltage suppression circuitry. These controllers
are also well suited for 48V telecom applications where
no galvanic isolation is required.
In a typical application for these controllers, the IC’s
supply voltage (INTVCC) is provided by the on-chip LDO.
This LDO produces 5V from input voltages up to 60V to
bias control circuitry and provide power FET gate drive.
Although simple, this built-in biasing scheme can be inefficient. Power losses can be significant in applications
where the input voltage is consistently high, such as in
48V telecom applications. Reducing the power losses
in the bias conversion can increase efficiency and also
reduce the controller case operating temperature.
Employing EXTVCC to Improve Efficiency
One of the attractive features of the LTC3890 and LTC3891
controllers is the external power input (EXTVCC). This is
a second on-chip LDO, which can be used to bias the
chip. When the input voltage is consistently high, it is
more efficient to produce the biasing voltage by stepping down the converter’s output voltage, which is fed
into EXTVCC, rather than generating 5V INTVCC from the
high input voltage.
08/11/493
Figure 1 shows a block diagram for this scheme. The output
can be directly connected to the EXTVCC pin of the chip as
long as the output voltage is above 4.7V. However, extra
circuitry (described in the following section) is required
for outputs below 4.7V.
INTVCC
VIN (4.5V to 60V)
LTC3890
LTC3891
VOUT (< 4.7V)
EXTVCC
VOLTAGE DOUBLER OR BOOST
(FIGURE 2 OR FIGURE 3)
DN493 F01
Figure 1. Block Diagram Showing External Bias
Voltage Doubler for Output Voltages Below 4.7V
When the controller’s output is below 4.7V, it must be
stepped up to allow the built-in LDO to work. A simple
voltage doubler solves this problem as long as the output
is higher than 2.5V. Below 2.5V output, a multivibratorbased circuit can be used.
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owners.
Figure 2 shows a simple, low cost solution for output
voltages between 2.5V and 4.7V. This is a voltage doubler scheme based on small P-channel and N-channel
MOSFETs, Q1 and Q2. The gates of these transistors are
controlled by the bottom gate driver, BG of the controller. When BG is high, Q2 is on, Q1 is off and capacitor
C1 charges from output voltage VOUT through D1. When
BG is low, Q2 is off, Q1 is on and capacitor C1 delivers
a voltage close to 2 • VOUT to EXTVCC.
D2
MBR0520
E3
EXTVCC
and output voltage VOUT is stepped up to 5V, which feeds
EXTVCC. The multivibrator frequency is set at 50kHz to
minimize the EMI signature. The pulse width is defined
by the ratio of resistors R1 and R2, as per the following
expressions:
D1
MBR0520
Q1
FDN340P
C1
2.2μF
C2
2.2μF
Q2
NDS331N
R1=
T • (1− D)
0.7 • C1
R2 =
T •D
0.7 • C2
EXTVCC − VOUT
EXTVCC
1
T=
ƒ
D=
E2
VOUT
E1
BG
Conclusion
DN493 F02
Figure 2. Voltage Doubler Allows External Bias
from VOUT in the Range of 2.5V to 4.7V
Figure 3 shows a solution for voltages below 2.5V. The
circuit consists of an astable multivibrator based on
transistors Q1 and Q2, and a boost based on N-channel
Q3 and inductor L1. Q1 and Q2 are biased from INTVCC
The efficiency of high input voltage DC/DC controllers
can be significantly improved by using the controller’s
output voltage to power the IC, instead of allowing the
internal LDO to produce the bias voltage. For input voltages above 30V, efficiency improvements of 2% to 3%
are realized when a voltage doubler circuit is used for a
3.3V at 5A output (see Figure 4). Similar efficiency improvements are shown for a 1.8V at 7A converter with a
multivibrator-based circuit.
92
E1
INTVCC
C1
220pF
R2
37.4k
RC2
1k
C2
220pF
Q1
FMMT2222ATA
L1
LPS5030-334ML
330μH
D1
MBR0520
90
89
E3
EXTVCC
Q2
FMMT2222ATA
Q3
TN0200K
88
STANDARD SOLUTION
87
86
6V
C3
2.2μF
DN493 F03
(%)
VOUT
R1
93.1k
RC1
1k
CHARGE PUMP
91
E2
85
84
VIN = 50V
VOUT = 3.3V
1
1.5
2
2.5
3
(A)
3.5
4
4.5
5
DN493 F04
Figure 3. Boost Controlled By Astable Multivibrator
Is Used for VOUT Lower Than 2.5V
Data Sheet Download
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Linear Technology Corporation
Figure 4. LTC3890/LTC3891
Efficiency Improvement
For applications help,
call (408) 432-1900, Ext. 2134
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