Aug 1998 LTC1627 Monolithic Synchronous Step-Down Regulator Maximizes Single or Dual Li-Ion Battery Life

DESIGN FEATURES
LTC1627 Monolithic Synchronous
Step-Down Regulator Maximizes
Single or Dual Li-Ion Battery Life
by Jaime Tseng
Introduction
Single and Double
Li-Ion Cell Operation
The LTC1627, with its operating supply range of 2.65V to 8.5V, can operate
from one or two Li-Ion batteries as
well as 3- to 6-cell NiCd and NiMH
battery packs. Figure 2 shows a typical discharge voltage profile of a single
Li-Ion battery. As shown, a fully
charged single-cell Li-Ion battery
begins the discharge cycle around 4V
(it may be slightly higher or lower,
depending upon the manufacturer’s
charge-voltage specifications). During the bulk of the discharge time, the
cell produces between 3.5V and 4.0V.
Finally, towards the end of discharge,
the cell voltage drops quickly below
3V. When the voltage drops further,
the discharge must be terminated to
prevent damage to the battery. A precision undervoltage lockout circuit
trips when the LTC1627’s supply voltage dips below 2.5V, shutting the part
down to only 5µ A of supply current.
Maximizing
Battery Run Time
The LTC1627 incorporates power saving Burst Mode operation and 100%
duty cycle for low dropout to maximize the battery operating time. In
Burst Mode operation, both power
VIN
LTC1627
2.5V
UVLO
350kHz
OSC
SYNC
5.0
4.5
Li-Ion CELL VOLTAGE (V)
The LTC1627 is a new addition to a
growing family of power management
products optimized for Li-Ion batteries. Li-Ion batteries, with their high
energy density, are becoming the
chemistry of choice for many handheld products. As the demand for
longer battery operating time continues to increase and the operating
voltages of submicron DSPs and
microcontrollers decreases, more
demands are placed on DC/DC conversion. The LTC1627 monolithic,
current mode synchronous buck regulator (Figure 1) was specifically
designed to meet these demands.
1C
4.0
0.2C
3.5
2C
3.0
SANYO 18650
1300mAh
TA = 25°C
2.5
2.0
1.5
0
200 400 600 800 1000 1200 1400
DISCHARGE CAPACITY (mAh)
Figure 2. Typical single-cell Li-Ion
discharge curve
MOSFETs are turned off for increasing intervals as the load current drops.
Along with the gate-charge savings,
unused circuitry is shut down between
burst intervals, reducing the quiescent current to 200µ A. This extends
operating efficiencies exceeding 90%
to over two decades of output load
range (see Figure 3). As the battery
discharges, the LTC1627 smoothly
shifts from a high efficiency switchmode DC/DC regulator to a low
dropout (100% duty cycle) switch. In
this mode, the voltage drop between
the battery input and the regulator
output is determined by the load current, the series resistance of the
100
RUN/SS
PDR
CONTROL
95
0.8V
SW
+
VFB
NDR
–
VOUT
EFFICIENCY (%)
VDR
90
85
80
75
ITH
Figure 1. LTC1627 block diagram
Linear Technology Magazine • August 1998
VIN = 5V
70
0.01
L1 = 15µH
VOUT = 3.3V
0.10
OUTPUT CURRENT (A)
1.00
Figure 3. Efficiency vs output load current
5
DESIGN FEATURES
0.9
TJ = 25˚C
0.8
0.7
RDS(ON) (Ω)
0.6
0.5
0.4
0.3
MAIN AND SYNCHRONOUS SWITCH
0.2
0.1
0
1
2
6
8
5
4
7
INPUT VOLTAGE (V)
3
9
10
Figure 4. RDS(ON) for both switches
vs input voltage
internal P-channel power MOSFET
and the inductor resistance.
The internal power MOSFET
switches provide very low resistance
even at low supply voltages. Figure 4
is a graph of switch resistance vs
supply voltage for both switches. The
RDS(ON) is typically 0.5Ω at 5V and
only rises to approximately 0.65Ω at
3V, for both switches. This low switch
RDS(ON) ensures high efficiency switching as well as low dropout DC
characteristics at low supply voltages.
Extending
Low Supply Operation
At low supply voltages, the LTC1627
is most likely to be running at high
duty cycles or in dropout, where the
P-channel main switch is on continuously. Hence, the I2R loss is due mainly
to the RDS(ON) of the P-channel MOSFET. When VIN is below 4.5V, the
RDS(ON) of the P-channel MOSFET can
be lowered further by driving its gate
below ground. The top P-channel
MOSFET driver makes use of a floating
return pin, VDR, to allow biasing below
1 I
TH
VIN ≤ 8.5V
CIN*
22µF
16V
+
Constant-Frequency,
Current Mode Architecture
The LTC1627 uses a constantfrequency, current mode step-down
architecture that provides excellent
rejection of input line and output load
transients and also provides cycleby-cycle current limiting. Input line
transients are rejected by the feedforward characteristics inherent in
current mode control. The output load
transients are rejected by the greater
error -amplifier bandwidth afforded in
current mode control. In current
mode, the circuit behaves as if there
were a constant current feeding the
parallel combination of the output
capacitor and output load, yielding
only a 90° rather than a 180° phase
lag. This simplifies the feedbackloop design and the circuitry around
the error amplifier required for
stabilization.
SYNC/ 8
FCB
7
V
2 RUN/SS
DR
LTC1627
6
VFB 3
VIN
4 GND
SW
1%
R4
80.6k
1%
D1
MBR0520LT1
+
VSEC†††
3.3V/100mA
22µF***
6.3V
5
25µH† R1
1:1 100k
*AVX TPSC226M016R0375
**AVX TPSC107M006R0150
***AVX TAJA226M006R
†COILTRONICS CTX25-1
††MMSZ4678T1
†††10mA MIN LOAD CURRENT
RECOMMENDED
1%
+
COUT**
100µF
6.3V
R2
80.6k
1%
Figure 6. Dual-output 1.8V/0.3A and 3.3V/100mA application
6
VIN
<5V
VIN
C1
0.1µF
LTC1627
VDR
L1
SW
C2
0.1µF
BAT54S
+
VOUT
COUT
100µF
D1
D2
Figure 5. Using a charge pump to bias VDR
Current mode limits the peak current cycle-by-cycle, protecting the
internal main switch and synchronous rectifier. In extreme cases, when
the output is shorted to ground, the
frequency of the oscillator is reduced
to one-tenth of its nominal frequency
to allow the inductor current time to
decay and prevent inductor-current
runaway. The oscillator’s frequency
gradually increases back to its nominal frequency when V FB rises above
0.3V.
The internal oscillator is set for a
fixed switching frequency of 350kHz,
allowing the use of small surface
mount inductors. In switching-noisesensitive applications, the LTC1627
can be externally synchronized to frequencies of up to 525kHz. During
synchronization, Burst Mode operation is inhibited and pulse-skipping
mode is used. In this mode, when the
output load is very low, the current
comparator remains tripped for more
than one cycle and forces the main
switch to stay off for the same number of cycles. Increasing the output
load slightly allows constant frequency
PWM operation to resume.
Minimal External
Components
R3
249k
CITH
47pF
CSS
0.1µF
GND. A simple charge pump bootstrapped to the SW pin realizes a
negative voltage at the VDR pin, as
shown in Figure 5. Each time the SW
node cycles from low to high and then
from high to low, charge is transferred from C2 to C1 producing a
negative voltage at VDR equal in magnitude to VIN – (2 • VDIODE). In dropout,
when the P-channel MOSFET is
turned on continuously, a dropout
detector counts the number of oscillator cycles that the P-channel
MOSFET remains on and periodically
forces a brief off period to allow C1 to
recharge. When 100% duty cycle is
desired, VDR can be grounded to disable the dropout detector.
D1††
1.8V
VOUT
1.8V/0.3A
Size is extremely important in modern portable electronics, so the
LTC1627 is designed to work with a
minimum number of external components. The loop compensation,
current sense resistor and the main
and synchronous switches are internal. An internal catch diode is also
provided across the internal synchronous switch, eliminating parasitic
currents or latch-up if the external
Schottky diode is omitted. Only an
Linear Technology Magazine • August 1998
DESIGN FEATURES
Typical Applications
CITH
47pF
1 I
TH
4 GND
CIN††
22µF
16V
1 or 2 Li-Ion
Step-Down Converter
2 RUN/SS
DR
LTC1627
3
6
VFB
VIN
CSS
0.1µF
VIN ≤ 8.4V
SYNC/ 8
FCB
7
V
SW
25µH*
5
+
R1
249k
1%
*SUMIDA CD54-250
†AVX TPSC107M006R0150
††AVX TPSC226M016R0375
Figure 7 is a schematic diagram showing the LTC1627 being powered by
one or two Li-Ion batteries. All the
components shown in this schematic
are surface mount and have been
selected to minimize the board space
and height. The output voltage is set
at 3.3V, but is easily programmed to
other voltages.
VOUT
3.3V/0.5A
+
R2
80.6k
1%
COUT†
100µF
6.3V
Figure 7. Dual lithium-ion to 3.3V/0.5A regulator
Single Li-Ion
Step-Down Converter
inductor, input and output filter
capacitors and two small resistors
and capacitors are needed to construct a high efficiency DC/DC
switching regulator (see Figure 7).
The 47pF filter capacitor connected
to the ITH pin (error-amplifier output)
filters out switching noise. If the loop
compensation needs to be adjusted
for a specific application, the ITH pin
can also be used for external
compensation.
Auxiliary-Winding Control
Using the SYNC/FCB Pin
Besides higher efficiency and lower
switching noise, synchronous switching provides a means of regulating a
secondary flyback winding. In nonsynchronous regulators, power must
CITH
47pF
C1
0.1µF
CSS
0.1µF
VIN
2.8V–4.5V
BAT54S**
CIN††
22µF
16V
+
1 I
TH
be drawn from the inductor primary
winding in order to extract power
from auxiliary windings. But with
continuous synchronous operation,
power can be drawn from the auxiliary windings without regard to the
primary output load.
The LTC1627, with its synchronous switching and attendant
circuitry, provides the means of easily
constructing a secondary flyback
regulator, as shown in Figure 6. This
flyback regulator is regulated by the
secondary feedback resistive divider
tied to the SYNC/FCB pin. This pin
forces continuous operation whenever
it drops below its ground-referenced
threshold of 0.8V. Power can then be
drawn from the secondary flyback
regulator whether the main output is
loaded or not.
SYNC/ 8
FCB
7
V
2 RUN/SS
DR
LTC1627
6
VFB 3
VIN
4 GND
C2
0.1µF
D1
D2
SW
5
* SUMIDA CD54-150
** ZETEX BAT54S
† AVX TPSC107M006R0150
††
AVX TPSC226M016R0375
15µH*
R1
169k
1%
R2
80.6k
1%
VOUT
2.5V/0.5A
+
The circuit in Figure 8 is intended for
input voltages below 4.5V, making it
ideal for single Li-Ion battery applications. Diodes D1 and D2 and
capacitors C1 and C2 comprise the
bootstrapped charge pump to realize
a negative supply at the VDR pin, the
return pin for the top P-channel
MOSFET driver. This allows Figure
8’s circuit to maintain low switch
RDS(ON) all the way down to the UVLO
trip voltage.
Conclusion
The new L TC1627 monolithic
synchronous buck regulator is a versatile, high efficiency, DC/DC
converter that is at home in a wide
range of low input voltage applications. Features such as precision
UVLO and optional bootstrapped gate
drive make it particularly well suited
to single-cell Li-Ion power.
for
the latest information
on LTC products,
visit
www.linear-tech.com
COUT✝
100µF
6.3V
Figure 8. Single lithium-ion to 2.5V/0.5A regulator
Linear Technology Magazine • August 1998
7
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