Aug 1998 New DC/DC Controller Enables High Step-Down Ratios

DESIGN FEATURES
New DC/DC Controller Enables
High Step-Down Ratios
by Greg Dittmer
Importance of
Minimum On-Time
As processor voltage requirements are
pushed lower and lower, input supply
voltages remain high, forcing DC/DC
converters to operate at lower and
lower duty cycles. Since operating
frequencies also remain high to minimize noise and the size of components,
the on-time of the topside switch in a
constant-frequency converter must
continue to decrease to regulate the
lower and lower output voltages. The
required on-time is given by TON =
VOUT/(VIN • f)
Unfortunately, there is a limit to
how small this time can be. In a
typical current mode DC/DC converter, once the main switch is turned
on at the beginning of each switching
cycle, the speed at which it can be
turned off is limited by the response
time of the current comparator, the
time required for the turn-off command to propagate through the logic
and output driver and the time
required to discharge the capacitance
of the topside gate. These delays add
up to a few hundred nanoseconds
and constitute the minimum time the
topside switch must stay on during
each switching cycle. If the maximum
VIN and frequency are fixed, this minimum on-time sets a lower limit on
output voltage. If an output voltage
below this limit is required, the only
choice is to lower the operating frequency, which is usually not desirable.
Figure 1 shows the on-times required
with VIN = 22V as a function of output
voltage for various frequencies.
Capabilities of the LTC1435
The LTC1435 high efficiency synchronous DC/DC controller has been
extremely popular for notebook computers and other battery-powered
equipment due to its low noise, constant-frequency operation and its dual
N-channel drive for outstanding high
current efficiency without sacrificing
low dropout operation. However, its
400ns–500ns minimum on-time
requires lower operating frequencies
(<150kHz) to regulate output voltages
below 2.0V if VIN is high.
What happens if minimum on-time
is violated in the LTC1435? If VIN is
increased so that the on-time falls
below TON(MIN), the LTC1435 will begin
to skip cycles to remain in regulation.
During this “cycle-skipping” mode,
the output remains in regulation but
the operating frequency decreases,
causing the inductor ripple current
and output ripple voltage to increase.
800
35
400
RECOMMENDED
REGION FOR
MIN ON-TIME
AND MAX
EFFICIENCY
30
600
f = 200kHz
500
400
f = 300kHz
300
25
MOSFET VDS LIMIT
20
LTC1435
15
10
f = 250kHz
ILOAD = 0A
L = 4.7µH
T = 25°C
200
5
100
1.5
1.75
2.0
2.25
OUTPUT VOLTAGE (V)
2.5
AN70 F52
Figure 1. Minimum on-time required
for VIN = 22V
Linear Technology Magazine • August 1998
MINIMUM ON-TIME (ns)
LTC1435A
MAXIMUM VIN (V)
MINIMUM ON-TIME (ns)
The operating envelope has been
substantially expanded with the
introduction of the new LTC1435A
DC/DC controller, which has all the
outstanding features of the LTC1435
with a reduced minimum on-time of
300ns or less and improved noise
immunity at low output voltages. With
these improvements, high perfor mance at output voltages down to
1.3V can be achieved with operating
frequencies in excess of 250kHz from
input supply voltages above 22V.
Figure 2 shows the resulting improvement of maximum VIN vs output
voltage as a result of the reduced
minimum on-time.
The LTC1435A’s minimum on-time
is dependent on the speed of the
internal current comparator, which
in turn is dependent on the amplitude of the signal the comparator is
monitoring: inductor ripple current.
Thus, the higher the ripple current,
the lower the minimum on-time. Figure 3 shows how minimum on-time
varies as a function of the inductor
ripple amplitude. At higher amplitudes, TON(MIN) is less than 250ns; at
low amplitudes it can be 350ns or
more. This means that for low duty
cycle applications where the on-time
f = 150kHz
700
0
1.25
Enter the LTC1435A
0
1.25
350
300
250
IMAX =
0.1
RSENSE
200
1.5
1.75
2.0
2.25
OUTPUT VOLTAGE (V)
2.5
AN70 F52
Figure 2. LTC1435/LTC1435A
maximum VIN comparison
0
10
20
30
40
50
60
70
INDUCTOR RIPPLE CURRENT (% OF IMAX)
AN70 F52
Figure 3. LTC1435A minimum on-time
vs inductor ripple current
11
DESIGN FEATURES
VIN
4.5V–22V
1
CSS
0.1µF
2
3
CC
330pF
COSC
VIN
RUN/SS
TG
ITH
SW
INTVCC
CC2
51pF
RC
10k
BOOST
5
100pF
this application, so no cycle skipping
will occur. If a 10µH inductor is used,
the ripple amplitude drops to 0.6A or
20% and the minimum on-time
increases to 280ns. This does not
provide much margin below the 291ns
on-time required, and thus the 4.7µ H
inductor is a better choice.
LTC1435A
COSC
43pF
6
SGND
BG
VOSENSE
PGND
13
+
M1
Si44412DY
16
CIN
10µF, 30V
×2
14
L1 4.7µH
DB*
12
VOUT
1.60V/3A
RSENSE
0.033Ω
35.7k
1% +
0.1µF
15
+
11
10
D1
MBRS
-140T3
4.7µF
M2
Si44412DY
102k
1%
COUT
100µF, 6.3V
×2
Intel Mobile Processor
VID Power Converter
Figure 5 shows the LTC1435A used
with an LTC1706-19 to implement an
Intel Mobile Pentium® II Processor
VID power converter. This DC/DC
converter provides digitally selectable
output voltages over the range of 1.3V
to 2.0V in 50mV increments at 250kHz
and a 7A load current. The selectable
output voltage is implemented by
replacing the conventional feedback
resistor network with the LTC170619, which provides the appropriate
feedback resistor ratios internally (see
the accompanying article in this
issue). The proper ratio is selected
with the 4-bit digital input pins.
SENSE– SENSE+
7
8
1000pF
*DB = CMDSH-3
CENTRAL
(516) 435-1110
Figure 4. LTC1435A 22V to 1.6V/3A converter (f = 250kHz)
is approaching TON(MIN), there may be
a minimum ripple current amplitude,
and hence, a maximum inductance
necessary to prevent cycle skipping.
Or, expressed differently, the lower
the inductance, the higher the maximum VIN that can be achieved before
the minimum on-time is violated and
cycle skipping occurs. For most
applications, 40% ripple not only
reduces the minimum on-time but
also optimizes efficiency.
oscillator frequency at 250kHz and
the 33mΩ sense resistor sets the
maximum load current at 3A. For a
22V to 1.6V converter, the on-time
required is:
TON = 1.6/(22 × 250kHz) = 291ns
Can the LTC1435A do this? At
maximum VIN the inductor ripple is
22V to 1.6V
Converter at 250kHz
VIN
4.5V–22V
1
CSS
0.1µF
2
3
CC2
220pF
COSC
RUN/SS
ITH
CC
1000pF
TG
SW
INTVCC
RC
10k
BOOST
5
51pF
VIN
6
SGND
VOSENSE
BG
PGND
The LTC1435A retains all the outstanding features of the LTC1435,
such as constant-frequency operation, dual N-channel MOSFET drive
and low dropout, while adding
enhancements such as reduced
minimum on-time and improved per formance at low output. With these
enhancements, the LTC1435A is a
perfect fit for notebook computers
and battery-powered equipment
requiring high frequency, low duty
cycle DC/DC converters.
= 1.3A
which is 43% of the 3A maximum
load. From Figure 3, 43% ripple gives
a minimum on-time of 235ns, which
is well below the 291ns required by
LTC1435A
COSC
43pF
1.6 • (1 – 1.6/22)
250kHz • 4.7µH
=
Figure 4 shows the LTC1435A configured in an all N-channel synchronous
buck topology as a 22V to 1.6V/3A
converter running at 250kHz. The
43pF COSC capacitor sets the internal
Conclusion
VOUT • (1 – VOUT/VIN)
F•L
∆IL =
RF
4.7Ω
13
CF
0.1µF
16
+
M1
Si4410DY
CIN
10µF, 30V
×2 R
Pentium is a registered trademark of Intel Corp.
SENSE
VOUT
1.30V–
2.00V/7A
0.015Ω
14
L1 3.3µH
DB*
12
6
0.22µF
3
15
10
SENSE– SENSE+
7
8
1000pF
VCC
+
+
11
SENSE
4.7µF
M2
Si4410DY
D1
MBRS
-140T3
5
FB
LTC1706-19
VID VID VID VID
0 1 2 3
7 8 1 2
COUT
820µF
4V
×2
GND
4
*DB = CMDSH-3
CENTRAL
(516) 435-1110
FROM µP
Figure 5. Intel Mobile Pentium II processor VID power converter
12
Linear Technology Magazine • August 1998
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