Adaptive On-Time DC-DC Converters Combine Speed and Efficiency

PET innovations
SAM DAVIS, Editor in Chief
Adaptive On-Time DC-DC Converters
Combine Speed and Efficiency
3.3 to 5V
BST
VIN
VOUT= 0.75 V TO 95%
LX
SC173
VDD
FB
PGOOD
Power Good
EMTECH CORP.’S newest EcoSpeed™ dc-dc conEnable/
TON
EN/PSV
verters are the SC173 and SC174 synchronous
Power Save
PGND
AGND
buck regulators. Their dc-dc converter platform
incorporates a patented adaptive on-time (AOT)
topology intended primarily for point-of-load (POL) applications. The SC173 and SC174 POL regulators satisfy
1. The SC173 is an integrated, synchronous 3A step-down regulator with an input
today’s “green” power requirements by efficiently managing
voltage range of 3V to 5.5V and a programmable output voltage from 0.75V up
both standby and full current demands. Typical applications
to 95% x VIN.
include office automation, networking and communications
equipment; set-top boxes; portable products; power supply
on the high-side MOSFET via the flip-flop and gate drive.
modules; and other embedded products.
The resulting output ripple triggers the one-shot multivibraFigure 1 is a typical configuration of the SC173 regulator, which sends a single pulse to the high-side MOSFET
tors. The SC174 configuration is similar, the only difference
by way of the flip-flop (F/F) and gate drive. One FB comis their output, the SC173 can supply 3A and the SC174
parator input accepts a percentage of VOUT determined by
can supply 4A. Their programmable switching frequency
the voltage divider consisting of R1 and R2, its other input
range of 200KHz to 1MHz enables circuit optimization for
accepts the reference voltage (VREF).
minimum board space and optimum efficiency.
You can configure the SC174 with various capacitor
types,
such as electrolytic, tantalum, POS and ceramic. For
The SC173 and SC174 converters exhibit advantages over
some applications that use low-ESR ceramic capacitors, a
conventional constant on-time control topologies, as shown in
signal injection circuit may be implemented to provide 10
Table 1 that compares their characteristics. The adaptive onmVp-p, as shown in Fig. 3. The on-time control regulates
time topology in common with hysteretic topologies, enables
fast transient response while eliminating
TABLE 1. CHARACTERISTICS COMPARISON OF CONSTANT
the need for external compensation comON-TIME AND ADAPTIVE ON-TIME TOPOLOGIES
ponents. This results in simple, spaceand cost-efficient power management
Function
Constant On-Time
Adaptive On-Time
solutions. However, compared with conLine Regulation
Excellent
Excellent
stant on-time and hysteretic topologies,
Load Regulation
Excellent
Excellent
the adaptive on-time technique provides
External Compensation
None
None
for more predictable switching frequenStable Operation with Wide
No
Yes
cies.
Range of Load Capacitance
Figure 2 shows the simplified circuit
Switching Frequency
Variable
Pseudo-Fixed
for pseudo-fixed frequency, adaptive
Transient Response
Ultra-Fast
Ultra-Fast
on-time control. When VFB is less than
Limited
Flexible
COUT
the internal 750mV reference, the FB
Power Save
Inherent
Inherent
Comparator output goes high, turning
S
3697E_POW_SEM.indd Sec1:41
10/5/10 2:13 PM
PET innovations
2. Simplified circuit of the
SC173 and SC174 employs a
comparator, one-shot multivibrator, flip-flop, gate drivers
and on-chip power MOSFETs.
Use of the one-shot provides
adaptive on-time control.
VIN
CIN
QHI
VFB
–
TON TON
Hi-Side
FB
Comparator
S
Q
F/F
+
R
VREF
Gate
Drivers
VLX
QLO
VOUT
L
VLX
R1
COUT
Lo-Side
ESR
FB
COUT
R2
VFB
FB Threshold
One-Shot
VIN
VOUT
RTON
the valley of the output ripple voltage. This ripple voltage is
the sum of the two voltages: one produced by the ESR of the
output capacitor, and the other due to capacitive charging
and discharging of the output capacitor during the switching
cycle. For most applications, the output capacitor dominates
total output ripple voltage.
In the one-shot multivibrator used in the SC173
and SC174, a current source determined by VIN and
RTON charges an internal 2.5pF capacitor. The highside MOSFET turns off when the capacitor voltage hits
VOUT, which is derived internally by the heavily filtered
LX voltage. Therefore, at the selected frequency, the IC
automatically anticipates the on-time needed to regulate
VOUT based on the existing VIN. This method automatically produces an on-time proportional to VOUT
and inversely proportional to VIN. Under steady-state
operation, you can program the switching frequency from
200kHz to 1MHz, according to:
f sw =
VOUT
TON × VIN
able frequency methods.
• Reduced component count by eliminating the error amplifier and external compensation components.
• Reduced component count by removing the need to sense
and control inductor current.
• Fast transient response, controlled by a fast comparator
rather than a typically slow error amplifier used in traditional regulator circuits. Figures 4a and 4b illustrate the
transient response of an SC174 during a rising and falling
load current.
• Operates with ceramic input and output capacitors.
• Reduced output capacitance due to fast transient
response
LOW STANDBY POWER
Very low power during standby mode is increasingly
important to comply with emerging “green” initiatives.
VIN
(1)
Q1
The SC173 and SC174 use external resistor RTON to set
the on-time, which indirectly sets the frequency. You can
determine RTON from the desired switching frequency:
R TON =
1
25pF × fsw
CFF
RL
(2)
The switcher output voltage (VOUT) is regulated by
comparing the internal 750mV reference voltage with the
percentage of VOUT determined by the R1 and R2 voltage
divider at FB. Adjusting the values of the voltage divider
controls VOUT.
Advantages of adaptive on-time control are:
• Predictable operating frequency compared to other vari-
3697E_POW_SEM.indd Sec1:42
L
VLX
R1
COUT
Q2
CC
CL
R2
3. Signal injection circuit provides 10 mVp-p at FB.
10/5/10 2:13 PM
PET innovations
Load Transient Response – Load Rising (SC 174)
VIN = 5V, VOUT = 1.2 V
Load Transient Response – Load Falling (SC 174)
VIN = 5V, VOUT = 1.2 V
Output Voltage
Output Voltage
50 mV
50 mV
5V
5V
Switching Node Waveform
Switching Node Waveform
2A
2A
Output Load Current
Output Load Current
20 μs
20μs
(a) Output voltage, output current, and switching node waveforms for rising
load with VIN = 5V and VOUT = 1.2V.
(b) Output voltage, output current, and switching node waveforms for
falling load with VIN = 5V and VOUT = 1.2V.
4. Transient response for the SC174
However, most electronic products also require fast “wakeup” capability, which demands fast recovery from low-power standby to full-power steady state mode. The SC173 and
SC174 converters solve this with an ultrasonic power-save
(UPSAVE) mode. UPSAVE reduces the switching frequency to meet low-power standby requirements, which cuts
power MOSFET switching losses and results in up to 95%
peak standby efficiency. Then, when the load changes from
standby to steady state, the EcoSpeed technology cycle-bycycle response quickly exits UPSAVE mode. To keep the
switching frequency from exceeding 25 kHz (high end of
the audible range) on-times are prevented from occurring at
intervals greater than 40μs.
You can see the effect of power-save in Figures 5 and 6
that show the SC173 and SC174 efficiency vs. output current, respectively. The red curve is with power-save and the
blue curve is without power-save.
6. Efficiency vs. output
current for the SC174.
Blue line is forced conduction mode efficiency.
Red line is efficiency with
Power-Save.
Efficiencyy vs. Output
p Current
100.0
0
95.0
0
90.0
0
90.0
0
85.0
0 VIN = 5 V
VO = 3.3 V
80.0
0 LOUT: DS86C-B992AS-2RON
COUT = 22μFx2
75.0
0 RTON = 80.6 kOhm
Red: PSAVE Mode
70.0
0 Blue: Forced Continuous PWM Mode
0.75
1.50
2.25
Output Current (A)
3.0
Efficiency (%)
95.0
0
65.0
0
0.0
3697E_POW_SEM.indd Sec1:43
Active loads may leak current from a higher voltage into the
switcher output. Under light load conditions with power-save
enabled, this can force VOUT to slowly rise and reach the
over-voltage threshold, resulting in a hard shutdown. Smart
Power Save prevents this condition. When the FB voltage
exceeds 10% above nominal (825mV), the ICs immediately
disable power-save, and turn on the low-side MOSFET. This
draws current from VOUT through the inductor and causes
VOUT to fall. When VFB drops back to the 750mV trip
point, a normal TON switching cycle begins. This method
prevents a hard over-voltage protection (OVP) shutdown
and also cycles energy from VOUT back to VIN.
These ICs feature fixed current limiting that employs the
RDS(ON) of the lower MOSFET for current sensing. While
the low-side MOSFET is on, the inductor current flows
through it and creates a voltage across its RDS(ON). During
Efficiencyy vs. Output
p Current
100.0
0
Efficiency (%)
5. Efficiency vs. output
current for the SC173.
Blue line is forced conduction mode efficiency.
Red line is efficiency with
Power-Save.
CIRCUIT PROTECTION
85.0
0 VIN = 5 V
VO = 3.3 V
80.0
0 LOUT: DS86C-B992AS-2RON
COUT = 22μFx2
75.0
0 RTON = 80.6 kOhm
Red: PSAVE Mode
70.0
0 Blue: Forced Continuous PWM Mode
0
65.0
0
1
3
2
Output Current (A)
4
10/5/10 2:13 PM
PET innovations
this time, the voltage across the MOSFET is negative with
respect to ground. If this MOSFET voltage drop exceeds
the internal reference voltage, it activates the current limit.
The current limit then keeps the low-side MOSFET on and
will not allow another high side on-time, until the current
in the low-side MOSFET reduces enough to drop below the
internal reference voltage once more.
ADDITIONAL PROTECTION FEATURES
The SC173 and SC174 synchronous buck regulators
incorporate a full range of protection features, including:
• Over-Voltage Protection (OVP)
• Over-Temperature Protection (OTP)
• Output short circuit protection
• Under-voltage lock-out (UVLO)
• Soft-Start
To indicate that the SC173 and SC174 are operating
properly, the regulators provide a power good (PGOOD)
output, with open-drain that requires a pull-up resistor.
When the output voltage is 10% below the nominal voltage,
PGOOD goes low and stays low until the output voltage
returns to the nominal voltage. PGOOD is held low during soft start and activated approximately 1ms after VOUT
reaches regulation. The total PGOOD delay is typically
2ms.
PGOOD will transition low if VFB exceeds +20% of
nominal, which is also the over-voltage shutdown threshold
(900mV). PGOOD also pulls low if the EN/PSV pin is low
when VDD is present.
SUMMARY
Designers wishing to implement point-of-load power
circuits now have an energy-saving alternative in the SC173
and SC174 synchronous buck regulators. The parts efficiently manage current in standby and full current modes,
using an advanced adaptive on-time topology to provide fast
transient response, reduced output capacitance, and predictable switching frequencies.
■ ECOSPEED FAMILY PORTRAIT
THE SC173 AND SC174 are the newest introductions
of Semtech’s EcoSpeed family.
lists the other
members of the family that will follow. Some of these
ICs have internal Power MOSFETs, whereas others will
require external MOSFETs. Those intended for external
MOSFETs have an asterisk in the IOUT (max) column
of Table 2.
Similar to the other AOT converters, the SC493 is also
a synchronous buck power supply controller, except that it
has an I2C interface. This can be used to program output
voltage offset, power-on delay time, soft-start time, power
save operating mode, and can enable/disable the controller.
Also, a status register provides information on device state
and faults.
TABLE 2. SEMTECH’S ECOSPEED FAMILY OF DC-DC CONVERTERS
Part
No.
Type
On-Chip
LDO
VIN Range
(V)
VDD Range
(V)
VOUT Range
(V)
IOUT (max.)
(A)
PGOOD
Enable
Selectable
PSAVE
Package (mm)
SC173
Regulator
No
3.0-5.5
3.0-5.5
0.75 to 95%
VIN
3
Yes
Yes
Ultrasonic
MLPD-10
(3 x 3 x1)
SC174
Regulator
No
3.0-5.5
3.0-5.5
0.75 to 95%
VIN
4
Yes
Yes
Ultrasonic
MLPD-10
(3 x 3 x1)
SC414
Regulator
Yes
3.0-5.5
3.0-5.5
0.75 to 5.25
6
Yes
Yes
Ultrasonic
MLPQ-28
(4 x 4 x1)
MLPQ-32
(5 x 5 x1)
SC417
Regulator
Yes
3.0-28
4.5-5.5
0.5 to 5.25
10
Yes
Yes
Ultrasonic
MLPQ-32
(5 x 5 x1)
SC418
PWM
Controller
Yes
3.0-28
3.0-5.5
0.5 to 5.25
30*
Yes
Yes
Programmable
Ultrasonic or
Regular
MLPQ-UT-20
(3 x 3 x0.6)
SC419
PWM
Controller
No
3.0-28
4.5-5.5
0.5 to 5.25
30*
Yes
Yes
Programmable
Ultrasonic or
Regular
MLPQ-UT-20
(3 x 3 x1)
SC424
Regulator
Yes
3.0-28
3.0-5.5
0.75 to 5.25
6
Yes
Yes
Regular
MLPQ-28
(4 x 4 x1)
SC427
Regulator
Yes
3.0-28
4.5-5.5
0.5 to 5.25
10
Yes
Yes
Regular
MLPQ-32
(5 x 5 x1)
SC493
I2C
Controller
No
3.0-28
3.0-5.5
0.5 to 3.3
30*
Yes
Yes
Programmable
Ultrasonic or
Regular
MLPQ-UT-20
(3x 3 x0.6)
Copyright © 2010 by Penton Media, Inc.
3697E_POW_SEM.indd Sec1:44
10/5/10 2:13 PM
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