A4492 データシート

A4492
Triple Output Step-Down Switching Regulator
Features and Benefits
Description
▪ Three buck converters
▪ 4.5 to 34 V input voltage range
▪ 550 kHz fixed frequency
▪ Multiphase switching
▪ Independent control of each converter
▪ Power-on-reset flag
▪ Internal compensation
▪ 4 × 4 mm QFN Package, small PCB footprint
▪ RoHS compliant
Designed to provide the power supply requirements for a
wide range of applications, the A4492 provides three high
current, high performance, switching regulator outputs with
independent soft start.
High frequency switching allows selection of inexpensive
inductors and small ceramic output capacitors. The turn-on
cycles of the regulators are interleaved to minimize stresses on
the input capacitors and to reduce EMI. A charge pump is used
to provide the supply for driving the power switches, ensuring
operation at very wide operating duty cycles and avoiding the
need for power-draining clamp circuits.
A power-on-reset circuit with user configurable delay indicates
when enabled regulators are in specification. The power-onreset flag also indicates when the input voltage drops below
specification, giving the system controller advance warning
while the switchers continue to operate down to the shutdown
level.
Package: 20-contact QFN (suffix ES)
Internal diagnostics provide comprehensive protection against
overloads, input undervoltages, and overtemperatures.
Continued on the next page…
Approximate size
Typical Application
VBB
VDD
CP1 CP2
PORZ
VCP VBB1
LX1
CPOR
Microcontroller or
Controller Logic
VREG1
A4492
ENB1
ENB2
FB1
VBB2
LX2
ENB3
VREG2
FB2
PGND
GND
VBB3
LX3
VREG3
FB3
4492-DS, Rev. 1
A4492
Triple Output Step-Down Switching Regulator
Description (continued)
The A4492 is provided in a 20-contact, 4 mm × 4 mm, 0.75 mm
nominal overall height QFN, with exposed pad for enhanced thermal
dissipation. It is lead (Pb) free, with 100% matte tin leadframe
plating.
Selection Guide
Part Number
A4492EESTR-T
Operating Temperature Range
(°C)
Packing
1500 pieces per 7-in. reel
–40 to 85
Absolute Maximum Ratings (reference to GND)
Characteristic
Symbol
Notes
Rating
Units
Load Supply Voltage
VBB
36
V
LX1, LX2, and LX3 Pins
VLXn
–1 to 36
V
PORZ and VDD Pins
ENBx Pin Input Current
VIN
IENBx
–0.3 to 7
V
1
mA
Driven by a current-limited voltage source
Operating Ambient Temperature
TA
–40 to 85
ºC
Maximum Junction Temperature
TJ(max)
150
ºC
Tstg
–55 to 150
ºC
Storage Temperature
Range E
Recommended Operating Conditions
Characteristic
Symbol
Conditions
Min.
Typ.
Max.
Units
Load Supply Voltage
VBB
To operate at VBB < 6 V, connect VDD
supply to the VBB supply. See Powering
Configurations section.
4.5
–
34
V
LX1, LX2, and LX3 Pins
VLXn
–0.7
–
34
V
Operating Ambient Temperature
TA
–40
–
85
ºC
Junction Temperature
TJ
–40
–
125
ºC
Thermal Characteristics may require derating at maximum conditions, see application information
Characteristic
Package Thermal Resistance
Symbol
RθJA
Test Conditions*
On 4-layer PCB based on JEDEC standard
Value Units
37
ºC/W
*Additional thermal information available on the Allegro website.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
A4492
Triple Output Step-Down Switching Regulator
Functional Block Diagram
C2
47 nF
Regulator
VDD
CP2
C1
CP1
V BB
Charge Pump
VCP
C3
100 nF
47 nF
Switch
V BB
VBB1
V BB
C4
Bias Supply
10 μF
VCP
LX1
100 kΩ
ENB1
SS
V REG1
L1
Switcher #1
PWM Control
5 V / 1.5 A
15 μH
C5
D1
R1
10 μF
FB1
R2
VBB2
C6
VCP
LX2
ENB2
SS
V DD
Switcher #2
PWM Control
V REG2
10 μF
L2
3.3 V / 1.5 A
10 μH
C7
C8
10 μF
10 μF
D2
R3
FB2
R4
PORZ
POR Block
C14
VBB3
C9
CPOR
10 μF
VCP
470 nF
LX3
ENB3
SS
V REG3
1.0 V / 1.5 A
L3
4.7 μH
C10
C11
C12
C13
D3
Switcher #3
PWM Control
10 μF
10 μF
10 μF
10 μF
R5
FB3
PGND
GND
R6
Note: All capacitors ceramic X7R.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
3
A4492
Triple Output Step-Down Switching Regulator
16 LX3
17 VBB3
18 VBB1
19 LX1
20 ENB1
Pin-out Diagram
FB1
1
15
ENB3
VDD
2
14
FB3
GND
3
13
PGND
FB2
4
12
CP1
ENB2
5
11
CP2
7
8
9
CPOR
PORZ
VCP 10
6
LX2
VBB2
PAD
Terminal List
Number
Name
1
FB1
Feedback REG1
2
VDD
Bias supply
3
GND1
Ground
4
FB2
5
ENB2
Function
Feedback REG2
Enable REG2, logic input, active high
6
LX2
7
VBB22
Switch node REG2
Input supply for REG2
8
CPOR
POR delay adjustment
9
PORZ
Power on reset output, active low
10
VCP
Charge pump reservoir
11
CP2
Charge pump capacitor terminal
12
CP1
Charge pump capacitor terminal
13
PGND1
14
FB3
15
ENB3
16
LX3
17
VBB32
Input supply for REG3
18
VBB12
Input supply for REG1
Ground for charge pump circuitry
Feedback REG3
Enable REG3, logic input, active high
Switch node REG3
19
LX1
20
ENB1
Switch node REG1
Enable REG1, logic input, active high
–
PAD3
Exposed pad for enhanced thermal dissipation
1GND
and PGND should be connected externally.
2The three VBBx pins should be connected together externally.
3Thermal pad should be connected to the ground (0 V) plane using thermal vias.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
4
A4492
Triple Output Step-Down Switching Regulator
ELECTRICAL CHARACTERISTICS1 at TA = –40°C to 85°C, VBB = 6.0 to 34 V, VDD supplied externally, unless noted otherwise
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
–
1
2
mA
General
VBB Quiescent Current
IBBON
ENBx = high, ILOAD = 0 mA, VBB = 12 V,
current drawn by feedback resistors ignored
IBBOFF
ENBx = 0 V
VDD Supply Range
VDD
VDD Quiescent Current
IDD
–
1
–
μA
3.3
–
5.5
V
ENBx = high
–
–
6
mA
ENBx = 0 V
–
1
–
μA
–400
–100
100
nA
–
±1.5
–
%
REG1, REG2, and REG3
Feedback Input Bias Current
IBIAS
Feedback Voltage
VFB
Output Voltage
Regulation2
–3.5
–
3.5
%
fSW
450
550
630
kHz
Maximum Duty Cycle
Dmax
90
–
–
%
Minimum Duty Cycle
Dmin
–
5
–
%
PWM Frequency
Buck Switch On-Resistance
VOUT
With respect to 0.8 V target voltage
RDS(on)
VREGx = 5.0 V, IOUT = 0 to 1.5 A, measured in volts
TJ = –40°C, ILOAD = 1.5 A, VBB = 6.0 V
–
450
–
mΩ
TJ = 25°C, ILOAD = 1.5 A, VBB = 6.0 V
–
450
–
mΩ
TJ = 125°C, ILOAD = 1.5 A, VBB = 6.0 V
–
700
–
mΩ
TJ = –40°C, ILOAD = 1.5 A, VBB = 4.5 V
–
560
–
mΩ
TJ = 25°C, ILOAD = 1.5 A, VBB = 4.5 V
–
560
–
mΩ
TJ = 125°C, ILOAD = 1.5 A, VBB = 4.5 V
–
870
–
mΩ
2.0
–
–
A
0.625
1.25
2
ms
VIL
–
–
0.8
V
VIH
2.0
–
–
V
Current Limit Threshold
ILIM
Soft Start Duration
tss
Peak current through switch with D = 0.9
Logic Inputs and Outputs
ENBx Input Voltage
ENBx Input Hysteresis
ENBx Input Current
VI(hys)
IIL
VIH ≤ 5 V
–
500
–
mV
–1
–
1
μA
PORZ Output (Open Drain)
VPORZL
IPORZL = 1 mA, fault asserted
–
–
0.4
V
PORZ Output Leakage Current
IPORZH
VPORZ = 5 V, fault not asserted
–1
–
1
μA
CPOR = 470 nF
75
115
175
ms
Power-On Reset Duration
tPOR
Continued on the next page…
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
5
A4492
Triple Output Step-Down Switching Regulator
ELECTRICAL CHARACTERISTICS1 (continued) at TA = –40°C to 85°C, VBB = 6.0 to 34 V, VDD supplied externally, unless noted otherwise
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
Protection
VREGx Undervoltage Lockout Startup
VREGUV(su)
FB1, FB2, and FB3 rising, measured in volts (85%
of 0.8 V)
–
85
–
%VFB
VREGx Undervoltage Lockout Shutdown
VREGUV(sd)
FB1, FB2, and FB3 falling, measured in volts
(80% of 0.8 V)
–
80
–
%VFB
Measured in volts (5% of 0.8 V)
–
5
–
%
3.7
4.3
4.7
V
VREGx Undervoltage Lockout
Startup Hysteresis
VBB Undervoltage Lockout Startup
VBB Undervoltage Lockout Shutdown
VBB Undervoltage Lockout
Shutdown Hysteresis
VBB Undervoltage Warning Threshold
VREGUV(suhys)
VBBUV(su)
VBBCPUV(su)
VBBUV(sd)
External VDD supply, VBB rising
3.8
4.2
4.6
V
No external VDD supply, VBB falling
3.6
4.1
4.7
V
VBBCPUV(sd)
External VDD supply, VBB falling
3
3.5
4.3
V
VBBUV(sdhys)
No external VDD supply
–
500
–
mV
VBBCPUV(sdhys) External VDD supply
VBBUV(por)
Junction Overtemperature Shutdown
TJTSD
Junction Overtemperature Shutdown
Hysteresis
TJTSD(hys)
1For
No external VDD supply, VBB rising
–
600
–
mV
VBB falling (forces PORZ low); switchers continue
to operate
–
3.6
–
V
Temperature rising
–
165
–
°C
Recovery = TJTSD –TJTSD(hys)
–
15
–
°C
input and output current specifications, negative current is defined as coming out of (sourcing) the specified pin.
value of VOUT relative to target voltage. The effects of the feedback resistors are not taken into account.
2Average
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
A4492
Triple Output Step-Down Switching Regulator
Functional Description
Basic Operation
The A4492 contains three fixed frequency, buck switching converters with peak current-mode control, including slope compensation. Each converter can be independently turned on and off via
the enable inputs (EN1, EN2, and EN3), which are active high.
When enabled, the corresponding output is brought-up under the
control of a soft start routine, which avoids output voltage overshoot and minimizes input inrush current.
The output voltage is typically divided down by an external
potential divider, and is compared against an internal reference
voltage to produce an error signal, also known as the current
demand signal. The current signal through the buck switch is
converted into a voltage. This signal is then compared against the
current demand signal to create the required duty cycle.
At the beginning of each switching cycle, the buck switch is
turned on. When the current signal through the switch reaches the
level of the current demand signal, the on-time of the switch is
terminated. On the next switching cycle, the switch is turned on
again and the cycle is repeated.
One shared clock is used to define the switching frequency for
each regulator. Each of the three switching cycles (REG1, REG2,
and REG3) are phase shifted with respect to one another by 120°
in an attempt to minimize the pulsed current drawn from the
input filter capacitors. Under certain conditions, for example at
low VBB conditions and relatively high user-set output voltages,
switching overlap between channels is inevitable.
Under conditions, such as light loads or high VBB voltages, that
cause duty cycles (DC) of less than the minimum value, the
converter enters a pulse-skipping mode to ensure regulation is
maintained.
A charge pump regulator is provided to ensure a sufficient gate
drive is available for all three power switches across the full input
voltage range. This regulator allows operation even at very wide
operating duty cycles. On initial power-up, an internal regulator
is used to provide the bias supply for on-chip control functions.
Each regulator channel utilizes pulse-by-pulse current limiting in
the event of either a short circuit or an overload. If the overload
is applied long enough, the IC temperature may rise sufficiently
to cause the thermal shutdown circuit to operate. The part will
auto-restart under control of the soft start circuit after the thermal
disable condition is removed, and assuming all other conditions
are met. See the Shutdown section for more information.
Power Configuration
The A4492 supports alternative schemes for providing logic supply voltage on the VDD pin. In addition, the IC can be powered
up and down using either the VBB or ENB pins.
Powering VDD To minimize power dissipation, especially at
high input voltages, it is recommended that an external supply be applied to the VDD input pin. Typically, this voltage is
derived from one of the three regulated outputs that are set-up for
between 3.3 and 5 V (VREGx).
Another advantage of powering the VDD externally is that the
VBB undervoltage lockout level is lowered. To maximize the
run time of the switchers during a VBB power-down condition,
two alternative undervoltage shutdown conditions are supported,
depending on which VDD-powering configuration has been
implemented. When no external VDD is applied, the minimum
VBB, VBBUV(sd) , is 4.2 V typical. When an external VDD is
applied, the minimum VBB, VBBCPUV(sd) , is 3.4 V typical.
One note of caution when deriving VDD from a VREG output:
during initial application of VBB, the internal bias supply automatically starts from the internal regulator because VREG has not
yet reached regulation. This means the startup threshold is determined by VBBUV(su) (4.3 V typical) because there is no external
VDD. When VREG has begun to supply VDD externally, the
shutdown threshold reduces to VBBCPUV(sd) (3.4 V typical). This
assumes that VREG is present.
Powering Up and Down with VBB Referring to figure 1,
each of the enable inputs (ENBx) are held high by being tied to
the VBB rail via a 100 kΩ resistor and the VDD is supplied from
one of the regulator outputs. When the VBB voltage reaches the
minimum threshold, VBBUV(su) , the charge pump supply (VCP)
ramps up. When VBB + VCP has reached the minimum threshold VBBCPUV(su), the soft start routines are initiated (tSS) for all
three regulator channels (VREGx). When all three regulators
have reached the 85% FBx threshold, the power-on-reset timer
is initiated. After the power-on-reset period, tPOR , has elapsed,
PORZ goes high, indicating that all the regulators and VBB are in
specification.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
7
A4492
Triple Output Step-Down Switching Regulator
V BBUV(su)
VBB
V BBUV(por)
0V
V BB+5.5 V
V BBCPUV(su)
VBB+VCP
VREG1
V BBCPUV(sd)
85%FB1
VREG2
85%FB2
VREG3
85%FB3
PORZ
t POR
t SS
Figure 1. Timing diagram for powering up and down using the VBB pin
VBB
0V
ENB1
ENB2
ENB3
VREG1
VREG2
VREG3
85%FB1
80%FB2
85%FB2
85%FB2
85%FB3
PORZ
tSS
tSS
tPOR
tSS
tPOR
tPOR
Figure 2. Timing diagram for powering up and down using the ENB pin
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
8
A4492
Triple Output Step-Down Switching Regulator
When the VBB voltage starts to fall below the undervoltage warning level, VBBUV(por) , of 3.6 V typical, the PORZ flag resets.
This gives advance warning to the system controller that the VBB
voltage is falling. Note that this feature is only guaranteed when
VDD is supplied externally. During this interval, the three switchers continue to operate.
While VBB falls further, the VCP supply also tends to fall, which
degrades the drive voltage to the series switches. In addition,
the higher voltage rails start to fall out of regulation first, as the
corresponding maximum duty cycle (Dmax) for these particular
converters is reached.
The regulators that have the lower output voltages achieve some
level of steady state, before the A4492 powers down when all
of the corresponding VBB undervoltage thresholds have been
reached. For example, it may be possible for a 1 V output to
continue to operate down to a VBB of 3.4 V typical, if the VDD
supply is derived externally. The extent of this effect depends on
a myriad of factors, including input and output filter capacitance,
output loads, gate drive amplitude, MOSFET RDS(on), and so
forth.
Powering Up and Down with Enable Referring to figure 2,
VBB is present and the UVLO start-up thresholds, VBBUV(su)
and VBBCPUV(su) have been reached. Each of the regulators are
enabled in turn. Initially, VREG1 is enabled and is brought-up
under the control of the soft start circuit (tSS). Before VREG1
reaches 85% FB1, VREG2 is enabled and is brought-up under a
separate soft start control.
When both regulators have reached their respective 85% FB
thresholds, the power-on-reset (POR) timer is initiated. Note that
the POR timer is only enabled after all of the enabled regulators
reach their corresponding 85% FB levels. After the power-onreset time, tPOR , has elapsed, if the FB levels of VREG1 and
VREG2 are not below their respective 80% FB levels, then the
PORZ signal will go high.
At some point later, if VREG3 is enabled, then the PORZ is
reset and VREG3 is brought-up under the control of the soft start
circuit. When the 85% FB3 threshold is reached, the POR timer
is initiated. After tPOR has elapsed, if all the FB levels are above
their respective 80% FB levels, then the PORZ signal will go
high.
Note that if any regulator channel is not enabled, the channel
will not influence PORZ. To avoid multiple signal changes of the
PORZ signal, it is recommended that the system be designed such
that all three regulator channels are within specification before
tPOR has elapsed.
If any regulator channel drops below 80% FB, the PORZ signal
will be reset. If the voltage then recovers to within 85% FB, the
POR timer is initiated again. Note that a soft start is not initiated
when the feedback voltage drops below the 80% FB level. This is
to allow a rapid auto-restart in the event of an overload or similar
fault. If a soft start is required, it is recommended that on receipt
of the PORZ reset signal, the system controller disables and then
re-enables the relevant regulator channels again. As soon as the
last regulator is disabled the PORZ signal is reset.
Power on Reset The power-on-reset duration, tPOR , is determined by selecting an appropriate capacitor connected to the
CPOR pin. The value of tPOR can be determined by the following
formula:
tPOR = 2.131 ×105 × CPOR .
(1)
The PORZ output goes high when both VBB is above the undervoltage warning levels, and the FB pins of the regulators that are
enabled are > 85% of the VREG voltage.
Because the external capacitor is charged via a 5 μA current
source, care must be taken in the layout to avoid additional leakage paths. The capacitor should be positioned adjacent to the
CPOR pin, and the ground connection to the A4492 GND pin
should be as short as possible.
It is recommended that the tPOR period be set to exceed the
start-up phases of all three regulators, to avoid the possibility of
multiple triggerings of the PORZ output.
Output Voltage Selection The output voltage on each of the
three regulators is set by the following relationship, shown here
for the VREG1 channel:
⎛V
⎞
R1 = R2 ⎜⎜ REG1 – 1 ⎟⎟
,
(2)
⎝ VFB
⎠
where R2 (connected between GND and the FB1 pin) should
be a value between 4.7 and 12 kΩ. R1 is connected between the
output rail and the FB1 pin. VREG1 is the set output regulator
voltage. VFB is the reference voltage.
The tolerances of the feedback resistors influence the voltage setpoint. It is therefore important to consider the tolerance selection
when targeting an overall regulation figure.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
9
A4492
Triple Output Step-Down Switching Regulator
The bias current, IBIAS , flowing out of the FB1 node into R2, will
introduce a small voltage offset to the output.
to check the implications on the thermal performance. See the
Thermal Considerations section.
Enable Each regulator channel can be individually enabled via
the corresponding ENBx pin. If any channel is required to startup automatically after the VBB voltage is applied, that particular channel should have the ENB pin tied to the VBB rail via a
pullup resistor.
Component Selection
Soft Start Each regulator channel contains a soft start circuit. A
soft start cycle is initiated when the appropriate regulator enable
input is set to high; the VBB, charge pump, and bias supply voltages are above the minimum values; and no thermal shutdown
condition exists. Note that an overload or short circuit will not
cause a soft start cycle, unless a thermal shutdown event occurs.
During a soft start cycle, the reference voltage is ramped from
0 to 0.8 V typical, which in turn forces the current demand signal
to increase in a linear fashion.
Shutdown All converter channels are disabled in the event of
either a thermal shutdown event or an undervoltage on VBB
(VBBUV(sd) or VBBCPUV(sd)).
As soon as the above fault conditions have been removed, and
assuming the ENB inputs are enabled, the appropriate channels
will auto-restart under control of the soft start.
Current Limit The typical peak current limit for each channel is
specified as 2.5 A minimum, with a duty cycle of 0.9. The minimum current limit occurs at maximum duty cycle (0.9), because
the slope compensation has a maximum effect under this condition. As the duty cycle reduces, the current limit increases. This
means for applications that operate with a narrow duty cycle, it is
possible to operate with a load current greater than 2.0 A.
Figure 3 illustrates the typical peak current limit versus duty
cycle. For example, it is possible to operate with a peak current
limit of 3.75 A with a duty cycle of 0.3.
As well as ensuring the peak current limit is not exceeded, under
worst case load and input voltage conditions, it is also important
It is recommended that gapped ferrite solutions be used as
opposed to powdered iron solutions, the latter of which exhibit
relatively high core losses that can have a large impact on long
term reliability.
Inductors are typically specified at two current levels, rms current and saturation current. With regard to the rms current, it is
important to understand how the rms current level is specified,
in terms of ambient temperature. Some manufacturers quote an
ambient only, whilst others quote a temperature that includes a
self-induced temperature rise. For example, if an inductor is rated
for 125°C and includes a self-induced temperature rise of 25°C
at maximum load, then the inductor cannot be safely operated
beyond an ambient temperature of 100°C at full load. The rms
current can be assumed to be simply the maximum load current,
with perhaps some margin to allow for overloads, and so forth.
The first stage of determining the inductor value is to specify a
peak-to-peak ripple current of typically about 20% to 25% of the
maximum load.
Current Limit (A)
This resistor should be selected to limit the current to less than
the maximum specified value, 1 mA. This prevents the internal
protection clamps from turning on. It is recommended that a
100 kΩ pull-up resistor be used. This would ensure the current
remains below the maximum value when VBB = 36 V.
Inductor The inductance value, L, determines the ripple current.
It is important to ensure that the minimum current limit is not
exceeded under worst-case conditions: VBB(min), ILOAD(max),
fSW(min), and L(min).
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
0.5
1.0
0.5
0
0
20
40
60
80
100
Duty Cycle (%)
Figure 3. Current limit versus duty cycle
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
10
A4492
Triple Output Step-Down Switching Regulator
The maximum peak-to-peak ripple current, IRIPPLE , occurs at the
maximum input voltage. Therefore the duty cycle, D, should be
found under these conditions (for the VREG1 channel):
VREG1+Vf
D(min) =
VBB(max)+Vf
.
(3)
where Vf is the forward voltage drop of the recirculation diode.
The required inductance can be found:
1
VBB(max) – VREG1
× D(min) ×
L (min) =
IRIPPLE
fSW(min)
, (4)
Note that the manufacturers inductance tolerance should also
be taken into account. This value may be as high as ±20%. The
peak-to-peak current should not exceed 1 A, to avoid instability
in the innermost circuit loops due to insufficient slope compensation.
The maximum peak current can be found from to ensure that the
saturation current level of the chosen inductor is not exceeded:
Isat = ILOAD +
IRIPPLE
2
.
(5)
Output Capacitor In the interests of size, cost and performance, it is highly recommended that ceramic X7R capacitor
types be used. When using ceramic capacitors another important
consideration is the E-field effects on the actual value of the
capacitor. To minimize the effects of the capacitance reducing
with output voltage, it is recommended that the working voltage
of the capacitor be considerably more than the set output voltage.
As a suggestion, it is recommended that 6.3 V-rated capacitors
should be used for output voltages of 3.3 V and below. For output
voltages of 5 V, a 10 V-rated capacitor should be used.
The output capacitor determines the output voltage ripple and is
used to close the control loop. To guarantee stability, the capacitance has to increase as the output voltage is reduced. This is
actually reasonable from a ripple voltage point of view, as the
ripple voltage is typically specified as a percentage of output
voltage.
The following table outlines what the minimum output capacitance should be for a given output voltage:
Output Voltage
(V)
Minimum Output Capacitance
(μF)
5
10
3.3
20
1.8 to 2.5
30
<1.8
40
Capacitance values with greater than the above values can be
used with the effect of reducing the bandwidth. This may be necessary in systems that have extremely low ripple/noise requirements.
The output ripple is largely determined by the output capacitance
and the effects of ESR and ESL can largely be ignored assuming
good layout practice is observed.
The output voltage ripple can be approximated to:
VRIPPLE ≈
IRIPPLE
8 × fSW × COUT
,
(6)
When using ceramic capacitors, there is generally no need to consider the current carrying capability due to the negligible heating
effects of the ESR. Also, the rms current flowing into the output
capacitor is extremely low.
Input Capacitor Again it is highly recommended that ceramic,
X7R capacitors be used.
The value of the input capacitance determines the amount of
current ripple (EMI) that appears at the source (VBB supply)
terminals. The amounts of current flowing in and out of the input
capacitor depend on the relative impedances between the input
capacitor impedance and the source impedance. To achieve a low
impedance filter solution it is recommended to place at least two
capacitors in parallel.
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11
A4492
Triple Output Step-Down Switching Regulator
Again, there is generally no need to consider the heating effects
of the rms current flowing through the ESR. Also, the phaseshifting of the input current drawn by each of the regulators helps
to reduce the overall rms current.
The following steps can be used as a guideline for determining a
suitable thermal solution. It should be noted that this process is
usually an iterative one to achieve the optimum solution. These
factors can be considered as follows:
Flyback Diode This diode conducts during the switch off-time.
A Schottky diode is recommended to minimize both the forward
drop and switching losses.
Step 1. Estimate the maximum ambient temperature, TA(max) , of
the application.
The worst case dissipation occurs at maximum VBB , when the
duty cycle, D, is a minimum. The average current through the
diode can be found:
IDIODE(av) = ILOAD × (1 – D(min)) .
(7)
The forward voltage drop, Vf , can be found from the diode
characteristics by using the actual load current (not the average
current).
The static power dissipation can be found:
PSTAT = ILOAD(av) × Vf .
Step 2. Define the maximum junction temperature, TJ(max). Note
that the absolute maximum is 150°C.
Step 3. Determine the worst case power dissipation, PD(max).
The evaluation should consider these at maximum load and minimum VBB. Contributors are switch static and dynamic losses, and
control losses. These are described in the following sections
Switch Static Losses The following steps can be used to
determine switch static losses:
Estimate the maximum duty cycle:
(8)
It is also important to take into account the thermal rating of
the package, RθJA , and the ambient temperature, to ensure that
enough heatsinking is provided to maintain the diode junction
temperature within the safe operating area for the device.
To minimize the heating effects from the A4492 on the diode and
vice-versa, it is recommended that the diode be mounted on the
reverse side of the printed circuit board.
Support Components POR capacitor (C11), charge pump
capacitor (C1), reservoir capacitor (C2) and VDD filter capacitor
(C12) should be ceramic X7R.
Thermal Considerations
To ensure the A4492 operates in the safe operating area, which
effectively means restricting the junction temperature to less than
150°C, several checks should be made. The general approach
is to work out what thermal impedance (RJA) is required to
maintain the junction temperature at a given level, for a particular
power dissipation.
Another factor worth considering is that other power dissipating
components on the system PCB may influence the thermal performance of the A4492. For example, the power loss contribution
from the recirculation diode and the sense resistor may cause the
junction temperature of the A4492 to be higher than expected.
D(max) =
VREG + Vf
VBB (min) + Vf
,
(9)
where Vf is the forward voltage drop of the Schottky diode under
the given load current.
Estimate the RDS(on) of the each regulator switch at the given
junction temperature:
⎛
TJ – 25 ⎞
⎟
.
RDS(on)TJ = RDS(on)25C ⎜⎜ 1+
(10)
200 ⎟
⎝
⎠
Note that if the VBB range is restricted to between 4.5 and 5.5 V,
the RDS(on) increases. For example, the RDS(on) at 25°C with a
VBB greater than 6 V is 450 mΩ typical, as stated in the Electrical Characteristics table. Under the same temperature conditions,
with the VBB = 4.5 V, the RDS(on) is 560 mΩ typical. For VBB
voltages between 4.5 and 6 V, the RDS(on) can be found by linear
approximation. For more information on operating the A4492
between a VBB voltage of 4.5 and 5.5 V, see the Power Configurations section.
The static loss for each switch can be determined:
PSTAT = ILOAD2 × D(max) × RDS(on)TJ ,
(11)
where ILOAD is the load for that particular regulator channel.
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12
A4492
Triple Output Step-Down Switching Regulator
Switch Dynamic Losses The following can be used to determine switch dynamic losses:
ILOAD
2
5+0.4
= 0.84
6+0.4
3.3+0.4
= 0.58
VREG2 duty cycle, D2 =
6+0.4
1.8+0.4
= 0.34
VREG3 duty cycle, D3 =
6+0.4
VREG1 duty cycle, D1 =
Both turn on and turn off losses can be estimated:
PDYN = VBB (min)
(a) Switch static losses
30 10–9
fSW
,
(12)
The RDS(on) of each switch can be found:
where fSW is the switching frequency.
Control Losses The following steps can be used to determine
control losses:
PVBB = IBBON × VBB ,
(13)
⎛ 115 – 25
RDS(on)TJ = 450×10–3 ⎜⎜ 1+
200
⎝
⎞
⎟ = 0.653 Ω
⎟
⎠
The static loss of each switch can be found:
PSTAT1 = 12 × 0.84 × 0.653 = 0.55 W
where IBBON is the quiescent current assuming all three regulators
PSTAT2 = 12 × 0.58 × 0.653 = 0.379 W
are on.
PSTAT3 = 0.82 × 0.34 × 0.653 = 0.14 W
PVDD = IVDD × VDD ,
(14)
Total Losses The total losses can now be estimated:
PTOTAL = PSTAT1 + PSTAT2 + PSTAT2
+PDYN1 +PDYN2 + PDYN3
(15)
Thermal Impedance The thermal impedance required for the
solution can now be determined:
RQJA =
T J – TA
PTOTAL
Example
Selected parameters:
VBB(min) = 6 V
VREG1 = 5 V at 1 A
VREG2 = 3.3 V at 1 A
VREG3 = 1.8 V at 800 mA
TA= 70°C
TJ = 115°C
Vf = 0.4 V
.
1
30 10–9
500 103 = 0.045 W
2
1
PDYN2 = 6
30 10–9
500 103 = 0.045 W
2
0.8
PDYN3 = 6
30 10–9
500 103 = 0.036 W
2
(c) Control losses
PVBB = 0.005 × 6 = 0.03 W
PDYN1 = 6
where IVDD and is the quiescent current on VDD.
+PVBB + PVDD .
(b) Switch dynamic losses
PVDD = 0.001 × 3.3 = 0.003 W
(d) The total power dissipation can now be found:
PTOTAL = 0.55 + 0.379 + 0.14 + 0.045
+ 0.045 + 0.036 + 0.03 + 0.003 = 1.228 W
(16)
(e) The thermal impedance required for the solution can be
found:
115 – 70
= 36.6 °C/W
RQJA =
1.228
For this particular solution a high thermal efficiency board is
required to ensure the junction temperature is kept below 115°C.
For maximum effectiveness, the PCB pad area underneath the
thermal pad of the A4492 should be exposed copper. It is recommended that nine thermal vias be used, arranged in an array of
3 × 3, to connect the thermal pad to the ground plane. The vias
should be of 0.5 mm diameter with a pitch of 0.9 mm between
vias. If possible, an additional thermal copper plane should be
applied to the bottom side of the PCB and connected to the thermal pad of the A4492 through the vias.
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13
A4492
Triple Output Step-Down Switching Regulator
This calculation assumes no thermal influence from other components. If possible, it is advisable to mount the flyback diodes on
the reverse side of the printed circuit board. Ensure low impedance electrical connections are implemented between board
layers.
PCB Layout Guidelines The ground plane is largely dictated
by the thermal requirements described in the previous section.
The ground referenced power components should be referenced
to a star ground, located away from the A4492 to minimize
ground bounce issues.
A small, local, relatively quiet ground plane near the A4492 should
be used for the ground referenced support components, to minimize interference effects of ground noise from the power circuitry.
Figure 4 illustrates the recommended grounding architecture.
A4492 Support
Components
Power Circuitry
Cin
D
A4492
Local “Quiet’
Ground Plane
GND
Cout
PGND
Star Connection
Thermal Vias
Internal Ground Plane
A local quiet ground plane around these components can be
implemented, however, this ground plane should have a high
impedance connection to the star connection of the power stages.
If a ground plane is used, it is recommended that it does not
overlap the switching nodes (LX1, LX2, and LX3) to avoid the
possibility of noise pick-up. To minimize the possibility of noise
injection issues, it is recommended to isolate the ground plane
around high impedance nodes such as: FBx, ENBx and CPOR.
In terms of grounding the power components, a star connection
should be made to minimize the ground loop impedances. Note
that although a ground plane may be required to meet the thermal
characteristics of the solution it is still imperative to implement
a ground star connection for the power components. The ground
for the charge pump (PGND) should be connected to the thermal
vias.
Figures 5 and 6 below illustrates the importance of keeping the
ground connections as short as possible and forming good star
connections.
Figure 5 also illustrates the current conduction paths during the
on-cycle of the switching FET. The following points should be
noted:
• The capacitor CIN should be placed as close as possible to the
VBB terminals. The capacitance should be split between the VBB
terminals for VREG1 and VREG3 and the VBB terminal for VREG2.
Figure 4. Ground plane configurations
Q
To avoid ground bounce and offset issues, it is highly recommended that the ground referenced feedback resistors (R2, R4,
and R6) should be connected as close to the GND connection of
the A4492 as possible.
L
LX
VREG
VBB
Q
VREG
VBB
D
CIN
L
LX
D
COUT
CIN
COUT
RLOAD
Star Connection
Figure 5. FET on-cycle current conduction paths
RLOAD
Star Connection
Figure 6. FET off-cycle current conduction paths
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14
A4492
Triple Output Step-Down Switching Regulator
The VBB terminals for VREG1 and VREG2 should be connected
via short and wide traces to the VBB terminal for VREG3.
• Support components: POR capacitor (C11), charge pump capacitor (C1), reservoir capacitor (C2), and VDD filter capacitor
• Each inductor should be connected as close as possible to the
respective switching FET (LX1, LX2, and LX3) and output
capacitors.
(C12) should be located as close as possible to their respective
Figure 6 shows the current conduction path during the off-cycle
of the switching FET. The following points should be noted:
Powering Configurations The following three diagrams show
• The diode D should be placed as close as possible to both the
switching FET and the inductor.
middle diagram corresponds to the typical application shown on
terminal connections. The ground referenced capacitors should
be connected as close to the GND terminal as possible.
typical configurations for providing power to the application. The
the front page.
VDD applied externally (first option)
Only VBB supplied
6 to 34 V
VDD applied externally (second option)
4.5 to 5.5 V
6 to 34 V
VDD
VBB
VBB
VBB
L
LX
VREG
D
Comments:
- Simple configuration, only one supply required
- Increased power losses at higher VBB voltages
- VBB start-up = 4.3 V (typical), shutdown = 4.2 V
(typical)
C
VDD
L
LX
VREG
D
Comments:
- Reduced power losses at higher VBB voltages
- VBB start-up = 4.3 V (typical), shutdown = 3.4 V
(typical). In this case, the start-up threshold
(VBBUV(su) ) is lower because VREG is not present
C
VDD
L
LX
VREG
D
C
Comments:
- Power restricted as VBB < 6 V, due to increase in
RDS(on) of buck switches
- VBB start-up = 4.1 V (typical), shutdown = 3.4 V (typical)
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15
A4492
Triple Output Step-Down Switching Regulator
Package ES, 20-Pin QFN
0.30
4.00 ±0.15
1
2
0.50
20
20
0.95
A
1
2
4.00 ±0.15
2.60
4.10
2.60
4.10
21X
D
SEATING
PLANE
0.08 C
+0.05
0.25 –0.07
0.75 ±0.05
0.50
C
C
PCB Layout Reference View
For Reference Only, not for tooling use (reference JEDEC MO-220WGGD)
Dimensions in millimeters
Exact case and lead configuration at supplier discretion within limits shown
A Terminal #1 mark area
B Exposed thermal pad (reference only, terminal #1
identifier appearance at supplier discretion)
+0.15
0.40 –0.10
B
2.60
2
1
C Reference land pattern layout (reference IPC7351
QFN50P400X400X80-21BM)
All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary
to meet application process requirements and PCB layout tolerances; when
mounting on a multilayer PCB, thermal vias at the exposed thermal pad land
can improve thermal dissipation (reference EIA/JEDEC Standard JESD51-5)
D Coplanarity includes exposed thermal pad and terminals
20
2.60
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16
A4492
Triple Output Step-Down Switching Regulator
Revision History
Revision
Revision Date
Rev. 1
September 11, 2013
Description of Revision
Update application information
Copyright ©2009-2013, Allegro MicroSystems, LLC
Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to
permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that
the information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its
use; nor for any infringement of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
www.allegromicro.com
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