APPLICATION NOTE - Skyworks Solutions, Inc.

APPLICATION NOTE
Setting the Current Limit for
the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family
Introduction
This application note describes the current limit network calculations for the different DC winding resistances (DCR) of
inductors selected for the Skyworks family of high-voltage step-down converters. This family includes the AAT1184,
AAT1185, AAT1189, AAT2687, AAT2688, and AAT2689. In order to protect the device from permanent damage during
over-current stress or short circuit events, the current is sensed through the output inductor DC winding resistance. An
external resistor (R1) and capacitor (C4) network adjusts the pre-set over-current threshold (IPRESET) based on the DCR
of the inductor.
The over-current offset voltage threshold (VOCP) is internally set to 100mV. The R1-C4 network senses the voltage drop
across the inductor and compares it to the over-current offset voltage threshold. As illustrated in Figure 2, in normal
operation the differential voltage (VRS – VOS) signal is a triangle waveform and in phase with the inductor current. The
over-current controller is triggered when the differential voltage signal from RS to OS exceeds 100mV (nominal). When
the over-current is triggered, the switching frequency and the output voltage reduce to limit the current to a user
designed value (see Figure 3). The designed current limit threshold (ILIMIT) can be set by the current limit network. The
operating frequency returns to the nominal setting when over-current conditions are removed.
The current limit network including components (R1, C4, R6, R7, and R8)1 can be calculated and configured according
to the DCR of the selected inductor. With the same inductor value, a small footprint inductor has a greater DCR compared to a larger footprint inductor. The inductor DCR variation results in the variation of the pre-set over current
threshold (IPRESET). There are three current limit network configurations used to set the design current limit (ILIMIT) for
different pre-set over-current thresholds (IPRESET) due to the inductor DCR selection.
U1
1
LX
LX
16
VOUT
5V/2.5A
L1
4.7µH
2
3
LX
LX
15
LX
RS
8
4
LX
5
LX
OS
D2
B340A
R1
6.34k
C7
330pF
C4
68nF
R8
open
R4
44.2k
R6
R3
6.34k 499
C8
22µF
9
C9
22µF
C10
open
AAT1189
C2
0.1µF
6
D1
BAS16
VCC
13
11
COMP
10
GND
12
VL
14
3
C11
C1
100µF open
25V
C12
1µF
25V
IN
EP
EN
R2
C5
24.3k 220pF
R7
649k
R5
6.04k
C6
56pF
C3
2.2µF
VIN
6.0V -25.0V
+
BST
FB
7
2
1
EN
TDFN34-16
Figure 1: AAT1189 Typical Application Schematic with Current Limit Network (R1, C4, R6, R7, R8).
1. R1, C4, R6, R7, R8 may have different names in AAT1184, AAT1185, AAT2687, AAT2688, and AAT2689 schematics.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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1
APPLICATION NOTE
Setting the Current Limit for
the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family
Figure 2: AAT1189 (VRS – VOS) Signal and Inductor Current are In Phase.
CH2 (Blue): RS signal; CH3 (Pink): OS signal; CHM (Red): (RS-OS) signal; CH4: Inductor Current (1A/div).
Figure 3: AAT1189 Short Circuit Test.
CH1: Output Voltage (2V/div); CH2: Switching Node (LX); CH4: Inductor Current (1A/div).
2
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012
APPLICATION NOTE
Setting the Current Limit for
the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family
Current Limit Network Calculation
In order to correctly sense the inductor current, the R1-C4 network time constant needs to be matched to the inductor
time constant as shown in the following equation:
Eq. 1: R1 · C4 =
L1
DCR
The pre-set over-current threshold with the matching R1-C4 network is the ratio of the over-current offset voltage
threshold and the winding resistance (DCR) of the inductor:
Eq. 2: IPRESET =
VOCP 100mV
=
DCR
DCR
Where the over-current offset voltage threshold (VOCP) is internally set at 100mV with 20% tolerance.
1. Setting the Current Limit close to the Pre-set Over-Current (ILIMIT = IPRESET)
For example, for the 7447789004, 4.7µH Würth inductor with a typical DCR of 35mΩ is used in the AAT1189. The
pre-set over-current threshold is determined as Equation 2:
IPRESET =
VOCP 100mV
=
= 2.86A
DCR 35mΩ
If the current limit threshold is set around 3A, there is no need to use the divider R6 and R7 (Figure 4). The sensing capacitor C4 = 68nF and the sensing resistor R1 can be calculated from Equation 1:
R1 =
L
4.7µH
=
= 1.97kΩ; choose R1 = 1.91kΩ
DCR · C4 35mΩ · 68nF
ILIM
LX
DCR
L1
35mΩ
4.7µH
R1
C4
1.91k
+
-
+
RS
68nF
VL1
V OUT
5V/2.5A
-
+
100mV
OS
Figure 4: Setting the Current Limit Close to the Pre-set Over-Current for the AAT1189 (R6 = 0, R7 =
Open).
Note: The value of R1 should be adjusted from the calculation value to derive the designed current limit due to the
variation of over-current offset voltage threshold (VOCP) between parts.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012
3
APPLICATION NOTE
Setting the Current Limit for
the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family
2. Setting the Current Limit less than the Pre-set Over-Current (ILIMIT < IPRESET)
For example, the RCH108NP-4R7M, 4.7μH Sumida inductor with a typical DCR of 11.7mΩ is used with the AAT1189.
The pre-set over-current threshold is determined by Equation 2:
IPRESET =
VOCP
100mV
=
= 8.5A > ILIMIT = 3A
DCR 11.7mΩ
If the AAT1189 current limit is set to 3A, the divider (R6 and R7) needs to be added into the network (Figure 5). If
the sensing capacitor C4 = 68nF, then the sensing resistor R1 value can be calculated using Equation 1:
R1 =
L
4.7µH
=
= 5.9kΩ; choose R1 = 5.9kΩ
DCR · C4 11.7mΩ · 68nF
I LIM
LX
DCR
L1
12mΩ
4.7µH
R1
C4
5.9kΩ
+
-
RS
+
100mV
OS
+
VOUT
5V/2.5A
68nF
VL1 +
VR6
-
R6
R7
Figure 5: Setting the Current Limit Less than the Pre-set Over-Current for the AAT1189
(R6 = 5.9k, R7 = 464k).
Neglecting the current that sinks into RS and OS terminals of the comparator, the voltage drop on the inductor (VL1)
should be identical to the voltage drop across C4, hence:
Eq. 3: VL1 + VR6 = 100mV
Eq. 4: ILIM · DCR +
R6 · VOUT
= 100mV
R6 + R7
Eq. 5: ILIM · DCR +
R6 · R7 · VOUT
= 100mV
(R6 + R7) · R7
4
To balance the impedance between the two terminals of the comparator, R1 should be equal to the equivalent resistance of R6 and R7 in parallel:
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012
APPLICATION NOTE
Setting the Current Limit for
the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family
Eq. 6: R1 =
Substituting Equation 6 into Equation 5 and solving for R7:
R7 =
VOUT · R1
5V · 5.9kΩ
=
= 454.5kΩ; choose R7 = 464kΩ
VOCP - ILIMIT · DCR
0.1V - 3A · 11.7mΩ
Substituting R1 and R7 into Equation 6 and solving for R6:
Eq. 7: R6 =
R6 · R 7
R6 + R7
R1 · R 7
5.9kΩ · 464kΩ
=
= 5.9kΩ
R7 - R1
464kΩ - 5.9kΩ
Note: The value of R1, R6 and R7 should be adjusted from the calculated values to get the designed current limit
due to the variation of over-current offset voltage threshold (VOCP) between parts.
3. Setting the Current Limit Greater than the Pre-set Over-Current (ILIMIT> IPRESET)
The AAT2687 and AAT2688 can provide up to 4.5A constant output current which is equivalent to a 5.0A peak
inductor current. That requires at least 5.5A or above for the designed current limit. If the IHLP-2525CZ-01, 4.7µH
Vishay inductor with a maximum DCR of 40mΩ is used, the pre-set over-current threshold is determined by
Equation 2:
IPRESET =
VOCP
100mV
=
= 2.5A < ILIMIT = 6A
DCR
40mΩ
If the current limit is set to 6A, R6 and R8 need to be added to the network (see Figure 6). If the sensing capacitor
C4 = 56nF, then the sensing resistor R1 can be calculated from Equation 1:
R1 =
L
4.7µH
=
= 2.098kΩ; choose R1 = 2.05kΩ
DCR · C4 35mΩ · 56nF
VL1
+
LX
DCR
L1
40mΩ
4.7µH
R1
C4
2.05k
+
-
56nF
- V OUT
5V/2.5A
I LIM
+ VC4 = VR8
RS
+
100mV
-
R8
R6
OS
Figure 6: Setting the Current Limit Greater than the Pre-set Over-Current
(R6 = 1k, R8 = 2.05k) for the AAT2687 and AAT2688.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012
5
APPLICATION NOTE
Setting the Current Limit for
the AAT1184/85/89, AAT2687/88/89 High Voltage Step-Down Family
Neglecting the current that sinks into the RS and OS terminals of the comparator, the over-current offset voltage
threshold is identical to the voltage drop across R8:
Eq. 8: VR8 = VC4 = VOCP = 100mV
Where:
R8 · VL1
R · I · DCR
= 8 LIM
= VOCP = 100mV
R8 + R1
R8 + R1
Eq. 9: VR8 =
Solve for R8:
Eq. 10: R8 =
VOCP · R1
100mV · 2.05kΩ
=
= 1.47kΩ
ILIM · DCR - VOCP
6A · 40mΩ - 100mV
To balance the impedance between two terminals of the comparator, R6 should equal the equivalent resistance of R1
and R8 in parallel:
Eq. 11: R6 =
R1 · R 8
2.05kΩ · 1.47kΩ
=
= 856Ω
R1 + R8 2.05kΩ + 1.47kΩ
Note: The value of R1, R6 and R8 should be adjusted from the calculated values to derive the designed current limit due
to the variation of over-current offset voltage threshold (VOCP) between parts.
Sensing Capacitor Selection and
Current Limit Network Layout Considerations
The sensing capacitor C4 should be large enough to hold the sensing voltage from noise interference. A typical value of
C4 is not smaller than 47nF for adequate filtering.
The sensing components C4 and R1 should be connected as close as possible to the RS1 and OS1 pins and can be placed
on the bottom side of the layout to avoid noise coupling from the inductor. The trace that connects from RS and OS to
the inductor terminals should be kept as short as possible to prevent adding more DCR to the inductor.
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6
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202375A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • September 21, 2012