Intersil ISL78236DUALEVAL1Z Dual 3a current sharing 2.5mhz high efficiency synchronous buck regulator Datasheet

Dual 3A Current Sharing 2.5MHz High Efficiency
Synchronous Buck Regulator
ISL78236
Features
The ISL78236 is a dual output 3A/3A integrated FET buck
regulator for point-of-load power applications. The supply
voltage range is from 2.8V to 6V, allowing for the use of a
single Li+ cell, three NiMH cells or a regulated 3V/5V bus
input. It is optimized for regulating output voltages down to
1.2V. Each channel provides an output current up to 3A which
can be combined to form a single 6A output in current sharing
mode. The two channels operate 180° out-of-phase to reduce
input RMS current and EMI.
• Dual 3A/3A independent outputs
The ISL78236 integrates a pair of low ON-resistance P-channel
and N-channel internal MOSFETs to maximize efficiency and
minimize external component count. It can operate up to
100% duty cycle to maximize operating life as battery voltage
drops out. When supplying 3A on each channel, the 100% duty
cycle operation limits the dropout voltage to less than 250mV.
Other features include internal digital soft-start, independent
channel enable for power sequencing, overcurrent protection,
and thermal shutdown.
The ISL78236 is offered in a 24 Ld 4mmx4mm QFN package
with 1mm maximum height. The complete converter occupies
less than 1.5cm2 area.
The ISL78236 is AEC - Q100 qualified and is rated for the
automotive temperature range (-40°C to +105°C).
• 2.5MHz synchronous buck regulator with internal MOSFETs
up to 95% efficiency
• 2% voltage reference accuracy over-temperature
• 6A current sharing mode operation
• Internal or external compensation
• Peak current limiting and hiccup mode short circuit
protection
• Reverse overcurrent protection
• Over-temperature protection shutdown
• AEC - Q100 qualified
Applications
• DSP and embedded processor power supply
• Infotainment system power
• Automotive point of load power
Related Literature
• AN1927, “ISL78236DUALEVAL1Z Dual 3A Low Quiescent
Current High Efficiency Synchronous Buck Regulator”
• AN1928, “ISL78236CRSHEVAL1Z Current Sharing 6A Low
Quiescent Current High Efficiency Synchronous Buck
Regulator”
100
2.5VOUT
LX1
1.2V/3A
EN1
PG1
FPGA OR DSP
POWER
ISL78236
EN2
PG2
LX2
1.5V/3A
ASIC POWER
EFFICIENCY (%)
90
3V INPUT
80
1.2VOUT
70
1.5VOUT
1.8VOUT
60
50
40
0.0
FIGURE 1. TYPICAL APPLICATION BLOCK DIAGRAM: DUAL OUTPUT
3A/3A BUCK REGULATOR
April 28, 2014
FN8624.0
1
0.5
1.0
1.5
2.0
OUTPUT LOAD (A)
2.5
3.0
FIGURE 2. EFFICIENCY vs LOAD CURRENT, VIN = 3.3V, TA = +25°C
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Copyright Intersil Americas LLC 2014. All Rights Reserved
Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries.
All other trademarks mentioned are the property of their respective owners.
ISL78236
Table of Contents
Typical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Pin Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Absolute Maximum Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Thermal Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Recommended Operating Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Electrical Specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Typical Operating Performance for Dual PWM Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Typical Performance for Current Sharing PWM Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Theory of Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PWM Control Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synchronization Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Current Sharing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overcurrent Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power-Good (PG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
UVLO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Enable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Soft Start-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Discharge Mode (Soft-Stop) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power MOSFETs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
100% Duty Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermal Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
18
18
18
18
19
19
19
19
19
19
19
19
Applications Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Inductor and Capacitor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Voltage Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Minimum Output Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Input Capacitor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Loop Compensation Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PCB Layout Recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermal Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
19
20
20
20
20
21
21
Revision History. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
About Intersil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Package Outline Drawing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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ISL78236
Typical Applications
L1
0.6µH
INPUT 2.8V TO 5V
VIN1, 2
LX1
VDD
C1
2x22µF
O UTPUT
1.8V/6A
R2
124k
PGND
C3
12pF
EN1
ISL78236
EN2
FB1
R3
100k
PG1
SGND
L2
0.6µH
SYNC
LX2
PG2
C2
4x22µF
SS
PGND
C5
22nF
TO FB1
FB2
COM P
C6
150pF
R6
50k
SGND
FIGURE 3. TYPICAL APPLICATION DIAGRAM - SINGLE 6A
L1
0.6µH
INPUT 2.8V TO 5V
VIN
LX1
C2
2x22µF
VDD
C1
2x22µF
PGND
SS
EN1
OUTPUT1
1.8V/3A
R2
124k
C3
12pF
ISL78236
FB1
R3
100k
PG1
SGND
L2
0.6µH
SYN C
OUTPUT2
1.8V/3A
LX2
C4
2x22µF
EN2
PGND
R5
124k
C5
12pF
PG2
FB2
R6
100k
COMP
SGND
FIGURE 4. TYPICAL APPLICATION DIAGRAM - DUAL 3A OUTPUTS
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ISL78236
TABLE 1. COMPONENT VALUE SELECTION FOR DUAL OUTPUT OPERATION
VOUT
1.2V
1.5V
1.8V
2.5V
3.3V
C1
2x22µF
2x22µF
2x22µF
2x22µF
2x22µF
C2
4x22µF
4X22µF
4x22µF
4x22µF
4x22µF
L1 (or L2)
0.5~1.1µH
0.5~1.1µH
0.5~1.68µH
0.5~1.68µH
0.5~2.2µH
R2 (or R5)
50k
87.5k
124k
212.5k
312.5k
R3 (or R6)
100k
100k
100k
100k
100k
TABLE 2. COMPONENT VALUE SELECTION FOR CURRENT SHARING OPERATION
VOUT
1.2V
1.5V
1.8V
2.5V
3.3V
C1
2x22µF
2x22µF
2x22µF
2x22µF
2x22µF
C2 (or C4)
2x22µF
2x22µF
2x22µF
2x22µF
2x22µF
L1 (or L2)
0.5~1.1µH
0.5~1.1µH
0.5~1.68µH
0.5~1.68µH
0.5~2.2µH
R2
50k
87.5k
124k
212.5k
312.5k
R3
100k
100k
100k
100k
100k
R6
33k
31k
30k
29k
28k
C6
180pF
150pF
150pF
150pF
150pF
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ISL78236
Block Diagram
COMP
2 7p F
SS
0 .3 p F
SOFTSTAR
S TA R T
T
SHUTDOW N
EN1
BANDG AP
0.8 V
3 9 0k
SHUTDO W N
V IN 1
+
PW M
L O G IC
C O N T R O LL E R
P R O T E C T IO N
D R IV E R
+
EAMP
PW M
COMP
LX1
3pF
PGND
S LO P E
COMP
+
+
FB1
CSA1
1 .6 k
0.5 V
0.8 6 4V
SCP
+
+
+
V IN 1
O S C IL L A T O R
0 .7 3 6V
1M
PG1
SYNC
1m s
DELAY
SGND
SHUTDO W N
THERMAL
SHUTDOW N
O CP
THRESHOLD
L O G IC
27pF
3 9 0k
SS
S O F TSTAR
START
T
SHUTDO W N
0 .3 p F
SHUTDO W N
V IN 2
EN2
BANDGAP
0.8 V
+
+
COMP
EAMP
3pF
PWM
L O G IC
C O N TR O LL E R
P R O T E C T IO N
D R IV E R
LX2
PGND
S LO P E
COMP
+
+
FB2
CSA2
1 .6 k
SCP
0.5 V
+
0.8 6 4 V
+
+
V IN 2
0 .7 3 6V
1M
PG2
1m s
DELAY
SGND
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ISL78236
Pin Configuration
LX2
PGND2
PGND2
PGND1
PGND1
LX1
ISL78236
(24 LD QFN)
TOP VIEW
24
23
22
21
20
19
LX2
1
18 LX1
VIN2
2
17 VIN1
VIN2
3
EN2
4
PG2
5
14 SS
FB2
6
13 EN1
16 VIN1
25
PAD
8
9
10
11
NC
FB1
SGND
PG1
12
SYNC
7
COMP
15 VDD
Pin Descriptions
PIN
NUMBER
SYMBOL
1, 24
LX2
Switching node connection for Channel 2.
4
EN2
Regulator Channel 2 enable pin. Enable the output, VOUT2, when driven to high. Shutdown VOUT2 and discharge output
capacitor when driven to low. Do not leave this pin floating.
5
PG2
Active high Power-Good (PG) indicator for Channel 2. After power-up or EN2 High, this output is a 1ms delayed
Power-Good signal for the Channel 2 output voltage.
6
FB2
The feedback network of the Channel 2 regulator. FB2 is the negative input to the transconductance error amplifier.
The output voltage is set by an external resistor divider from VOUT2 connected to FB2. The power-good output and
undervoltage lockout protection circuitry uses FB2 to monitor the Channel 1 regulator output voltage.
7
COMP
COMP pin is treated as a No Connect in dual output mode operation, using only the internal compensation. If the SS pin
is tied to a soft-start capacitor, external compensation is automatically used. An additional external network across
COMP and SGND is required to improve the loop compensation of the amplifier in parallel current sharing operation.
Connect an external RC network on COMP pin for parallel mode operation.
8
NC
No connect pin; please tie to GND for thermal relief.
9
FB1
The feedback network of the Channel 1 regulator. FB1 is the negative input to the transconductance error amplifier.
The output voltage is set by an external resistor divider from VOUT1 connected to FB1. The power-good output and
undervoltage lockout protection circuitry uses FB1 to monitor the Channel 1 regulator output voltage.
10
SGND
11
PG1
Active high Power-Good (PG) indicator for Channel 1. After power-up or EN1 High, this output is a 1ms delayed
power-good signal for the Channel 1 output voltage.
12
SYNC
Connect to logic high or input voltage VIN. Connect to an external function generator for external synchronization.
Negative edge trigger. Do not leave this pin floating. Do not tie this pin low (or to SGND).
13
EN1
Regulator Channel 1 enable pin. Enable the output, VOUT1, when driven to high. Shut down VOUT1 and discharge
output capacitor when driven to low. Do not leave this pin floating.
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DESCRIPTION
System ground.
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ISL78236
Pin Descriptions (Continued)
PIN
NUMBER
SYMBOL
DESCRIPTION
14
SS
SS is used to adjust the soft-start time. When SS pin is tied to VIN, SS time is 1.5ms. SS pin is tied to VIN only in dual
output mode operation. SS pin is tied to a soft-start capacitor only in parallel current sharing mode operation. Connect
a capacitor from SS to SGND to adjust the soft-start time. CSS should not be larger than 33nF. This capacitor, along
with an internal 5µA current source, sets the soft-start interval of the converter, tSS.
(EQ. 1)
C SS  F  = 6.25  t SS  s 
15
VDD
Input supply voltage for the logic. VDD to be at the same potential as VIN +0.3/-0.5V.
16, 17
2, 3
VIN1,
VIN2
Input supply voltage. Connect 22µF ceramic capacitor to power ground per channel.
18, 19
LX1
20, 21
PGND1
Negative supply for the power stage of Channel 1.
22, 23
PGND2
Negative supply for the power stage of Channel 2.
25
PAD
Switching node connection for Channel 1.
The exposed pad must be connected to the SGND pin for proper electrical performance. Add as many vias as possible
to connect the PAD to a ground plane for optimal thermal performance.
Ordering Information
PART NUMBER
(Notes 1, 2, 3)
PART
MARKING
ISL78236ARZ
782 36ARZ
ISL78236DUALEVAL1Z
Evaluation Board
ISL78236CRSHEVAL1Z
Evaluation Board
TEMP. RANGE
(°C)
-40 to +105
PACKAGE
(Pb-free)
24 Ld 4x4 QFN
PKG.
DWG. #
L24.4x4D
NOTES:
1. Add “-T*” suffix for tape and reel. Please refer to TB347 for details on reel specifications.
2. These Intersil Pb-free plastic packaged products employ special Pb-free material sets, molding compounds/die attach materials, and 100% matte tin plate plus
anneal (e3 termination finish, which is RoHS compliant and compatible with both SnPb and Pb-free soldering operations). Intersil Pb-free products are MSL
classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020.
3. For Moisture Sensitivity Level (MSL), please see product information page for ISL78236. For more information on MSL, please see tech brief TB363.
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ISL78236
Absolute Maximum Ratings
Thermal Information
(Reference to SGND)
VIN1, VIN2, VDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6.5V
LX1, LX2 (Note 4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6.5V
EN1, EN2, PG1, PG2, SYNC, SS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6.5V
FB1, FB2, COMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +2.7V
ESD Ratings
Human Body Model (Tested per AEC-Q100-002) . . . . . . . . . . . . . . . . 4kV
Machine Model (Tested per AEC-A100-003) . . . . . . . . . . . . . . . . . . . 300V
Charged Device Model (Tested per AEC-Q100-011). . . . . . . . . . . . . . 2kV
Latch Up (Per JESD-78D; Class 2, Level A; AEC-Q100-004) . . . . . . 100mA
Thermal Resistance (Typical)
JA (°C/W) JC (°C/W)
24 Ld 4x4 QFN (Notes 5, 6). . . . . . . . . . .
36
2
Junction Temperature Range . . . . . . . . . . . . . . . . . . . . . . .-55°C to +150°C
Storage Temperature Range. . . . . . . . . . . . . . . . . . . . . . . .-65°C to +150°C
Ambient Temperature Range . . . . . . . . . . . . . . . . . . . . . . .-40°C to +105°C
Pb-Free Reflow Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . see TB493
Recommended Operating Conditions
VIN Supply Voltage Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . +2.85V to +6V
Load Current Range per Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . 0A to 3A
Ambient Temperature Range . . . . . . . . . . . . . . . . . . . . . . .-40°C to +105°C
CAUTION: Do not operate at or near the maximum ratings listed for extended periods of time. Exposure to such conditions may adversely impact product
reliability and result in failures not covered by warranty.
NOTES:
4. Values shown for continuous voltage. Absolute Maximum Rating of 7V for a duration less than 20ms. Absolute Maximum Rating of -1.5V for duration
of less than 100ns.
5. JA is measured in free air with the component mounted on a high effective thermal conductivity test board with “direct attach” features. See Tech
Brief TB379.
6. For JC, “case temperature” location is at the center of the exposed metal pad on the package underside.
Electrical Specifications Unless otherwise noted, the typical specifications are measured at the following conditions:
TA = -40°C to +105°C, VIN = 3.6V, EN1 = EN2 = VDD, L = 1.5µH, C1 = C2 = C4 = 2x22µF, IOUT1 = IOUT2 = 0A to 3A, unless otherwise noted. Typical values
are at TA = +25°C. Boldface limits apply across the operating temperature range, -40°C to +105°C.
PARAMETER
SYMBOL
TEST CONDITIONS
MIN
(Note 7)
TYP
MAX
(Note 7)
2.5
2.85
40
100
UNITS
INPUT SUPPLY
VIN Undervoltage Lockout Threshold
VUVLO
Rising
Hysteresis
V
mV
Quiescent Supply Current
IVDD
SYNC = VDD, EN1 = EN2 = VDD, no load at the
output
30
70
mA
Shutdown Supply Current
ISD
VIN = VDD = 6V, EN1 = EN2 = SGND
8
35
µA
0.8
0.810
V
OUTPUT REGULATION
FB1, FB2 Regulation Voltage
VFB
FB1, FB2 Bias Current
IFB
Load Regulation
0.784
VFB = 0.75V
1
µA
SYNC = VDD, output load from 0A to 6A
2
mV/A
Line Regulation
VIN = VO + 0.5V to 6V (minimal 2.85V)
0.1
%/V
Soft-start Ramp Time Cycle
SS = VDD
1.5
ms
Soft-start Charging Current
ISS
4
5
6
µA
COMPENSATION
Error Amplifier Trans-Conductance
SS = VDD
20
µA/V
SS with Capacitor
100
µA/V
Current Sense Amplifier Gain
CSA_GAIN
0.172
Ch1/Ch2 Amplifier Gain Matching
GAINMATCH
-0.05
0.2
0.228
V/A
+0.05
V/A
OVERCURRENT PROTECTION
Dynamic Current Limit ON-time
tOCON
17
Clock pulses
Dynamic Current Limit OFF-time
tOCOFF
8
SS cycle
Positive Peak Overcurrent Limit
Negative Peak Overcurrent Limit
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Ipoc1
4.1
4.8
5.5
A
Ipoc2
4.1
4.8
5.5
A
Inoc1
-3.5
-2.5
-1.5
A
Inoc2
-3.5
-2.5
-1.5
A
FN8624.0
April 28, 2014
ISL78236
Electrical Specifications Unless otherwise noted, the typical specifications are measured at the following conditions:
TA = -40°C to +105°C, VIN = 3.6V, EN1 = EN2 = VDD, L = 1.5µH, C1 = C2 = C4 = 2x22µF, IOUT1 = IOUT2 = 0A to 3A, unless otherwise noted. Typical values
are at TA = +25°C. Boldface limits apply across the operating temperature range, -40°C to +105°C. (Continued)
PARAMETER
SYMBOL
TEST CONDITIONS
MIN
(Note 7)
TYP
MAX
(Note 7)
UNITS
LX1, LX2
P-Channel MOSFET ON-Resistance
N-Channel MOSFET ON-Resistance
VIN = 5.5V, IO = 200mA
50
75
mΩ
VIN = 2.85V, IO = 200mA
70
100
mΩ
VIN = 5.5V, IO = 200mA
50
75
mΩ
VIN = 2.85V, IO = 200mA
70
100
mΩ
2.85
MHz
8
MHz
140
ns
LX_ Maximum Duty Cycle
100
PWM Switching Frequency
FS
Synchronization Frequency Range
FSYNC
2.15
(Note 8)
Channel 1 to Channel 2 Phase Shift
Rising edge to rising edge timing
LX Minimum On Time
SYNC = High (PWM mode)
Soft Discharge Resistance
RDIS
LX Leakage Current
EN = LOW
2.5
6
%
180
80
VIN = VDD = 6V
°
100
120
Ω
0.1
1
µA
0.3
V
PG1, PG2
Output Low Voltage
Sinking 1mA, VFB = 0.7V
PG Pin Leakage Current
PG = VIN = 6V
0.01
0.1
µA
Internal PGOOD Low Rising Threshold
Percentage of nominal regulation voltage
88
92
95
%
Internal PGOOD Low Falling Threshold
Percentage of nominal regulation voltage
85
88
92
Delay Time (Rising Edge)
Time from VOUT_ reached regulation
1
Internal PGOOD Delay Time
(Falling Edge)
7
%
ms
15
µs
0.4
V
EN1, EN2, SYNC
Logic Input Low
Logic Input High
1.5
V
SYNC Logic Input Leakage Current
ISYNC
VIN = VDD = 6V
0.1
1
µA
Enable Logic Input Leakage Current
IEN
VIN = VDD = 6V
0.1
1
µA
Thermal Shutdown
150
°C
Thermal Shutdown Hysteresis
25
°C
NOTES:
7. Parameters with MIN and/or MAX limits are 100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization and
are not production tested.
8. The operational frequency per switching channel will be half of the SYNC frequency.
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FN8624.0
April 28, 2014
ISL78236
Typical Operating Performance for Dual PWM Operation
Unless otherwise noted, operating conditions are: TA = +25°C; VOUT1 = 1.8V; VOUT2 = 1.8V; IOUT1 = 0A to 3A; IOUT2 = 0A to 3A, L1 = L2 = 0.6µH,
COUT1 = 2x22µF, COUT2 = 2x22µF, FSW = 2.5MHz.
100
100
2.5VOUT
1.2VOUT
1.5VOUT
EFFICIENCY (%)
80
1.8VOUT
70
60
80
1.8VOUT
70
60
50
50
40
40
0
0.5
1.0
1.5
2.0
2.5
3.0
0
0.5
OUTPUT LOAD (A)
1.6
1.6
T = +125°C
1.4
T = +105°C
1.2
POWER DISSIPATION (W)
POWER DISSIPATION (W)
2.5
3.0
1.8
1.8
T = +25°C
1.0
T = -40°C
0.8
0.6
0.4
0.2
T = +125°C
1.4
T = +105°C
1.2
T = +25°C
1.0
T = -40°C
0.8
0.6
0.4
0.2
0
0.5
1.0
1.5
2.0
2.5
0
3.0
0
0.5
FIGURE 7. POWER DISSIPATION, VIN = 3.3V, VOUT = 1.8V
2.0
2.5
3.0
0
VOUT
REFERENCED FROM
VOUT AT VIN = 2.7V
-1
0A LOAD
VOUT (mV)
6A LOAD
3A LOAD
0.05
5VIN
-2
0.10
-3
-4
3.3VIN
-5
VOUT
REFERENCED FROM
VOUT AT IOUT = 0A
0.00
-6
-0.05
2.5
1.5
FIGURE 8. POWER DISSIPATION, VIN = 5V, VOUT = 1.8V
0.20
0.15
1.0
OUTPUT LOAD (A)
OUTPUT LOAD (A)
VOUT (%)
1.0
1.5
2.0
OUTPUT LOAD (A)
FIGURE 6. EFFICIENCY, VIN = 5V, TA = +25°C
FIGURE 5. EFFICIENCY, VIN = 3.3V, TA = +25°C
0
3.3VOUT
90
90
EFFICIENCY (%)
2.5VOUT
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
FIGURE 9. LINE REGULATION, VOUT = 1.8V, TA = +25°C
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6.0
-7
0
0.5
1.0
1.5
2.0
2.5
3.0
OUTPUT CURRENT (A)
FIGURE 10. LOAD REGULATION, VOUT = 1.8V, TA = +25°C
FN8624.0
April 28, 2014
ISL78236
Typical Operating Performance for Dual PWM Operation
Unless otherwise noted, operating conditions are: TA = +25°C; VOUT1 = 1.8V; VOUT2 = 1.8V; IOUT1 = 0A to 3A; IOUT2 = 0A to 3A, L1 = L2 = 0.6µH,
COUT1 = 2x22µF, COUT2 = 2x22µF, FSW = 2.5MHz. (Continued)
200ns/DIV
VIN = 5V
LX1 2V/DIV
200ns/DIV
VIN = 5V
LX2 2V/DIV
VOUT RIPPLE 20mV/DIV
VOUT RIPPLE 20mV/DIV
IL1 2A/DIV
IL2 2A/DIV
FIGURE 11. STEADY STATE OPERATION AT NO LOAD CHANNEL 1
200ns/DIV
VIN = 5V
LX1 2V/DIV
FIGURE 12. STEADY STATE OPERATION AT NO LOAD CHANNEL 2
200ns/DIV
VIN = 5V
LX2 2V/DIV
VOUT RIPPLE 20mV/DIV
VOUT RIPPLE 20mV/DIV
IL1 2A/DIV
IL2 2A/DIV
FIGURE 13. STEADY STATE OPERATION AT 3A LOAD CHANNEL 1
VOUT1 RIPPLE 100mV/DIV
VIN = 5V
VIN = 5V
100µs/DIV
CH2 IOUT = 0A
100µs/DIV
CH1 IOUT = 0A
IL1 2A/DIV
FIGURE 15. LOAD TRANSIENT CHANNEL 1
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FIGURE 14. STEADY STATE OPERATION AT 3A LOAD CHANNEL 2
VOUT2 RIPPLE 100mV/DIV
IL2 2A/DIV
FIGURE 16. LOAD TRANSIENT CHANNEL 2
FN8624.0
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ISL78236
Typical Operating Performance for Dual PWM Operation
Unless otherwise noted, operating conditions are: TA = +25°C; VOUT1 = 1.8V; VOUT2 = 1.8V; IOUT1 = 0A to 3A; IOUT2 = 0A to 3A, L1 = L2 = 0.6µH,
COUT1 = 2x22µF, COUT2 = 2x22µF, FSW = 2.5MHz. (Continued)
VIN = 3.3V
VIN = 3.3V
EN1
VOUT
EN2
VOUT
LX1 VOLTAGE
LX2 VOLTAGE
PG1
PG2
FIGURE 17. SOFT-START WITH NO LOAD CHANNEL 1
EN1
VIN = 3.3V
FIGURE 18. SOFT-START WITH NO LOAD CHANNEL 2
EN2
VIN = 3.3V
VOUT
VOUT
LX1 VOLTAGE
LX2 VOLTAGE
PG2
PG1
FIGURE 20. SOFT-START AT 3A LOAD CHANNEL 2
FIGURE 19. SOFT-START AT 3A LOAD CHANNEL 1
VIN = 3.3V
VIN = 3.3V
EN1
EN2
VOUT
VOUT
LX1 VOLTAGE
LX2 VOLTAGE
PG1
PG2
FIGURE 21. SOFT-DISCHARGE SHUTDOWN CHANNEL 1
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FIGURE 22. SOFT-DISCHARGE SHUTDOWN CHANNEL 2
FN8624.0
April 28, 2014
ISL78236
Typical Operating Performance for Dual PWM Operation
Unless otherwise noted, operating conditions are: TA = +25°C; VOUT1 = 1.8V; VOUT2 = 1.8V; IOUT1 = 0A to 3A; IOUT2 = 0A to 3A, L1 = L2 = 0.6µH,
COUT1 = 2x22µF, COUT2 = 2x22µF, FSW = 2.5MHz. (Continued)
200ns/DIV
VIN = 5V
LX1 2V/DIV
200ns/DIV
VIN = 5V
SYNC
SYNC
VOUT2 RIPPLE 50mV/DIV
VOUT1 RIPPLE 50mV/DIV
FIGURE 23. STEADY STATE OPERATION CHANNEL 1 AT NO LOAD
WITH FSW = 4MHz
200ns/DIV, VIN = 5V
LX1 2V/DIV
FIGURE 24. STEADY STATE OPERATION CHANNEL 2 AT NO LOAD
WITH FSW = 4MHz
200ns/DIV, VIN = 5V
SYNC
VOUT2 RIPPLE 50mV/DIV
OUT1 2A/DIV
OUT2 2A/DIV
FIGURE 26. STEADY STATE OPERATION CHANNEL2 3A LOAD WITH
FSW = 4MHz
VIN = 5V
VIN = 5V
LX2 2V/DIV
SYNC
VOUT1 RIPPLE 50mV/DIV
FIGURE 25. STEADY STATE OPERATION CHANNEL1 3A LOAD WITH
FSW = 4MHz
LX2 2V/DIV
LX1 5V/DIV
LX1 5V/DIV
VOUT1 1V/DIV
IL1 1A/DIV
VOUT1 1V/DIV
IL1 1A/DIV
PG1 5V/DIV
FIGURE 27. OUTPUT SHORT CIRCUIT CHANNEL 1
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PG1 5V/DIV
FIGURE 28. OUTPUT SHORT CIRCUIT RECOVERY (FROM HICCUP)
CHANNEL 1
FN8624.0
April 28, 2014
ISL78236
Typical Operating Performance for Dual PWM Operation
Unless otherwise noted, operating conditions are: TA = +25°C; VOUT1 = 1.8V; VOUT2 = 1.8V; IOUT1 = 0A to 3A; IOUT2 = 0A to 3A, L1 = L2 = 0.6µH,
COUT1 = 2x22µF, COUT2 = 2x22µF, FSW = 2.5MHz. (Continued)
VIN = 5V
VIN = 5V
LX2 5V/DIV
LX2 5V/DIV
VOUT2 1V/DIV
VOUT2 0.5V/DIV
IL2 1A/DIV
IL2 1A/DIV
PG2 5V/DIV
PG2 5V/DIV
FIGURE 29. OUTPUT SHORT CIRCUIT CHANNEL 2
1V/DIV
1V/DIV
4ns/DIV
4ns/DIV
FIGURE 32. LX JITTER AT 3A LOAD, VIN = 3V
FIGURE 31. LX JITTER AT NO LOAD, VIN = 3V
FIGURE 33. LX JITTER AT NO LOAD, VIN = 5V
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FIGURE 30. OUTPUT SHORT CIRCUIT RECOVERY (FROM HICCUP)
CHANNEL 2
1V/DIV
1V/DIV
4ns/DIV
4ns/DIV
FIGURE 34. LX JITTER AT 3A LOAD, VIN = 5V
FN8624.0
April 28, 2014
ISL78236
Typical Performance for Current Sharing PWM Operation
Unless otherwise noted, operating conditions are: TA = +25°C; VOUT = 1.8V, IOUT1 + IOUT2 = 0A to 6A, L1 = L2 = 0.6µH, COUT = 4x22µF, FSW = 2.5MHz.
100
100
3.3VOUT
90
90
80
1.5VOUT
EFFICIENCY (%)
EFFICIENCY (%)
2.5VOUT
1.8VOUT
1.2VOUT
70
60
80
2.5VOUT
70
60
50
50
40
40
0
1
2
3
4
5
6
0
1
2
4.0
3.5
3.5
POWER DISSIPATION (W)
POWER DISSIPATION (W)
4.0
T = +125°C
T = +105°C
2.5
2.0
T = +25°C
T = -40°C
1.5
1.0
0.5
5
6
3.0
T = +125°C
2.5
T = +105°C
2.0
T = +25°C
1.5
T = -40°C
1.0
0.5
0
1
2
3
4
5
0
6
0
1
2
3
4
OUTPUT LOAD (A)
OUTPUT LOAD (A)
FIGURE 37. POWER DISSIPATION, VIN = 3.3V, VOUT = 1.8V
0.35
6
0
VOUT
REFERENCED FROM
VOUT AT VIN = 2.7V
-1
0.30
-2
0A LOAD
VOUT (mV)
0.25
6A LOAD
0.20
0.15
3A LOAD
-5
0.05
-7
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
FIGURE 39. LINE REGULATION, VOUT = 1.8V, TA = +25°C
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6.0
5VIN
-4
-6
3.0
3.3VIN
-3
0.10
0
2.5
5
FIGURE 38. POWER DISSIPATION, VIN = 5V, VOUT = 1.8V
0.40
VOUT (%)
4
FIGURE 36. EFFICIENCY vs LOAD, VIN = 5V, TA = +25°C
FIGURE 35. EFFICIENCY vs LOAD, VIN = 3.3V, TA = +25°C
3.0
3
OUTPUT LOAD (A)
OUTPUT LOAD (A)
0
1.8VOUT
-8
0
VOUT
REFERENCED FROM
VOUT AT IOUT = 0A
1
2
3
4
OUTPUT CURRENT (A)
5
6
FIGURE 40. LOAD REGULATION, VOUT = 1.8V, TA = +25°C
FN8624.0
April 28, 2014
ISL78236
Typical Performance for Current Sharing PWM Operation
Unless otherwise noted, operating conditions are: TA = +25°C; VOUT = 1.8V, IOUT1 + IOUT2 = 0A to 6A, L1 = L2 = 0.6µH, COUT = 4x22µF, FSW = 2.5MHz.
VIN = 5V
VOUT RIPPLE 100mV/DIV
VIN = 5V
VOUT RIPPLE 50mV/DIV
LX1 5V/DIV
LX1 5V/DIV
LX2 5V/DIV
LX2 5V/DIV
200ns/DIV
200ns/DIV
FIGURE 42. STEADY STATE OPERATION AT 6A LOAD
FIGURE 41. STEADY STATE OPERATION AT NO LOAD
VIN = 5V
VIN = 3.3V
VOUT RIPPLE 200mV/DIV
EN
OUTPUT CURRENT 2A/DIV
VOUT
LX1 VOLTAGE
PG1
200µs/DIV
FIGURE 43. LOAD TRANSIENT RESPONSE
VIN = 3.3V
FIGURE 44. SOFT-DISCHARGE SHUTDOWN
EN
VIN = 3.3V
EN
VOUT
LX1 VOLTAGE
VOUT
LX1 VOLTAGE
PG1
FIGURE 45. SOFT-START AT NO LOAD
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PG1
FIGURE 46. SOFT-START AT 6A LOAD
FN8624.0
April 28, 2014
ISL78236
Typical Performance for Current Sharing PWM Operation
Unless otherwise noted, operating conditions are: TA = +25°C; VOUT = 1.8V, IOUT1 + IOUT2 = 0A to 6A, L1 = L2 = 0.6µH, COUT = 4x22µF, FSW = 2.5MHz.
VIN = 5V
IL2 1A/DIV
IL2 1A/DIV
LOAD RAMP = 6A/ms
FIGURE 47. CURRENT SHARE BALANCING, 0A TO 6A
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VIN = 5V
IL2 1A/DIV
IL2 1A/DIV
LOAD RAMP = 6A/ms
FIGURE 48. CURRENT SHARE BALANCING, 2A TO 6A
FN8624.0
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ISL78236
Theory of Operation
The ISL78236 is a dual 3A or single current sharing 6A step-down
switching regulator optimized for low output ripple point-of-load
power in automotive applications. The regulator operates at
2.5MHz internally fixed switching frequency allowing small output
filter components while maintaining up to 95% efficiency. The two
channels are 180° out-of-phase operation to reduce input ripple
currents and EMI. The supply current is typically only 8µA when the
regulator is shutdown.
PWM Control Scheme
Pulling the SYNC pin HI (>1.5V) forces the converter into PWM mode
in the next switching cycle regardless of output current. Each of the
channels of the ISL78236 employ the current-mode pulse-width
modulation (PWM) control scheme for fast transient response and
pulse-by-pulse current limiting, as shown in the “Block Diagram” on
page 5 with waveforms in Figure 49. The current loop consists of
the oscillator, the PWM COMP comparator, current sensing circuit,
and the slope compensation for the current loop stability. The
current sensing circuit consists of the resistance of the P-channel
MOSFET when it is turned on and the current sense amplifier CSA.
The gain for the current sensing circuit is typically 0.2V/A. The
control reference for the current loops comes from the error
amplifier, EAMP, of the voltage loop.
The PWM operation is initialized by the clock from the oscillator.
The P-channel MOSFET is turned on at the beginning of a PWM
cycle and the current in the MOSFET starts to ramp up. When the
sum of the current amplifier CSA1 (or CSA2 on Channel 2) and the
compensation slope (0.46V/µs) reaches the control reference of
the current loop, the PWM COMP comparator sends a signal to the
PWM logic to turn off the P-MOSFET and to turn on the N-channel
MOSFET. The N-MOSFET stays on until the end of the PWM cycle.
Figure 49 shows the typical operating waveforms during the PWM
operation. The dotted lines illustrate the sum of the compensation
ramp and the current-sense amplifier CSA_ output.
voltage loop is internally compensated with the 27pF and 390kΩ
RC network. The maximum EAMP voltage output is precisely
clamped to the bandgap voltage (1.172V).
Synchronization Control
The synchronization frequency can be operated to a range of
6MHz to 8MHz by an external signal applied to the SYNC pin. The
SYNC pin has logic threshold levels of 0.4V and 1.5V for LOW and
HIGH respectively, to allow for external clock signals to be of
different magnitude regardless of supply voltage to ISL78236.
The 1st falling edge on the SYNC triggers the rising edge of the
PWM ON pulse of Channel 1. The 2nd falling edge of the SYNC
triggers the rising edge of the PWM ON pulse of Channel 2.
Typically, the pulse width of the SYNC signal should be 50% duty
cycle, however, it is recommended that the pulse width be in the
range of 50ns to 100ns for valid synchronization. This process
alternates indefinitely allowing 180°output phase operation
between the two channels. It is important to note that this
operation makes the switching frequency of each channel 1/2 of
the SYNC frequency. Thus, Channel 1 and Channel 2 have a
synchronized switching frequency of 3MHz to 4MHz.
Output Current Sharing
The ISL78236 dual outputs are paralleled for multi-phase
operation in order to support 6A output. Channel 1 and Channel 2
switches 180° out-of-phase to reduce input ripple currents. In
parallel configuration, external soft-start should be used to ensure
proper full loading start-up. Connect the FBx pins together and
connect a soft-start capacitor from SS pin to GND. External
compensation using the COMP pin is required for current sharing
operation. Please see Table 2 for recommended values in current
sharing mode. The current sharing balancing is dependent on the
current sense amplifier matching between the two channels. The
matching is internally trimmed and provides excellent balancing
of output currents. See Figures 47 and 48 for typical output
current matching.
Overcurrent Protection
VEAMP
CSA1 and CSA2 are used to monitor Output 1 and Output 2
channels respectively. The overcurrent protection is realized by
monitoring the CSA output with the OCP threshold logic, as
shown in the “Block Diagram” on page 5. The current sensing
circuit has a gain of 0.2V/A, from the P-MOSFET current to the
CSA output. When the CSA output reaches the threshold, the
OCP comparator is tripped to turn off the P-MOSFET immediately.
The overcurrent function protects the switching converter from a
shorted output by monitoring the current flowing through the
upper MOSFETs.
VCSA1
Duty
Cycle
IL
VOUT
FIGURE 49. PWM OPERATION WAVEFORMS
The output voltage is regulated by controlling the reference
voltage to the current loop. The bandgap circuit outputs a 0.8V
reference voltage to the voltage control loop. The feedback
voltage signal comes from the FB pin. The soft-start block only
affects the operation during the start-up and will be discussed
separately. The error amplifier is a transconductance amplifier
that converts the voltage error signal to a current output. The
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Upon detection of overcurrent condition, the upper MOSFET will
be immediately turned off and will not be turned on again until
the next switching cycle. Upon detection of the initial overcurrent
condition, the Overcurrent Fault Counter is set to 1 and the
Overcurrent Condition Flag is set from LOW to HIGH. If, on the
subsequent cycle, another overcurrent condition is detected, the
OC Fault Counter will be incremented. If there are 17 sequential
OC fault detections, the regulator will be shut down under an
Overcurrent Fault Condition. An Overcurrent Fault Condition will
result with the regulator attempting to restart in a hiccup mode
with the delay between restarts being 8 soft-start periods. At the
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April 28, 2014
ISL78236
end of the eighth soft-start wait period, the fault counters are
reset and soft-start is attempted again. If the overcurrent
condition goes away prior to the OC Fault Counter reaching a
count of four, the Overcurrent Condition Flag will set back to LOW.
The ISL78236 also features current sense amplifiers on the
N-MOSFET for Negative Overcurrent Protection. If the negative
output current reaches -2.5A, the part enters Negative OCP. At
this point, all switching stops and the part enters tri-state mode
while the pull-down FET is discharging the output until it reaches
normal regulation voltage, then the IC restarts.
Power-Good (PG)
There are two independent power-good signals. PG1 monitors
the Output Channel 1 and PG2 monitors the Output Channel 2.
When powering up, the open-collector Power-Good output holds
low for about 1ms after the output reaches within 8% of the
preset output voltage. The PG pin will pull low under fault
conditions when an overcurrent, OTP or UVLO event occurs.
UVLO
When the input voltage is below the undervoltage lockout (UVLO)
threshold (2.85V MAX), the regulator is disabled and the PG pin
will pull low.
Enable
The enable (ENx) inputs allows the user to control the turning on
or off each channel of the regulator for purposes such as low
power shutdown or power-up sequencing. When the regulator is
enabled, there is typically a 600µs delay for waking up the
bandgap reference, afterwards the soft start-up sequence
begins.
C SS  F 
t SS  S  = ----------------------6.25
(EQ. 2)
Discharge Mode (Soft-Stop)
When a transition to shutdown mode occurs, or the input UVLO
fault latch is set, the LX pin discharges to PGND through an
internal 100 switch.
Power MOSFETs
The integrated high-side and low-side power MOSFETs are
optimized for best efficiency while delivering up to 3A current.
The ON-resistance for the P-MOSFET is typically 50m and the
ON-resistance for the N-MOSFET is typical 50m.
100% Duty Cycle
The ISL78236 features 100% duty cycle operation to maximize
the battery life in portable applications. When the battery voltage
drops to a level that the ISL78236 can no longer maintain the
regulation at the output, the regulator completely turns on the
P-MOSFET. The maximum dropout voltage under the 100% duty
cycle operation is the product of the load current and the
ON-resistance of the P-MOSFET.
Thermal Shutdown
The ISL78236 has built-in thermal protection. When the internal
temperature reaches +150°C, the regulator is completely shut
down. As the temperature drops to +125°C, the ISL78236 resumes
operation by stepping through a soft start-up.
Applications Information
Output Inductor and Capacitor Selection
Soft Start-Up
The soft-start-up eliminates the in-rush current during the
start-up. The soft-start block outputs a ramp reference to both
the voltage loop and the current loop. The two ramps limit the
inductor current rising speed as well as the output voltage speed
so that the output voltage rises in a controlled fashion. When the
FB voltage is less than 0.2V, the PWM operating frequency is 1/2
of the normal frequency.
When the SS soft-start pin is tied to the VIN pin, the soft start-up
time is internally set to 1.5ms. This internal soft-start mode is
only for dual output operation. In current sharing mode, for
externally programmable soft-start time, connect a capacitor
from the SS pin to GND. A 5µA current source charges up the
soft-start capacitor and sets the soft-start ramp time. The
soft-start capacitor, CSS, should not be larger than 33nF. See
Equation 2 for calculating the soft-start ramp time.
To consider steady state and transient operation, ISL78236
typically uses a 0.6µH output inductor. Higher or lower inductor
value can be used to optimize the total converter system
performance. For example applications with output voltage
>3.3V, in order to decrease the inductor current ripple and output
voltage ripple, the output inductor value can be increased. The
inductor ripple current can be expressed in Equation 3:
VO 

V O   1 – ---------
V IN

I = --------------------------------------L  fS
(EQ. 3)
The inductor’s saturation current rating needs to be larger than
the peak current. The ISL78236 overcurrent protection threshold
is typically 4.8A. The saturation current needs to be over 4.8A for
maximum output current application.
It is recommended to operate the internal soft-start ramp time
only in Dual Output Mode. In Current Share Mode external
soft-start should be used.
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ISL78236
ISL78236 uses an internal compensation network and the output
capacitor value is dependent on the output voltage. The ceramic
capacitor is recommended to be X5R or X7R. The recommended
minimum output capacitor values for the ISL78236 are shown in
Table 3.
TABLE 3. MINIMUM OUTPUT CAPACITOR VALUE vs VOUT
Output Voltage Selection
The output voltage of the regulator can be programmed via an
external resistor divider, which is used to scale the output voltage
relative to the internal reference voltage and feed it back to the
inverting input of the error amplifier. Refer to Figures 3 and 4.
VOUT
(V)
COUT
(µF)
L
(µH)
1.2
2 x 22
0.5~1.1
The output voltage programming resistor, R2 (or R5 in
Channel 2), will depend on the desired output voltage of the
regulator. The value for the feedback resistor is typically between
50k and 312.5k. Setting R2 and VOUT, R3 will be:
1.6
2 x 22
0.5~1.1
(EQ. 6)
1.8
2 x 22
0.5~1.68
2.5
2 x 22
0.5~1.68
3.3
2 x 6.8
0.5~2.2
3.6
10
0.5~2.2
R 2 x0.8V
R 3 = ---------------------------------V OUT – 0.8V
For better performance, add 12pF in parallel with R2 (or R5) for
faster transient response
Minimum Output Voltage
In Table 3, the minimum output capacitor value is given for
different output voltages to make sure the converter system is
stable.
While ceramic capacitors offer excellent overall performance
and reliability, the actual in-circuit capacitance must be
considered. Ceramic capacitors are rated using large
peak-to-peak voltage swings and with no DC bias. In the DC/DC
converter application, these conditions do not reflect reality. As a
result, the actual capacitance may be considerably lower than
the advertised value. Consult the manufacturers data sheet to
determine the actual in-application capacitance. Most
manufacturers publish capacitance vs DC bias so that this effect
can be easily accommodated. The effects of AC voltage are not
frequently published, but an assumption of ~20% further
reduction will generally suffice. The result of these
considerations may mean an effective capacitance 50% lower
than nominal and this value should be used in all design
calculations. Nonetheless, ceramic capacitors are a very good
choice in many applications due to their reliability and extremely
low ESR.
Equations 4 and 5 allow calculation of the required capacitance
to meet a desired ripple voltage level. Additional capacitance
may be used.
For the ceramic capacitors (low ESR):
I
V OUTripple = --------------------------------------8 F SW C OUT
(EQ. 4)
where I is the inductor’s peak-to-peak ripple current, FSW is the
switching frequency and COUT is the output capacitor.
If using electrolytic capacitors then:
V OUTripple = I*ESR
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(EQ. 5)
20
The ISL78236 switching frequency FS (2.5MHz typical,
2.85MHz max) and the minimum LX pin ON Time (140ns, max)
sets a minimum duty cycle of the converter under worst case
scenario of 0.4 across temperature. Because of this minimum
duty cycle, there is an input VIN to output VOUT range the
ISL78236 is capable of regulating to. The ratio of output to input
(VOUT/VIN) must be higher than 0.4 to maintain output voltage
regulation. For example, it is not recommended to regulate below
2.0V for VIN = 5V and below 1.2V for VIN = 3V as the minimum
duty cycle limitation will impact output voltage. Note that when
external synchronization is used, the switching frequency is
higher than 2.85MHz which further restricts the VOUT/VIN range
of operation.
Input Capacitor Selection
The main functions for the input capacitor are to provide
decoupling of the parasitic inductance and to provide filtering
function to prevent the switching current flowing back to the
battery rail. One 22µF X5R or X7R ceramic capacitor is a good
starting point for the input capacitor selection per channel.
Loop Compensation Design
When a soft-start capacitor is connected to the SS pin, the COMP
pin is active for external loop compensation. The ISL78236 uses
constant frequency peak current mode control architecture to
achieve a fast loop transient response. An accurate current
sensing pilot device in parallel with the upper MOSFET is used for
peak current control signal and overcurrent protection. The
inductor is not considered as a state variable since its peak
current is constant, and the system becomes a single order
system. It is much easier to design a type II compensator to
stabilize the loop than to implement voltage mode control. Peak
current mode control has an inherent input voltage feed-forward
function to achieve good line regulation. Figure 50 shows the
small signal model of the synchronous buck regulator.
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ISL78236
^
iin
+
^
Vin
d^
ILd^ 1:D Vin
^
iL LP
RLP
+
2f c V o C o R t
3
R 6 = ---------------------------------- = 15.7 10  f c V o C o
GM  V FB
Ro
Where Rt is the current sense amplifier gain (0.2V/A) and GM is
the trans-conductance, gm, of the voltage error amplifier in each
phase (see “Electrical Specification” Table for “Error Amplifier
Trans-Conductance” on page 8). Compensator capacitor C6 and
C7 is then given by Equation 9.
Rc
RT
GAIN (VLOOP (S(fi))
vo^
Co
Ti(S)
d^
Ro Co Vo Co
Rc Co 1
C 6 = --------------- = --------------- ,C 7 = max of (--------------,----------------)
R6
Io R6
R 6 f s R 6
K
Fm
+
v^comp
-Av(S)
FIGURE 50. SMALL SIGNAL MODEL OF SYNCHRONOUS BUCK
REGULATOR
Vo
R2
C3
VFB
R3
VREF
(EQ. 9)
An optional zero can boost the phase margin. CZ2 is a zero due
to R2 and C3
Tv(S)
He(S)
(EQ. 8)
1
C 3 = ---------------f c R 2
(EQ. 10)
Example: VIN = 5V, VO = 1.8V, IO = 3A, Fs = 2.5MHz, R2 = 124k,
R3 = 100k, Co = 2X22µF/3mΩ, L = 0.6µH, fc = 100kHz, then
compensator resistance R6:
3
(EQ. 11)
R 6 = 15.7 10  100kHz  1.8V  44F = 124k
-
VCOMP
Use a standard 124kΩ 1% tolerance or better resistor.
GM
+
R6
1.8V  44 F
C 6 = -------------------------------- = 213pF
3A  124k
(EQ. 12)
3m  44F-,---------------------------------------------------1
C 7 = max (-------------------------------) = (1pF,1pF)
124k
  2.5MHz  124k
(EQ. 13)
C7
C6
Use the closest standard values for C6 and C7. There is
approximately 2pF parasitic capacitance from VCOMP to GND;
Therefore, C7 is optional. Use C6 = 220pF and C7 = OPEN.
FIGURE 51. TYPE II COMPENSATOR
Figure 51 shows the type II compensator and its transfer function
is expressed, as shown in Equation 7:
S 
S
 1 + ------------ 1 + -------------

GM  R 3
 cz1 
 cz2
v̂ COMP
A v  S  = -------------------- = -------------------------------------------------------- -------------------------------------------------------------- C6 + C7    R2 + R3  
S
S
v̂ FB
S 1 + -------------  1 + -------------





cp1
cp2
(EQ. 7)
where,
R2 + R3
C6 + C7
1
1
 cz1 = --------------- ,  cz2 = ---------------  cp1 = -----------------------  cp2 = ----------------------R6 C6 C7
C3 R2 R3
R6 C6
R2 C3
Compensator design goal:
High DC gain
Choose Loop bandwidth fc 100kHz or below
Gain margin: >10dB
Phase margin: >40°
The compensator design procedure is as follows:
The loop gain at crossover frequency of fc has a unity gain.
Therefore, the compensator resistance R6 is determined by
Equation 8.
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21
1
C 3 = ---------------------------------------------------- = 26pF
  100kHz  124k
(EQ. 14)
Use C3 = 22pF. Note that C3 may increase the loop bandwidth
from previous estimated value.
PCB Layout Recommendation
The PCB layout is a very important converter design step to make
sure the designed converter works well. For ISL78236, the power
loop is composed of the output inductor L’s, the output capacitor
COUT1 and COUT2, the LX’s pins, and the PGND pin. It is necessary to
make the power loop as small as possible and the connecting traces
among them should be direct, short and wide. The switching node of
the converter, the LX pins, and the traces connected to the node are
very noisy, so keep the voltage feedback trace away from these
noisy traces. The FB network should be as close as possible to its FB
pin. SGND should have one single connection to PGND. The input
capacitor should be placed as closely as possible to the VIN pin.
Also, the ground of the input and output capacitors should be
connected as closely as possible. The heat of the IC is mainly
dissipated through the thermal pad. Maximizing the copper area
connected to the thermal pad is preferable. In addition, a solid
ground plane is helpful for better EMI performance. It is
recommended to add at least 5 vias ground connection within the
pad for the best thermal relief.
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ISL78236
Thermal Performance
PGND2
PGND2
PGND1
PGND1
LX1
24
23
22
21
20
19
LX2
1
18 LX1
VIN2
2
17 VIN1
VIN2
3
EN2
4
PG2
5
14 SS
FB2
6
13 EN1
16 VIN1
25
PAD
7
8
9
10
11
12
FB1
SGND
PG1
SYNC
15 VDD
NC
It is essential to have the package thermal pad connected to a
top layer PCB ground pad with the via connecting to additional
ground planes. This is where most of the thermal relief will occur.
The four PGND pins of the ISL78236 should be connected to the
thermal pad also. These connections provide the extra thermal
relief to minimize theta JA allowing the ISL78236 to maintain full
output current up to +105°C. See Figure 52 for an example
layout of the thermal relief pad.
LX2
The theta JA (JA) spec shown in the “Thermal Information” on
page 8 is based upon JEDEC standard JESD51-5. However, real
world application boards will differ from the JEDEC standard thus
producing different theta JA results. For example, the JESD51-5
specifies the thermal attach pad via only to the top buried layer.
Most practical applications will have the via connect to all layers
of the PCB board ground plane. JESD51-5 also requires that
buried planes use 1 oz. copper while the outer planes use 2 oz.
copper. It is recommended to have 2 oz. or greater copper on all
layers in application boards.
PIN 1
COMP
Delivering a full load output current of 6A across the ambient
operating temperature is strongly dependent on the thermal
characteristic of the PCB layout. The power dissipation of the IC
and the thermal impedance of the board will result in a
temperature gradient between ambient and junction. Power
dissipation curves for typical application can be found in
Figures 7 and 8 for dual output operation and Figures 37 and 38
for current sharing operation. If the junction temperature
exceeds the +150°C over-temperature protection threshold the
regulator will be disabled.
FIGURE 52. RECOMMENDED THERMAL PAD LAYOUT
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ISL78236
Revision History
The revision history provided is for informational purposes only and is believed to be accurate, but not warranted. Please go to web to make sure you
have the latest Rev.
DATE
REVISION
April 28, 2014
CHANGE
FN8624.0 Initial Release.
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ISL78236
Package Outline Drawing
L24.4x4D
24 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE
Rev 3, 11/13
4X 2.5
4.00
A
20X 0.50
B
PIN 1
INDEX AREA
PIN #1 CORNER
(C 0 . 25)
24
19
1
4.00
18
2.45 (+ 0.10mm)
(- 0.15mm)
13
0.15
(4X)
12
7
0.10 M C A B
0 . 07
24X 0 . 23 +- 0
. 05 4
24X 0 . 4 ± 0 . 1
TOP VIEW
BOTTOM VIEW
SEE DETAIL "X"
0.10 C
C
0 . 90 ± 0 . 1
BASE PLANE
( 3 . 8 TYP )
SEATING PLANE
0.08 C
SIDE VIEW
(
2 . 50 )
( 20X 0 . 5 )
C
0 . 2 REF
5
( 24X 0 . 25 )
0 . 00 MIN.
0 . 05 MAX.
( 24X 0 . 6 )
DETAIL "X"
TYPICAL RECOMMENDED LAND PATTERN
NOTES:
1. Dimensions are in millimeters.
Dimensions in ( ) for Reference Only.
2. Dimensioning and tolerancing conform to AMSE Y14.5m-1994.
3. Unless otherwise specified, tolerance : Decimal ± 0.05
4. Dimension b applies to the metallized terminal and is measured
between 0.15mm and 0.30mm from the terminal tip.
5. Tiebar shown (if present) is a non-functional feature.
6. The configuration of the pin #1 identifier is optional, but must be
located within the zone indicated. The pin #1 identifier may be
either a mold or mark feature.
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April 28, 2014