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SS6577
External NMOS Step-Down PWM Controller
FEATURES
N-Channel MOSFET Drive Operating input voltage from 4.5V to 24V Wide output Range : 0.8V to 20V Reference: 1.5% 0.8V Reference Low dropout operation : 95% duty cycle Fixed constant frequency - 500kHz Low standby current, I Q typically 720A Logic-control micropower shutdown Output overvoltage protection Internal diode for bootstrapped gate drive Current-mode operation for excellent line and load transient response Available in 8-lead SO or MSOP packages
DESCRIPTION
The SS6577 is a current-mode switching regulator controller that drives an external N-channel power MOSFET using a fixed frequency architecture. It uses an external divider to adjust the output voltage from 0.8V to 20V with excellent line and load regulation. A maximum high duty-cycle limit of 95% provides low dropout operation which extends operating time in battery-operated systems. A constant switching frequency of 500KHz is used thus allowing smaller sized filter components. The operating current level is user-programmable via an external current sense resistor. It also provides output overvoltage protection under fault conditions. A multifunction pin (ITH/RUN) allows external compensation for optimum load step response plus shutdown. Soft start can also be implemented with this pin to properly sequence supplies. Packages available are SOP-8 and MSOP-8 for SMD.
APPLICATIONS
LCD Monitor Palmtop Computers, PDAs Wireless Modems On-Card Switching Regulators DC Power Distribution Systems
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SS6577
TYPICAL APPLICATION CIRCUIT
1 2 C5 330pF R3 24k 3 4 8 7 6 5 C3 0.1F D1 SL43 1000pF C1 C2 0.1F RS 33m
+
VIN 6V~24V CIN1 22F
+
CS
VIN
ITH/RUN BOOST FB DRI
CIN2 22F
SW GND SS6577
M1 L1 10H COUT 220F C6 2.2F VOUT 3.3V 3A
C4 R1 20k 1nF R2 62k
CIN1, CIN2: HER-MEI 22F/35V Electrolytic capacitors M1: N-MOSFET SSM6680M D1: GS SL43 L1: TDK SLF12555T-100M3R4 COUT: HER-MEI 220F /16V Electrolytic capacitor C6: TAIYO YUDEN LMK212BJ225KG-T Ceramic capacitor
ORDERING INFORMATION
SS6577C(X)XXX Packing type TB: Tube TR: Tape and reel Package outline S: SO-8 O: MSOP-8 G: Pb-free lead finish Example: SS6577COTR in MSOP package shipped in tape and reel SS6577CGOTR in MSOP package with Pb-free lead finish shipped in tape and reel
PIN CONFIGURATION
TOP VIEW CS 1 ITH/RUN 2 FB 3 GND 4 8 7 6 5 VIN BOOST DRI SW
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ABSOLUTE MAXIMUM RATINGS
(Note 1) Supply Voltage (VIN) .........................................................................................25V Drive Supply Voltage (BOOST) .......................................................................32V Switch Voltage (SW) ........................................................................... 25V Differential Boost Voltage (BOOST to SW ) ........................................................8V ITH/RUN,VFB Voltages ............................................................................7V Peak Drive Output Current < 10S (DRI ) ..........................................................2A Operating Temperature Range ........................................................-40C ~ 85C Thermal Resistance (JA) (Assuming no ambient airflow, no heatsink)
SOP8 MSOP8
..............................................................................160C/W .............................................................................. 180C/W ............................................ -65C ~ 150C ...........................................300C
Storage Temperature Range Lead Temperature ( Soldering, 10sec )
TEST CIRCUIT
Refer to Typical Application Circuit.
ELECTRICAL CHARACTERISTICS
PARAMETER
Input Voltage Input Supply Current Feedback Voltage Output Overvoltage Lockout Reference Voltage Line Regulation Output Voltage Load Regulation Run Threshold Maximum Current Sense Threshold Oscillator Frequency VFB=0.72V
(TA=25C, VIN=15V, unless otherwise noted.)
TEST CONDITIONS
MIN.
4.5
TYP.
MAX.
24
UNIT
V A A V mV %/V
Normal Mode (Note 2) Shutdown Mode, VITH/RUN=0V 0.788 VFB connect to Vout, VOVL=VOVL-VFB VIN= 4.5V to 20 V ITH Sinking 5A ITH Sourcing 5A 0.6 125 450 20
720 16 0.8 55 0.002 0.7 -0.4 0.8 150 500
900 20 0.812 90 0.015 1.1 -0.8 0.9 175 550
% V mV kHz
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SS6577
ELECTRICAL CHARACTERISTICS (Continued)
PARAMETER
DRI Rise Time DRI Fall Time BOOST Voltage Maximum Duty Cycle Soft Start Time Run Current Source Run Pullup Current VITH/RUN=0V, VFB=0V VITH/RUN=1V
TEST CONDITIONS
CLOAD = 3000PF CLOAD = 3000PF VIN=8V, IBOOST=5mA, SW=0V
MIN.
TYP.
50 50
MAX.
75 75 5.7
UNIT
ns ns V % ms
4.9 90 5 1.0 100
5.3 94 7.5 2.3 190
4.0 250
A A
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency.
TYPICAL PERFORMANCE CHARACTERISTICS
100 100
VOUT=3.3V
95
VOUT=5V VIN=6V Efficiency (%) VIN=12V VIN=19V
95 90 85 80 75 70
VIN=6V VIN=12V VIN=19V
Efficiency (%)
90
85 80 75 70
Load Current (mA) Fig. 1 Efficiency vs Load Current (VOUT=3.3V)
1
10
100
1000
10000
1
10
100
1000
10000
Load Current (mA) Fig. 2 Efficiency vs Load Current (VOUT=5.0V)
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TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
100 100
95
VOUT=3.3V Efficiency (%)
95 90
VOUT=5V
Efficiency (%)
90
85 80
ILOAD=1A
85
ILOAD=1A ILOAD=0.1A
80
75
ILOAD=0.1 A
0 5 10 15 20 25 30
75
70
70 0 5 10 15 20 25 30
Input Voltage (V) Fig. 3 Efficiency vs Input Voltage
900 800 7
Input Voltage (V) Fig. 4 Efficiency vs Input Voltage
Normal Mode Boost Voltage (V)
6 5 4 3 2 1 0
Supply Current (A)
700 600 500
VCC=15V VCC=5V
40 20 0 0 5 10 15 20 25 30
Shutdown
VPHASE=0V
0 5 10 15 20
Input Voltage (V) Fig. 5 Supply Current vs Input Voltage
7 6
0.805 0.804
Boost Load Current (mA) Fig. 6 Boost Load Regulation
Reference Voltage (V)
VCC UP
0.803 0.802 0.801 0.800 0.799 0.798 0.797 0.796 0.795 0.794 0.793 0.792 0.791 0.790 -40
Boost Voltage (V)
5 4 3 2 1 0
VCC DOWN IBOOST=2mA VPHASE=0V
0 5 10 15 20 25 30
-20
0
20
40
60
80
100
120
140
Input Voltage (V) Fig. 7 Boost Line Regulation
Temperature (C) Fig. 8 Reference Voltage vs Temperature
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TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
6.0
500
Boost Voltage (V)
Frequency (KHz)
140
5.5
480
460
5.0
440
4.5
IBOOST=1mA VPHASE=0V
420
4.0 -40
400
-20
0
20
40
60
80
100
120
-40
-20
0
20
40
60
80
100
120
140
Temperature (C) Fig. 9 Boost Voltage vs Temperature
160
Temperature (C) Fig. 10 Operating Frequency vs Temperature
Current Sense Threshold (mV)
155 150 145 140 135 130 125 120 -40
-20
0
20
40
60
80
100
120
140
Temperature (C) Fig. 11 Maximum Current Sense Threshold vs Temperature
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SS6577
BLOCK DIAGRAM
CS VIN
VIN
VINT VINT
+
R1 + R2 40mV + LC_COMP VIN VINT INTVCC VIN VIN
2.5A
Q2
*
ICOMP
1.2V SD
Q1
*
Slope LEB Blank SD SD Thermal Switching Logic Floating Driver
Clock SS Q3 Q4 2.4V 0.8V + + Burst_Mode Clock R S
BOOST
DRI
+
1.33V ITH Buffer_ITH + EA 0.8V
SW Q VINT Dropout DET 1_SHUT M1 *
ITH
ITH
VIN 0.855V FB FB OVDT
+
REF 0.8V
OSC
Slope GND
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PIN DESCRIPTIONS
PIN 1: CS - Current sense comparator inverting input, not to exceed VIN voltage. Built in offsets between the CS and VIN pins in conjunction with RSENSE set the current trip thresholds. PIN4: GND - Singal GND for IC. All voltage levels are measured with respect to this pin. PIN 5: SW - Switch node connection to inductor. In buck converter applications the voltage swing at this pin is from a schottky diode voltage drop below ground to VIN - External high-side N-MOSFET gate drive pin. Connect DRI to gate of the external high-side NMOSFET.
PIN 2:
ITH/RUN -Combination of error amplifier compensation point and run control inputs. The current comparator threshold increases with this control voltage. Forcing this pin below 0.8V causes the device to be shutdown. - Feedback error amplifier input, to compare the feedback voltage with the internal reference voltage. Connecting a resistor R2 to converter output node and a resistor R1 to ground yields the output voltage: VOUT=0.8 x (R1+R2)/ R1
PIN 6: DRI
PIN 3: FB
PIN 7: BOOST - Supply to high-side floating driver. The bootstrap capacitor C3 is returned to this pin. PIN 8: VIN - The chip power supply pin. It also provides the gate bias charge for all the MOSFETs controlled by the IC. Recommend supply voltage is 4.5V~24V.
APPLICATION INFORMATION
Introduction The SS6577 is a current mode switching regulator controller that drives an external N-channel power MOSFET with constant frequency architecture. It uses an external divider to adjust output voltage with excellent line regulation and load regulation. A maximum high duty cycle limit of 95% provides low dropout operation, which extends operating time in battery-operated system. Wide input voltage ranges from 4.5V to 24V, and a switching frequency of 500KHz allows smaller sized filter components. The operating current level is user-programmable via an external current sense resistor and it automatically enters PFM operation at low output current to boost circuit efficiency. A multifunction pin (ITH/RUN) allows external compensation plus shutdown. A built-in soft start can properly provide sequenced supplies. Available packages are SOP8 and MSOP8 for SMD.
Principle of Operation The SS6577 uses a current mode with a constant frequency architecture. Normally high-side MOSFET turns on each cycle when oscillator sets RS latch and it turns off when internal current comparator resets RS latch. Voltage on ITH/RUN pin, which is the output voltage of voltage error amplifier, will control peak inductor current. The output voltage feeds back to VFB pin so that the error amplifier receives a voltage through external resistor divider. When load current increases, it causes a slight decrease
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SS6577
in the voltage of VFB pin. Thus the ITH/RUN voltage remains increasing until the average inductor current matches new load current. While the high-side MOSFET turns off, the low-side MOSFET is turned on to recharge bootstrap capacitor C3. Main control loop is shutdown when ITH/RUN goes below 0.8V. When ITH/RUN pulled up to 0.8V or up by error amplifier, main control loop is enabled. Low Current Operation During heavy load current operation, the SS6577 operates in PWM mode with a frequency of 500KHz. Decreasing of the current will cause a drop in ITH/RUN below 1.33V so that the SS6577 enters PFM mode operation for better efficiency. If the voltage across RS does not exceed the offset of current comparator within a cycle, then the high-side and internal MOSFETs will disable until ITH/RUN goes over 1.33V.
RS = 100mV IMAX
Inductor Selection With the high operating frequency of 500KHz, smaller inductor values are possible. In general, operating at high frequency will cause low efficiency because of large MOSFET switching loss. Thus the effect of inductor value on ripple current and low current operation must be considered as well. The inductor value has a direct influence on ripple current ( IL), which decreases with high inductance and increases with high VIN or VOUT:
IL = VIN - VOUT f xL
VOUT + VD V +V IN D
VD is the drop voltage of the output Schottky diode. Accepting a large value ofIL allows the use of low inductance, but yields high output ripple voltage and large core loss. The inductor value also has an effect on low current operation. Low inductor value causes the PFM operation to begin at high load current. The efficiency of the circuit decreases at the beginning of low current operation. Generally speaking, low inductance in PFM mode will cause the efficiency to decrease. Power MOSFET Selection For an application of SS65577, an external Nchannel power MOSFET, used as the high-side switch, must be properly selected. To prevent M OS FE T d a m a g e d u r i n g h i g h i n p u t vo l ta g e operation, attention should be given to the BV DSS specification of the MOSFET. Other important selection criteria for the power MOSFET include the "ON" resistance RDS(ON), input voltage and maximum output current.
Component Selection The SS6577 can be used in many switching regulator applications, such as step-down, step-up, SEPIC and positive-to-negative converters. Among these step-down converter is the most common application. External component selection, beginning with selecting RS, depends on load requirement of the application. Once RS is decided, the choice of inductor, which is followed by selecting power MOSFET and diode, can be easily chosen. Finally, CIN and COUT can be determined. RS Selection The choice of RS has substantial connection with required output current. The threshold voltage of current comparator decides peak inductor current, which yields a maximum average output current (IMAX), and the peak current is less than half of the peak-to-peak ripple current, IL. Allowing a margin for variation of the SS6577, external components can be calculated as:
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Output Diode Selection In order not to exceed the diode ratings, it is important to specify the diode peak current and average power dissipation. CIN and COUT Selection To prevent the high voltage spike resulted from high frequency switching, a low ESR input capacitor for the maximum RMS current must be used. Usually capacitors may be paralleled to meet size or height requirements in the design. The selection of COUT depends on the required effective series resistance (ESR). In general once the ESR requirement is met, the capacitance is suitable for filtering. The output ripple voltage (VOUT) is determined by: 1 VOUT IL ESR + 4fC OUT where f = operating frequency, COUT = output capacitance and IL = ripple current of the inductor. Once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement.
R2 VOUT = 0.8 V 1 + R1
The feedback reference voltage 0.8V allows low output voltages from 0.8V to input voltage. A small capacitor at 1nF in parallel to the upper feedback resistor is required for a stable feedback. ITH/RUN Function The ITH/RUN pin, also as a dual-purpose pin, provides loop compensation as well as shutdown function. An internal current source at 2.5A charges up the external capacitor C5. When the voltage on ITH/RUN pin reaches 0.8V, the SS6577 begins to operate.
VIN 4.5V~24V R4 1.2M
D2
ITH/RUN
C5
C7 1F
LL4148
330pF
R3
24k
Fig. 12 ITH/RUN pin interfacing Topside MOSFET Driver Supply (C3) External bootstrap capacitor C3 connecting to BOOST pin supplies the gate drive voltage for highside MOSFET. C3 is charged from INTVCC when SW pin is low. When the high-side MOSFET turns on, the driver places the C3 voltage across the gate to the source of MOSFET. It will enhance the MOSFET and turn on the high-side switch. Then the switch node voltage SW rises to VIN and BOOST pin rises to VIN + INTVCC. In general, 0.1F is acceptable. Output Voltage Programming The typical SS6577 application circuit is shown in figure17. A resistive divider, as in the following formula, sets the output voltage.
Over Current Protection Over current protection occurs when the peak inductor current reaches maximum current sense threshold divided by sense resistor. The maximum current under over current protection can be calculated by the following formula. 150mV(Maxi mum current sense threshold) IMAX = RS At the same time, the frequency of oscillator will be reduced to sixteenth of original value, 500kHz. This lower frequency allows the inductor current to safely discharge, thereby preventing current runaway. The frequency of oscillator will automatically
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return to its designed value when the peak inductor value no longer exceeds over current protection point. short as possible. In addition, in the case of a large current loop, the track width of each component in the loop should maintain as wide as possible. In order to prevent the effect from noise, the GND pin should be placed close to the ground. Also keep the IC's GND pin and the ground leads in the shortest distance. Recommended layout diagrams and component placement are as shown as figures 13 to 16. No sensitive components, which may cause noise interference to the circuit, should be allowed to be close to SW pin. Furthermore, the SS6577 is a current-mode controller. Keeping the sense resistor close to both VIN and CS pins is recommended for better efficiency and output performance. In addition, all filtering and decoupling capacitors, such as C1 and C2, should connect to the SS6577 as close as possible.
Over Voltage Protection Over voltage protection occurs when the FB pin voltage (the negative input of error amplifier) exceeds 0.855V. The over voltage comparator will force driver to pull low until output over voltage is removed.
PCB Layout Since operating at a high switching frequency, 500KHz, proper PCB layout and component placement may enhance the performance of the SS6577 application circuit. For a better efficiency, major loop from input terminal to output terminal should be as
Fig. 12 Top Layer
Fig. 13 Bottom Layer
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Fig. 14 Placement (Top Overlay)
Fig. 15 Placement (Bottom Overlay)
APPLICATION EXAMPLES
** VIN 6V~24V
R4 1.2M 1 2 C5 330pF R3 24k 3 4 8 7 6 5 C3 0.1F 1000pF C1 C2 0.1F RS 33m
+
VIN 6V~24V CIN1 22F
+
D2
CS
VIN
ITH/RUN BOOST FB DRI
C7 1F
LL4148
CIN2 22F
SW GND SS6577
M1 SSM6680M D1 SL43
L1
VOUT 3.3V 3A
10H COUT 220F C6 2.2F
C4 R1 20k R2 1nF 62k
Fig. 16 3.3V Step-Down Converter with External Soft-Start Circuit
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SS6577
1000pF C1 C2 0.1F RS 33m
+
1 2 C5 C7 1nF R3 24k 330pF 3 4
CS
VIN
8 7 6 5
VIN 5V CIN1 22F
+
ITH/RUN BOOST FB DRI
D2
CIN2 22F
SW GND SS6577
LL4148 C3
M1 SSM6680M D1 SL43
L1
VOUT 3.3V 3A COUT 220F C6 2.2F
10H
C4 R1 20k R2 1nF 62k
Fig. 17 5V to 3.3V Step-Down Converter
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PHYSICAL DIMENSIONS (unit: mm)
8 LEAD PLASTIC SO (CS)
D
SYMBOL A
H E
MIN 1.35 0.10 0.33 0.19 4.80 3.80 5.80 0.40
MAX 1.75 0.25 0.51 0.25 5.00 4.00 6.20 1.27
A1 B C D E
e A C A1
e H L
L
1.27(TYP)
B
MSOP 8 (CO)
D
SYMBOL A
H E
MIN 0.76 -0.28 0.13 2.90 2.90 0.65 4.80 0.40
MAX 0.97 0.20 0.38 0.23 3.10 3.10 5.00 0.66
A1 B C D E
e A C A1
e H L
L
B
Information furnished by Silicon Standard Corporation is believed to be accurate and reliable. However, Silicon Standard Corporation makes no guarantee or warranty, express or implied, as to the reliability, accuracy, timeliness or completeness of such information and assumes no responsibility for its use, or for infringement of any patent or other intellectual property rights of third parties that may result from its use. Silicon Standard reserves the right to make changes as it deems necessary to any products described herein for any reason, including without limitation enhancement in reliability, functionality or design. No license is granted, whether expressly or by implication, in relation to the use of any products described herein or to the use of any information provided herein, under any patent or other intellectual property rights of Silicon Standard Corporation or any third parties.
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