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SC4502/SC4502H 1.4Amp, 2MHz Step-Up Switching Regulator with Soft-Start
POWER MANAGEMENT Description
The SC4502/SC4502H is a high-frequency current-mode step-up switching regulator with an integrated 1.4A power transistor. Its high switching frequency (programmable up to 2MHz) allows the use of tiny surface-mount external passive components. Programmable soft-start eliminates high inrush current during start-up. The internal switch is rated at 32V (40V for the SC4502H) making the converter suitable for high voltage applications such as Boost, SEPIC and Flyback. The operating frequency of the SC4502/SC4502H can be set with an external resistor. The ability to set the operating frequency gives the SC4502/SC4502H design flexibilities. A dedicated COMP pin allows optimization of the loop response. The SC4502/SC4502H is available in thermally enhanced 10-pin MLPD package.
Features
Low saturation voltage switch: 210mV (250mV for the SC4502H) Constant switching frequency current-mode control Programmable switching frequency up to 2MHz Soft-Start function Input voltage ranges from 1.4V to 16V Output voltage up to 32V (40V for the SC4502H) Low shutdown current Adjustable undervoltage lockout threshold Small low-profile thermally enhanced lead free package. This product is fully WEEE and RoHS compliant.
Applications
Flat screen LCD bias supplies TFT bias supplies XDSL power supplies Medical equipment Digital video cameras Portables devices White LED power supplies
Typical Application Circuit
VIN 5V 8 OFF ON C1 2.2F 3 10 IN SHDN SC4502 SS GND C3 47nF 4,5 COMP ROSC 9 R4 C4 6,7 SW FB 2 1 R3 C6 R2 100K C2 10F L1 D1 10BQ015 VOUT 12V R1 866K
Efficiency
95 90 85 10.5H, 700KHz 5.3H, 1.4MHz
Efficiency (%)
80 75 70 65 60 55 50 VIN = 5V VOUT = 12V 0.0 0.1 0.2 0.3 0.4 0.5 3.3H, 2MHz
All Capacitors are Ceramic.
f (MHz) 0.7 1.4 2
R3 (K ) R4 (K ) 33.2 59.0 73.2 23.7 9.53 5.36
C4 (pF) 1500 560 330
C6 (pF) 22
L1 (H) 10.5 (Falco D08019) 5.3 (Sumida CDRH5D28) 3.3 (Coilcraf t DO1813P)
Load Current (A)
Figure 1(b). Efficiencies of 5V to 12V Boost Converters at 700KHz, 1.4MHz and 2MHz.
Figure 1(a). 5V to 12V Boost Converter.
Revision: July 25, 2005
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SC4502/SC4502H
POWER MANAGEMENT Absolute Maximum Rating
Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied.
Parameter Supply Voltage SW Voltage SW Voltage (SC4502H) FB Voltages SHDN Voltage Operating Temperature Range Thermal Resistance Junction to Ambient (MLPD-10) Maximum Junction Temperature Storage Temperature Range Lead Temperature (Soldering)10 sec
Symbol VIN V SW V SW V FB VSHDN TA J A TJ TSTG TLEAD ESD
Typ -0.3 to 16 -0.3 to 32 -0.3 to 40 -0.3 to 2.5 -0.3 to VIN + 1 -40 to +85 40 160 -65 to +150 260 2000
Units V V V V V C C/W C C C V
Electrical Characteristics Electrical Characteristics
Parameter Minimum Operating Voltage Maximum Operating Voltage Feedback Voltage
ESD Rating (Human Body Model)
Unless other specified: VIN = 2V, SHDN = 1.5V, ROSC = 7.68k, -40C < TA = TJ < 85C
Test Conditions
Min
Typ 1.3
Max 1.4 16
Unit V V V V %
TA = 25C -40C < TA < 85C 1.5V < VIN < 16V
1.224 1.217
1.242
1.260 1.267
Feedback Voltage Line Regulation FB Pin Bias Current Error Amplifier Transconductance Error Amplifier Open-Loop Gain COMP Source Current COMP Sink Current VIN Quiescent Supply Current VIN Supply Current in Shutdown Switching Frequency Maximum Duty Cycle Minimum Duty Cycle Switch Current Limit Switch Saturation Voltage Switch Saturation Voltage (SC4502H)
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0.01 40 60 49 80
nA -1 dB A A
VFB = 1.1V VFB = 1.4V VSHDN = 1.5V, VCOMP = 0 ( Not Switching ) VSHDN = 0 1.3 85
5 5 1.1 10 1.5 90 0 1.4 2 210 250 340 390 1.6 18 1.7
mA A MHz % % A mV mV
ISW = 1.3A ISW = 1.3A
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SC4502/SC4502H
POWER MANAGEMENT Electrical Characteristics (Cont.)
Unless other specified: VIN = 2V, SHDN = 1.5V, ROSC = 7.68k, -40C < TA = TJ < 85C
Parameter Swi tch Leakage C urrent Shutdown Threshold Voltage Shutdown Pi n C urrent Soft-Start C hargi ng C urrent Thermal Shutdown Temperature Thermal Shutdown Hysteresi s
Test C onditions V S W = 5V
Min
Typ 0.01
Max 1 1.18
U nit mA V A
1.02 VSHDN = 1.2V VSHDN = 0 VSS = 0.3V
1.1 -4.6 0 1.5 160 10
0.1
A A C C
Pin Configurations
TOP VIEW
Ordering Information
Device(1)(2) SC4502MLTRT SC4502HMLTRT S C 4502E V B SC4502HEVB P ackag e MLPD-10 MLPD-10 Temp. Range( TA) -40 to 85C -40 to 85C
Evaluation Board Evaluation Board
(10 Pin - MLPD, 3 x 3mm)
Notes: (1) Only available in tape and reel packaging. A reel contains 3000 devices for MLP package. (2) Lead free product. This product is fully WEEE and RoHS compliant.
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SC4502/SC4502H
POWER MANAGEMENT Pin Descriptions
Pin 1 2 Pin Name COMP FB Pin Function The output of the internal transconductance error amplifier. This pin is used for loop compensation. The inverting input of the error amplifier. Tie to an external resistive divider to set the output voltage. Shutdown Pin. The accurate 1.1V shutdown threshold and the 4.6uA shutdown pin current hysteresis allow the user to set the undervoltage lockout threshold and hysteresis for the switching regulator. Pulling this pin below 0.1V causes the converter to shut down to low quiescent current. Tie this pin to IN if the UVLO and the shutdown features are not used. This pin should not be left floating. Ground. Tie both pins to the ground plane. Pins 4 and 5 are not internally connected. Collector of the internal power transistor. Connect to the boost inductor and the rectifying diode. Power Supply Pin. Bypassed with capacitors close to the pin. A resistor from this pin to the ground sets the switching frequency. Soft-Start Pin. A capacitor from this pin to the ground lengthens the start-up time and reduces startup current. The exposed pad must be soldered to the ground plane on the PCB for good thermal conduction.
3
SHDN
4,5 6,7 8 9 10
GND SW IN ROSC SS Exposed Pad
Block Diagram
IN 8 4.6A SW SW 6 7
SHDN 3
+
1.1V
CMP
INTERNAL SUPPLY ENABL E CLK
REG
VOLTAGE REFERENCE
THERMAL SHUTDOWN
FB 2 COMP 1 SS 10
1.242V
+
-
EA REG 1.5A
-
R S Q
+
PWM
+
ILIM
-
I-LIMIT
REG_GOOD ENABL E
RSENSE
+ +
+
ISEN
4 5 GND GND
ROSC 9
CLK
OSCILLATOR
SLOPE COMP
Figure 2. SC4502/SC4502H Block Diagram.
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SC4502/SC4502H
POWER MANAGEMENT Typical Characteristics
Feedback Voltage vs Temperature
1.3
100
ROSC vs Switching Frequency
1.7
Switching Frequency vs Temperature
ROSC = 7.68K
Feedback Voltage (V)
1.25
VIN = 2V ROSC (K ) 25C
10
Frequency (MHz)
1.6
VIN = 12V
1.5
VIN = 2V
1.4
1.2
1.15 -50 -25 0 25 50 75 100 125
1 0.0 0.5 1.0 1.5 2.0 2.5 3.0
1.3 -50 -25 0 25 50 75 100 125
Temperature (C)
Frequency (MHz)
Temperature (C)
Switch Saturation Voltage vs Switch Current
500 500
Switch Saturation Voltage vs Switch Current
1.5
Minimum VIN vs Temperature
SC4502
400 400
SC4502H
1.4
300
300
Input Voltage (V)
1.5 2
VCESAT (mV)
VCESAT (mV)
1.3
85C
200
85C
200
25C
100
25C
100
1.2
1.1
0 0 0.5 1 1.5 2
0 0 0.5 1
1 -50 -25 0 25 50 75 100 125
Switch Current (A)
Switch Current (A)
Temperature (C)
VIN Quiescent Current vs Temperature
1.3 50
VIN Current in Shutdown vs Input Voltage
1.20
Shutdown Threshold vs Temperature
VIN = 2V Shutdown Threshold (V)
Not Switching
1.2 40
1.15
VIN Current (mA)
1.1
VIN Current (A)
VIN = 16V
-40C
30
125C
20
1.10
1
VIN = 2V
0.9
10 VSHDN = 0
1.05
0.8 -50 -25 0 25 50 75 100 125
0 0 5 10 15 20
1.00 -50 -25 0 25 50 75 100 125
Temperature (C)
Input Voltage (V)
Temperature (C)
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SC4502/SC4502H
POWER MANAGEMENT Typical Characteristics
VIN Current vs SHDN Pin Voltage
1.2 VIN = 2V 1
0.08 0.1 VIN = 2V
VIN Current vs SHDN Pin Voltage
-3
Shutdown Pin Current vs Temperature
VSHDN = 1.25V
VIN Current (mA)
0.06
Current (A)
0.8 0.6 0.4 0.2 0 0 0.5 1
VIN Current (mA)
125C
25C
-4
VIN = 2V
-5
0.04
-40C
125C -40C
VIN = 12V
0.02
0
-6 0 0.2 0.4 0.6 0.8 1 1.2 -50 -25 0 25 50 75 100 125
1.5
SHDN Voltage (V)
SHDN Voltage (V)
Temperature (C)
Soft-Start Charging Current vs Temperature
2
Transconductance vs Temperature
80 2.4
Switch Current Limit vs Temperature
V SS = 0.3V
VIN = 2V Transconductance ( )
70
-1
1.8
1.6
60
Current Limit (A)
2.2
Current (A)
2
1.4
50
1.2
1.8 40
1 -50 -25 0 25 50 75 100 125
30 -50 -25 0 25 50 75 100 125
1.6 -50 -25 0 25 50 75 100
Temperature (C)
Temperature (C)
Temperature (C)
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SC4502/SC4502H
POWER MANAGEMENT Operation
The SC4502/SC4502H is a programmable constantfrequency peak current-mode step-up switching regulator with an integrated power transistor. As shown in the block diagram in Figure 2, the power transistor is turned on at the trailing edge of the clock. Switch current is sensed with an integrated sense resistor. The sensed current signal is summed with the slope-compensating ramp before compared to the output of the error amplifier EA. The PWM comparator trip point determines the switch turn-on pulse width. The current-limit comparator ILIM turns off the power switch when the switch current exceeds the 2A current-limit threshold. ILIM therefore provides cycle-by-cycle current limit. Current-limit is not affected by slope compensation because the current limit comparator ILIM is not in the PWM signal path. Current-mode switching regulators utiilize a dual-loop feedback control system. In the SC4502/SC4502H the amplifier output COMP controls the peak inductor current. This is the inner current loop. The double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop, easing loop compensation. Fast transient response can be obtained with a simple Type-2 compensation network. In the outer loop, the error amplifier regulates the output voltage. The switching frequency of the SC4502/SC4502H can be programmed up to 2MHz with an external resistor from the ROSC pin to the ground. For converters requiring extremely low or high duty cycles, the operating frequency can be lowered to maintain the necessary minimum on time or the minimum off time. The SC4502/SC4502H requires a minimum input of 1.4V to operate. A voltage higher than 1.1V at the shutdown pin enables the internal linear regulator REG in the SC4502/SC4502H. After VREG becomes valid, the softstart capacitor is charged with a 1.5A current source. A PNP transistor clamps the output of the error amplifier as the soft-start capacitor voltage rises. Since the COMP voltage controls the peak inductor current, the inductor current is ramped gradually during soft-start, preventing high input start-up current. Under fault conditions (VIN<1.4V or over temperature) or when the shutdown pin is pulled below 1.1V, the soft-start capacitor is discharged to ground. Pulling the shutdown pin below 0.1V reduces the total supply current to 10A.
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Applications Information
Setting the Output Voltage An external resistive divider R1 and R2 with its center tap tied to the FB pin (Figure 3) sets the output voltage.
V R1 = R 2 OUT - 1 1.242V
VOUT
(1)
R1 40nA 2
SC4502/SC4502H FB
R2
Figure 3. The Output Voltage is set with a Resistive Divider
The input bias current of the error amplifier will introduce an error of:
VOUT 40nA (R1//R 2 ) 100 = % VOUT 1.242V
(2)
The percentage error of a VOUT = 5V converter with R1 = 100K and R2 = 301K is
VOUT 40nA (100 K // 301K ) 100 = = 0.24% VOUT 1.242V
Operating Frequency and Efficiency Switching frequency of SC4502/SC4502H is set with an external resistor from the ROSC pin to the ground. A graph showing the relationship between R OSC and switching frequency is given in the "Typical Characteristics". High frequency operation reduces the size of passive components but switching losses are higher. The efficiencies of 5V to 12V converters operating at 700KHz, 1.4MHz and 2MHz are plotted in Figure 1(b) for SC4502. Duty Cycle The duty cycle D of a boost converter in continuous conduction mode is:
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SC4502/SC4502H
POWER MANAGEMENT Applications Information
VIN VOUT + VD D= VCESAT 1- VOUT + VD 1-
It is worth noting that IOUTMAX is directly proportional to the (3)
VIN ratio of V . Equation (4) over-estimates the maximum OUT
output current at high frequencies (>1MHz) since switching losses are neglected in its derivation. Nevertheless it is a useful first-order approximation. Using VCESAT = 0.3V, VD = 0.5V and ILIM = 1.4A in (3) and (4), the maximum output currents for three VIN and VOUT combinations are shown in Table 1.
VIN ( V ) 2.5 3.3 5 VOUT ( V ) 12 5 12 D 0.820 0.423 0.615 IOUTMAX ( A ) 0.25 0.80 0.53
where VCESAT is the switch saturation voltage and VD is the voltage drop across the rectifying diode. Maximum Output Current In a boost switching regulator the inductor is connected to the input. The DC inductor current is the input current. When the power switch is turned on, the inductor current flows through the switch. When the power switch is off, the inductor current flows through the rectifying diode to the output. The maximum output current is the average diode current. The diode current waveform is trapezoidal with pulse width (1 - D)T (Figure 4). The output current available from a boost converter therefore depends on the converter operating duty cycle. The power switch current in the SC4502/SC4502H is internally limited to 2A. This is also the maximum inductor or the input current. By estimating the conduction losses in both the switch and the rectifying diode, an expression of the maximum available output current of a boost converter can be derived as follows:
ILIM VIN D VD - D(VD - VCESAT ) - 1 - VOUT 45 VIN
Table 1. Calculated Maximum Output Current [ Equation (4)]
Considerations for High Frequency Operation The operating duty cycle of a boost converter decreases as VIN approaches VOUT. The PWM modulating ramp in a current-mode switching regulator is the sensed current signal. This current ramp is absent unless the switch is turned on. The intersection of this ramp with the output of the voltage feedback error amplifier determines the switch pulse width. The propagation delay time required to immediately turn off the switch after it is turned on is the minimum switch on time. Regulator closed-loop measurement shows that the SC4502/SC4502H has a minimum on time of about 150ns at room temperature. The power switch in the SC4502/SC4502H is either not turned on at all or on for at least 150ns. If the required switch on time is shorter than the minimum on time, the regulator will either skip cycles or it will start to jitter. Example: Determine the maximum operating frequency of a Li-ion cell to 5V converter using the SC4502. Assuming that VD=0.5V, VCESAT=0.3V and VIN=2.6V - 4.2V, the minimum duty ratio can be found using (3).
4.2 5 + 0.5 = 0.25 = 0.3 1- 5 + 0.5 1-
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IOUTMAX =
(4)
where ILIM is the switch current limit.
IIN Inductor Current ON OFF ON Switch Current
Diode Current
DT ON OFF
(1-D)T IOUT ON OFF ON
Figure 4. Current Waveforms in a Boost Regulator
DMIN
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SC4502/SC4502H
POWER MANAGEMENT Applications Information
The absolute maximum operating frequency of the operating in continuous-conduction mode is
DMIN 0.25 = = 1.67MHz . The 150ns 150ns actual operating frequency needs to be lower to allow for modulating headroom.
converter is therefore The power transistor inside the SC4502/SC4502H is turned off every switching cycle for an interval determined by the discharge time of the oscillator ramp plus the propagation delay of the power switch. This minimum off time limits the maximum duty cycle of the regulator at a given switching frequency. A boost converter with high
D (VIN - VCESAT) (5) f L where f is the switching frequency and L is the inductance. IL =
Substituting (3) into (5) and neglecting VCESAT ,
IL = VIN VIN 1 - V +V f L OUT D
(6)
VOUT VIn ratio requires long switch on time and high duty cycle.
If the required duty cycle is higher than the attainable maximum, the converter will operate in dropout. (Dropout is the condition in which the regulator cannot attain its set output voltage below current limit.) The minimum off times of closed-loop boost converters set to various output voltages were measured by lowering their input voltages until dropout occurs. It was found that the minimum off time of the SC4502/SC4502H ranged from 80ns to 110ns at room temperature. Beware of dropout while operating at very low input voltages (1.5V-2V) with off time approaching 110ns. Shorten the PCB trace between the power source and the device input pin, as line drop may be a significant percentage of the input voltage. A regulator in dropout may appear as if it is in current limit. The cycle-by-cycle current limit of the SC4502/SC4502H is duty-cycle and input voltage invariant and is typically 2A. If the switch current limit is not at least 1.4A, then the converter is likely in dropout. The switching frequency should then be lowered to improve controllability. Both the minimum on time and the minimum off time reduce control range of the PWM regulator. Bench measurement showed that reduced modulating range started to be a problem at frequencies over 2MHz. Although the oscillator is capable of running well above 2MHz, controllability limits the maximum operating frequency. Inductor Selection The inductor ripple current I L of a boost converter
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In peak current-mode control, the slope of the modulating (sensed switch current) ramp should be steep enough to lessen jittery tendency but not so steep that large flux swing decreases efficiency. Inductor ripple current DIL between 25%-40% of the peak inductor current limit is a good compromise. Inductors so chosen are optimized in size and DCR. Setting IL = 0.3*(1.4A) = 0.42A, VD=0.5V in (6),
L= VIN f IL VIN VIN VIN 1 - = 0.42A f 1 - V + 0.5V (7) VOUT + VD OUT
where L is in H and f is in MHz. Equation (6) shows that for a given VOUT, IL is the highest when VIN = . If VIN varies over a wide range, then 2 choose L based on the nominal input voltage. The saturation current of the inductor should be 20%30% higher than the peak current limit (2A). Low-cost powder iron cores are not suitable for high-frequency switching power supplies due to their high core losses. Inductors with ferrite cores should be used. Input Capacitor The input current in a boost converter is the inductor current, which is continuous with low RMS current ripples. A 2.2F-4.7F ceramic input capacitor is adequate for most applications. Output Capacitor Both ceramic and low ESR tantalum capacitors can be used as output filtering capacitors. Multi-layer ceramic capacitors, due to their extremely low ESR (<5m), are the best choice. Use ceramic capacitors with stable
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(VOUT + VD )
SC4502/SC4502H
POWER MANAGEMENT Applications Information
temperature and voltage characteristics. One may be tempted to use Z5U and Y5V ceramic capacitors for output filtering because of their high capacitance and small sizes. However these types of capacitors have high temperature and high voltage coefficients. For example, the capacitance of a Z5U capacitor can drop below 60% of its room temperature value at -25C and 90C. X5R ceramic capacitors, which have stable temperature and voltage coefficients, are the preferred type. The diode current waveform in Figure 4 is discontinuous with high ripple-content. In a buck converter, the inductor ripple current IL determines the output ripple voltage. The output ripple voltage of a boost regulator is however much higher and is determined by the absolute value of the inductor current. Decreasing the inductor ripple current does not appreciably reduce the output ripple voltage. The current flowing in the output filter capacitor is the difference between the diode current and the output current. This capacitor current has a RMS value of:
VOUT -1 VIN
Volts higher than the output voltage. For switching regulators operating at low duty cycles (i.e. low output voltage to input voltage conversion ratios), it is beneficial to use rectifying diodes with somewhat higher RMS current ratings (thus lower forward voltages). This is because the diode conduction interval is much longer than that of the transistor. Converter efficiency will be improved if the voltage drop across the diode is lower. The rectifying diodes should be placed close to the SW pins of the SC4502/SC4502H to minimize ringing due to trace inductance. Surface-mount equivalents of 1N5817, 1N5819, MBRM120, MBR0520 (ON Semi) and 10BQ015, 10BQ040 (IRF) are all suitable. Soft-Start Soft-start prevents a DC-DC converter from drawing excessive current (equal to the switch current limit) from the power source during start up. If the soft-start time is made sufficiently long, then the output will enter regulation without overshoot. An external capacitor from the SS pin to the ground and an internal 1.5A charging current source set the soft-start time. The soft-start voltage ramp at the SS pin clamps the error amplifier output. During regulator start-up, COMP voltage follows the SS voltage. The converter starts to switch when its COMP voltage exceeds 0.7V. The peak inductor current is gradually increased until the converter output comes into regulation. If the shutdown pin is forced below 1.1V or if a fault situation is detected, then the soft-start capacitor will be discharged to ground immediately. The SS pin can be left open if soft-start is not required. Shutdown The input voltage and shutdown pin voltage must be greater than 1.4V and 1.1V respectively to enable the SC4502/ SC4502H. Forcing the shutdown pin below 1.1V stops the SC4502/SC4502H from switching. Pulling this pin below 0.1V completely shuts off the SC4502/SC4502H. The total VIN shutdown current decreases to 10A at 2V. Figure 5 shows several ways of interfacing the control logic to the shutdown pin. Beware that the shutdown pin is a high impedance pin. It should always be driven from a low-impedance source or tied to a resistive divider. Floating the shutdown pin will result in undefined voltage. In Figure 5(c) the shutdown pin is driven from a logic
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IOUT
(8)
If a tantalum capacitor is used, then its ripple current rating in addition to its ESR will need to be considered. When the switch is turned on, the output capacitor supplies the load current IOUT (Figure 4). The output ripple voltage due to charging and discharging of the output capacitor is therefore:
VOUT = IOUT D T COUT
(9)
For most applications, a 10F - 22F ceramic capacitor is sufficient for output filtering. It is worth noting that the output ripple voltage due to discharging of a 10F ceramic capacitor (9) is higher than that due to its ESR. Rectifying Diode For high efficiency, Schottky barrier diodes should be used as rectifying diodes for the SC4502/SC4502H. These diodes should have a RMS current rating between 0.5A and 1A with a reverse blocking voltage of at least a few
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SC4502/SC4502H
POWER MANAGEMENT Applications Information
IN SC4502 SC4502H SHDN
IN SC4502 SC4502H SHDN
(a)
(b)
VIN 1N4148
IN SC4502 SC4502H SHDN
IN SC4502 SC4502H SHDN
(c)
(d)
Figure 5. Methods of Driving the Shutdown Pin (a) Directly Driven from a Logic Gate (b) Driven from an Open-drain N-channel MOSFET or an Open-Collector NPN Transistor (VOL < 0.1V) (c) Driven from a Logic Gate with VOH > VIN (d) Combining Shutdown with Programmed UVLO (See Section Below).
gate whose VOH is higher than the supply voltage of the SC4502/SC4502H. The diode clamps the maximum shutdown pin voltage to one diode voltage above the input power supply.
Programming Undervoltage Lockout
The SC4502/SC4502H has an internal VIN undervoltage lockout (UVLO) threshold of 1.4V. The transition from idle to switching is abrupt but there is no hysteresis. If the input voltage ramp rate is slow and the input bypass is limited, then sudden turn on of the power transistor will cause a dip in the line voltage. Switching will stop if VIN falls below the internal UVLO threshold. The resulting output voltage rise may be non-monotonic. The 1.1V disable threshold of the SC4502/SC4502H can be used
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in conjunction with a resistive voltage divider to raise the UVLO threshold and to add an UVLO hysteresis. Figure 6 shows the scheme. Both VH and VL (the desired upper and the lower UVLO threshold voltages) are determined by the 1.1V threshold crossings, VH and VL are therefore:
R VH = 1 + 3 (1.1V ) R 4 VL = VH - VHYS = VH - IHYSR3
(10)
Re-arranging,
R3 =
R4 =
VHYS IHYS
R3 VH -1 1.1V
(11)
(12)
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SC4502/SC4502H
POWER MANAGEMENT Applications Information
VL = VH - VHYS = 2.75V - 0.69V = 2.06V > 1.4V .
IN 6/8 I HYS R3
Frequency Compensation Figure 7 shows the equivalent circuit of a boost converter using the SC4502/SC4502H. The output filter capacitor and the load form an output pole at frequency:
p2 =
Y
4.6 A SWITCH CLOSED WHEN Y = "1"
SHDN 3 1.1V + -
2 IOUT 2 = VOUT C2 ROUT C2
(13)
where C2 is the output capacitance and ROUT = the equivalent load resistance.
R4
COMPARATOR
VOUT is IOUT
SC4502/SC4502H
Figure 6. Programmable Hysteretic UVLO Circuit
The zero formed by C2 and its equivalent series resistance (ESR) is neglected due to low ESR of the ceramic output capacitor. There is also a right half plane (RHP) zero with angular frequency:
ROUT (1 - D) L
2
with VL > 1.4 V . Example: Increase the turn on voltage of a VIN = 3.3V boost converter from 1.4V to 2.75V. Using VH = 2.75V and R4 = 100K in (12),
R3 = 150K .
Z2 =
(14)
z2 decreases with increasing duty cycle D and increasing IOUT. Using the 5V to 12V boost regulator (1.4MHz) in Figure 1(a) as an example,
R OUT 5V = 10 0.5A
The resulting UVLO hysteresis is:
VHYS = IHYSR3 = 4.6A * 150K = 0.69V
The turn off voltage is:
V IN POWER STAGE C5 R1
I
OUT V OUT
ESR C2
R OUT
COMP R3 C6 C4 RO
Gm
+
FB
1.242V VOLTAGE REFERENCE
R2
Figure 7. Simplified Block Diagram of a Boost Converter
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SC4502/SC4502H
POWER MANAGEMENT Applications Information
5 12 + 0.5 = 0.62 D= 0.3 1- 12 + 0.5 1-
p1 = 1 1 = ROC4 4.7M * 560pF = 380 rads -1 60Hz
C4 and R3 also forms a zero with angular frequency:
z1 = 1 1 = R3C4 59K * 560pF
Therefore
p2 2 = 20Krads-1 3.18KHz (10) (10F)
10 (1 - 0.62 ) = 272 Krads -1 43.4KHz 5.3H
2
= 30.3 Krads -1 4.8 KHz
and
z2
The poles p1, p2 and the RHP zero z2 all increase phase shift in the loop response. For stable operation, the overall loop gain should cross 0dB with -20dB/decade slope. Due to the presence of the RHP zero, the crossover frequency
The spacing between p2 and z2 is the closest when the converter is delivering the maximum output current from the lowest V IN . This represents the worst-case compensation condition. Ignoring C 5 and C 6 for the moment, C 4 forms a low frequency pole with the equivalent output resistance RO of the error amplifier:
RO = Amplifier Open Loop Gain 49dB = = 4.7M Transconductance 60 -1
z2 . Placing z1 near p2 nulls its 3 effect and maximizes loop bandwidth. Thus
should not be higher than
R3C4 VOUT C2 2 IOUT(MAX)
(15)
R3 determines the mid-band loop gain of the converter. Increasing R 3 increases the mid-band gain and the crossover frequency. However it reduces the phase margin. The values of R 3 and C 4 can be determined
GND
R3 C4 C6 C3 R4
R2
U1 C1
SHDN
R1 C5 C2
L1
VOUT
D1
VIN
Figure 8. Suggested PCB Layout for the SC4502/SC4502H. Notice that there is no via directly under the device. All vias are 12mil in diameter.
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SC4502/SC4502H
POWER MANAGEMENT Applications Information
empirically by observing the inductor current and the output voltage during load transient. Compensation is optimized when the largest R3 and the smallest C4without producing ringing or excessive overshoot in its inductor current and output voltage are found. Figures 9(b), 10(c), 11(b) and 11(c) show load transient responses of empirically optimized DC-DC converters. In a batteryoperated system, compensating for the minimum VIN and the maximum load step will ensure stable operation over the entire input voltage range. C5 adds a feedforward zero to the loop response. In some cases, it improves the transient speed of the converter. C6 rolls off the gain at high frequency. This helps to stabilize the loop. C5 and C6 are often not needed. Board Layout Considerations In a step-up switching regulator, the output filter capacitor, the main power switch and the rectifying diode carry switched currents with high di/dt. For jitter-free operation, the size of the loop formed by these components should be minimized. Since the power switch is integrated inside the SC4502/SC4502H, grounding the output filter capacitor next to the SC4502/SC4502H ground pin minimizes size of the high di/dt current loop. The input bypass capacitors should also be placed close to the input pins. Shortening the trace at the SW node reduces the parasitic trace inductance. This not only reduces the EMI but also decreases the sizes of the switching voltage spikes and glitches. Figure 8 shows how various external components are placed around the SC4502/SC4502H. The frequencysetting resistor should be placed near the ROSC pin with a short ground trace on the PC board. These precautions reduce switching noise pickup at the ROSC pin. To achieve a junction to ambient thermal resistance (JA) of 40C/W, the exposed pad of the SC4502/SC4502H should be properly soldered to a large ground plane. Use only 12mil diameter vias in the ground plane if necessary. Avoid using larger vias under the device. Molten solder may seep through large vias during reflow, resulting in poor adhesion, poor thermal conductivity and low reliability.
Typical Application Circuits
VIN 3.3V 8 OFF ON C1 2.2F C3 47nF IN 3 SHDN 10
L1 5.6H 6,7 SW FB COMP ROSC 9 R4 9.31K 4,5 2 1
D1 10BQ015
VOUT 12V, 0.3A R1 174K
SC4502 SS GND
C2 10F R3 40.2K C4 1.8nF R2 20K
40s/div L1: Sumida CR43 Figure 9(a). 1.35 MHz All Ceramic Capacitor 3.3V to 12V Boost Converter. Upper Trace : Output Voltage, AC Coupled, 1V/div Lower Trace : Inductor Current, 0.5A/div Figure 9(b). Load Transient Response of the Circuit in Figure 9(a). ILOAD is switched between 0.1A and 0.3A at 1A/s.
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SC4502/SC4502H
POWER MANAGEMENT Typical Application Circuits
Efficiency vs Load Current
95
2.6 - 4.2V L1 2.5H 8 OFF ON 3 1-CELL LI-ION C1 2.2F 10 IN SHDN SC4502 SS GND C3 47nF 4,5 COMP ROSC 9 R4 7.68K 6,7 SW FB 2 1 R3 34.8K C4 1nF R2 100K C2 10F D1 10BQ015 VOUT 5V, 0.5A
90 85 Efficiency (%) 80 75 70 65 60 55
1.5MHz
R1 301K
VIN = 4.2V
VIN = 3.6V
VIN = 2.6V VOUT = 5V
50 0.001
0.010
0.100
1.000
L1: Sumida CDRH5D28 Figure 10(a). 1.5 MHz All Ceramic Capacitor Single Li-ion Cell to 5V Boost Converter.
Load Current (A)
Figure 10(b). Efficiency of the Single Li-ion Cell to 5V Boost Converter in Figure 10(a).
VIN=2.6V
40s/div Upper Trace : Output Voltage, AC Coupled, 0.5V/div Lower Trace : Inductor Current, 0.5A/div Figure 10(c). Load Transient Response of the Circuit in Figure . 10(a). ILOAD is switched between 90mA and 0.5A at 1A/s.
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SC4502/SC4502H
POWER MANAGEMENT Typical Application Circuits
4-CELL 3.6 - 6V
L1 3.3H 8 OFF ON 3 C1 2.2F 10 IN SHDN SC4502 SS GND C3 47nF 4,5 COMP ROSC 9 R4 7.68K 6,7 SW FB 2 1
C6 2.2F
D1 10BQ015
VOUT 5V, 0.5A
R1 60.4K
C2 10F R3 35.7K C4 1.5nF L2 3.3H C6 22pF R2 20K
L1 and L2: Coiltronics DRQ73-3R3 Figure 11(a). 1.5 MHz All Ceramic Capacitor 4-Cell to 5V SEPIC Converter.
VIN=3.6V
VIN=6V
40s/div Upper Trace : Output Voltage, AC Coupled, 0.5V/div Lower Trace : Input Inductor Current, 0.2A/div Figure 11(b). Load Transient Response of the Circuit in Figure 11(a). I LOAD is switched between 150mA and 450mA at 1A/s.
40s/div Upper Trace : Output Voltage, AC Coupled, 0.5V/div Lower Trace : Input Inductor Current, 0.2A/div Figure 11(c). Load Transient Response of the Circuit in Figure 11(a). I LOAD is switched between 250mA and 700mA at 1A/s.
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SC4502/SC4502H
POWER MANAGEMENT Typical Application Circuits
D2 D3 D4 D5 OUT2 23V (10mA) C8 1F
C5 0.1F
C6 0.1F
C7 0.1F
3.3V
L1 3.3H R5 150K 3 8 IN SHDN SC4502 SS GND C3 47nF 4,5 COMP ROSC 9 R4 7.68K 1 R3 33.2K C4 1.5nF 6,7 SW FB 2
D1 10BQ015
OUT1 8V (0.4A) R1 274K
C1 2.2F R6 100K
10
C9 0.1F
C2 10F R2 49.9K
D7
OUT3 -8V (10mA) C10 1F
L1 : Coiltronics SD18-3R3 D2 - D7 : BAT54S
D6
Figure 12(a). 1.5MHz Triple-Output TFT Power Supply.
CH4
CH4
CH1
CH1 CH2
CH2
CH3
CH3
4ms/div CH1 : OUT1 Voltage, 5V/div CH2 : OUT2 Voltage, 10V/div CH3 : OUT3 Voltage, 5V/div CH4 : Input Voltage, 2V/div Figure 12(b). TFT Power Supply VIN Start-up Transient.
2ms/div CH1 : OUT1 Voltage, 5V/div CH2 : OUT2 Voltage, 10V/div CH3 : OUT3 Voltage, 5V/div CH4 : SHDN Voltage, 2V/div Figure 12(c). TFT Power Supply Start-up Transient as the SHDN Pin is stepped from 0 to 2V.
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SC4502/SC4502H
POWER MANAGEMENT Typical Application Circuits
VIN 14V 8 OFF ON 3 SHDN C1 2.2F 10 SC4502H SS GND C3 47nF 4,5 COMP ROSC 9 R4 C4 1.5nF IN 6,7 SW FB 2 1 R3 5.11K R2 649 C2 10F L1 D1 SS14 VOUT 35V R1 17.4K
Efficiency (%) Efficiency at 14V input 90 85 80 75 70 65 60 55 50 0 0.05 0.1 Load (A) 0.15 0.2
1.4MHz 6.8H
1.0MHz 10H
Figure 13(a). All Ceramic Capacitor High Voltage Application
Figure 13(b). Efficiency of the All Ceramic Capacitor High Voltage Application
f (MHz )
1.0 1.4
R 4(K) 15.8 10
L1 10uH IHLP-2525BD _01 6.8uH IHLP-2525BD _01
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SC4502/SC4502H
POWER MANAGEMENT Outline Drawing - MLPD-10, 3 x 3mm
A E B
DIM
A A1 A2 b C D E e L N aaa bbb
DIMENSIONS INCHES MILLIMETERS MIN NOM MAX MIN NOM MAX
.039 .031 .002 .000 (.008) .007 .009 .011 .074 .079 .083 .042 .048 .052 .114 .118 .122 .020 BSC .012 .016 .020 10 .003 .004 0.80 1.00 0.00 0.05 (0.20) 0.18 0.23 0.30 1.87 2.02 2.12 1.06 1.21 1.31 2.90 3.00 3.10 0.50 BSC 0.30 0.40 0.50 10 0.08 0.10
E PIN 1 INDICATOR (LASER MARK)
A aaa C C 1 LxN 2 A1 A2 C
SEATING PLANE
D
N e bxN bbb CAB
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS TERMINALS.
Land Pattern - MLPD-10, 3 x 3mm
K
DIM
(C) H G Y X P C G H K P X Y Z
DIMENSIONS INCHES MILLIMETERS
(.112) .075 .055 .087 .020 .012 .037 .150 (2.85) 1.90 1.40 2.20 0.50 0.30 0.95 3.80
Z
NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET.
Contact Information
Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804
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