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FUJITSU SEMICONDUCTOR
DS04-27209-3E
DATA SHEET
ASSP
SWITCHING REGULATOR
CONTROLLER
MB3788
nnnn DESCRIPTION
The MB3788 is a dual-channel PWM-type switching regulator controller; it incorporates a reference voltage.
The MB3788 has a PWM circuit and an output circuit as well as a reference voltage power supply with a voltage
accuracy of – 1%. The maximum operating frequency is 1 MHz. It is designed for a voltage-drop output switching
regulator suitable for a logic power supply or speed control o
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MB3788 (Continued) • High-frequency operation: 100 kHz to 1 MHz • On-chip timer and latch-type short-circuit detection circuit • Wide error amplifier input voltage range: -0.2 V to VCC - 1.8 V • On-chip high-accuracy reference voltage circuit: 2.50 V – 1% • Output circuit PNP transistor drive output pin: Push-pull type ON/OFF current values set independently • On-chip standby function and output control function • High-density packaging: SSOP-24P nnnn PIN ASSIGNMENT (TOP VIEW) VCC(out) 1 24
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MB3788 nnnn PIN DESCRIPTION Pin No. Pin name I/O Descriptions Channel 1 push-pull type output 2OUT1 O Channel 1 output current setting 3VE1 I 4Ca1 — Channel 1 output transistor OFF current setting: Output transistor OFF The current is set by connecting a capacitor between pins Ca1 and Cb1. 5Cb1 — Channel 1 Channel 1 error amplifier output 6FB1 O Channel 1 error amplifier inversion input 7-IN1(E) I Channel 1 error amplifier non-inversion input 8+IN1(E) Channel 1 comparator inversion input 9-IN1
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MB3788 nnnn BLOCK DIAGRAM Cb1 Channel 1 4 5 Ca1 Error amplifier 1 OFF current setting + 1 VCC(out) +IN1 (E) 8 PWM comparator 1 + - -IN1 (E) 7 - 2 OUT1 FB1 6 Comparator 1 + 3 VE1 0.6 V 1.5 V - -IN1 (C) 9 Ca2 Channel 2 20 21 Cb2 Error amplifier 2 OFF current setting + +IN2 (E) 17 PWM comparator 2 + - 18 -IN2 (E) - 23 OUT2 FB2 19 Comparator 2 + 22 0.6 V VE2 1.5 V - -IN2 (C) 16 SCP comparator 14 CTL2 - - 1.9 V + Timer circuit 1.3 V 2.1 V 1 m A 11 VCC SCP 15 VREF Low input Power/channel Reference SR
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MB3788 nnnn FUNCTIONAL DESCRIPTION 1. Major Functions (1) Reference voltage power circuit The reference voltage power supply produces a reference voltage (» 2.50 V) which is temperature-compensated by the voltage supplied from the power pin (pin 11); it is used as the IC internal circuit operating power supply. The reference voltage can also be output externally at 1 mA from VREF pin (pin12). (2) Error amplifier The error amplifier detects the switching regulator output voltage and outputs a PWM
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MB3788 3. Protection Functions (1) Timer and latch-type short-circuit protection circuit The SCP comparator detects the output voltage levels of two comparators to detect an output short circuit. If the output voltage of one comparator increases to 2.1 V, the transistor of the timer circuit is turned off and the short circuit protection capacitor connected externally to the SCP pin (pin 15) starts charging. The latch circuit turns off the output transistor and simultaneously clears the duty cyc
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MB3788 n ABSOLUTE MAXIMUM RATINGS (TA = +25 C) Ratings Parameter Symbol Conditions Unit Min Max —— Supply voltage VCC 20 V —— Control input voltage VICTL 20 V — Allowable loss PD Ta £ +25 C 500* mW — Operating ambient temperature TOP -30 +85 C — Storage temperature Tstg -55 +125 C *: Value obtained when mounted on 4 cm · 4 cm double-sided epoxy substrate WARNING: Semiconductor devices can be permanently damaged by application of stress (voltage, current, temperature, etc.) in excess of a
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MB3788 nnnn ELECTICAL CHARACTERISTICS (VCC =6V, TA = +25 C) Value Parameter Symbol Conditions Unit Min Typ Max Reference voltage VREF 2.475 2.500 2.525 V IOR = -1 mA Output voltage temperature D VREF/ TA = -30 to +85 C-2 – 0.2 2 % variation VREF Reference voltage Input stability Line VCC = 3.6 V to 18 V — 2 10 mV Load stability Load IOR = -0.1 mA to 1 mA — 3 10 mV Short-circuit output current IOS VREF = 2 V -20 -8 -3 mA VtH ——2.65—V Threshold voltage Low voltage VtL ——2.45—V malfunction fai
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MB3788 (Continued) Values Parameter Symbol Conditions Unit Min Typ Max Vt0 Duty cycle = 0 % 1.05 1.3 — V Threshold voltage Vt100 Duty cycle = 100 % — 1.9 2.25 V PWM Input sink current IIM+ —— 120 — m A comparator Input source current IIM- —— -2 —mA Input bias current IIB VI = 0 V -1.0 -0.5 — m A Threshold voltage Vth — 0.7 1.4 2.1 V Control IIH VCTL = 5 V — 100 200 m A Input current IIL VCTL = 0 V -10 — 10 m A Source current IO —— -40 —mA Output Sink current IO RB = 50 W 18 30 42 mA Output leak
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MB3788 n STANDARD CHARACTERISTIC CURVES 1. Power current - supply voltage characteristic 2. Reference voltage - supply voltage characteristic TA = +25 C TA = +25 C 2.5 CTL1 = 6 V 5 2.0 4 CTL1, 2 = 6 V 1.5 Reference 3 Power voltage current VREF (V) lCC (mA) 1.0 2 0.5 1 0 0 04 8121620 0 4 8 12 16 20 Supply voltage VCC (V) Supply voltage VCC (V) 3.Reference voltage, output current setting pin voltage 4. Reference voltage - ambient temperature characteristic - supply voltage characteristic 2.56
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MB3788 (Continued) 8.Gain - frequency characteristic and phase - frequency 7. Duty - input oscillating frequency characteristic characteristic 100 Input waveform VCC = 6 V TA = +25 C 180 40 1.9V VFB = 1.6 V 80 1.3V TA = +25 C 90 20 60 Duty Gain Phase Dtr (%) (dB) f (deg) 0 40 0 20 -90 -20 0 -180 0 5K 10K 50K 100K 500K 1M -40 Input oscillating frequency (Hz) 1K 10K 100K 1M 5M 10M f (Hz) 9. Power dissipation - ambient temperature characteristic Circuit for measuring gain - frequency charact
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MB3788 nnnn HOW TO SET OUTPUT VOLTAGE VREF VOUT VREF VOUT = (R1 + R2) 2 · R2 R R1 + - R R2 RNF Note: Set the output voltage in the positive range (VOUT > 0). 12
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MB3788 nnnn HOW TO SET OUTPUT CURRENT The output circuit is configured in a push-pull type as shown in Figure 1. The ON current value of the output current waveform shown in Figure 2 is a constant current and the OFF value set by RE is set by a time constant. Each output current can be calculated from the following expression: • ON current = 1.5/RE (A) (Output current setting pin voltage: VE » 1.5 V) • The OFF current time constant is proportional to the value of CB. Drive Tr ON current CB OFF
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MB3788 nnnn HOW TO SET TIME CONSTANT FOR TIMER & LATCH-TYPE SHORT-CIRCUIT PROTECTION CIRCUIT If the load conditions of the switching regulator are stable, the outputs of comparators 1 and 2 do not change, so the SP comparator outputs a High level. At this time, the SCP pin (pin 15) is held at about 50 mV. If the load conditions change suddenly due to a load short-circuit, for example, the output voltage of the com- parator of the channel becomes a High-level signal (more than 2.1 V). Then, the
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MB3788 nnnn PROCESSING WITHOUT USING SCP PIN If the timer and latch-type short-circuit protection circuit is not used, connect the SCP pin (pin 15) to GND as close as possible. Also, connect the input pin of each channel comparator to the VCC pin (pin 11). 11 VCC 9 -IN1 (C) 16 -IN2 (C) SCP GND 15 24 Fig. 6 Processing without using SCP Pin 15
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MB3788 nnnn EQUIVALENT SERIES RESISTANCE OF SMOOTHING CAPACITOR AND STABILITY OF DC/DC CONVERTER The equivalent series resistance (ESR) of the smoothing capacity in a DC/DC converter has a great effect on the loop phase characteristics. The ESR causes a small delay at the capacitor with a series resistance of 0 (Figures 8 and 9), thus improving system stability. On the other hand, using a smoothing capacitor with a low ESR reduces system stability. Therefore, attention should be paid to using s
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MB3788 (Reference Data) The phase margin is halved by changing the smoothing capacitor from an aluminum electrolytic capacitor (Rc = 1.0 W ) to a semiconductor electrolytic capacitor (OS capacitor: Rc = 0.2 W ) with a low ESR (Figures 11 and 12). VOUT VO+ CNF AV - f characteristic between VOUT and VIN R2 -IN + VIN +IN FB - R1 VREF/2 Error amplifier Fig. 10 DC/DC Converter AV - ffff Characteristic Measurement Diagram Aluminum electrolytic capacitor gain - frequency and phase - frequency charac
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MB3788 nnnn APPLICATION CIRCUIT 10 m H VCC + + - - 33 m F 33 m F 13 14 11 VCC CTL1 CTL2 4.7 kW 8 Cb1 4 +IN1 (E) 1000 pF (a) 4.7 kW Ca1 5 8.2 kW (a) Channel 1 7 -IN1 (E) 22 m H 5 V (dB) 1 VCC(out) 100 kW 0.22 m F 2.7 kW + 6 OUT1 2 FB1 - 10 m F 150 W (15 mA) VE1 3 9 -IN1 (C) 4.7 kW 17 +IN2 (E) (b) 20 Ca2 4.7 kW 1000 pF 3.8 kW 22 m H 3 V (b) 18 -IN2 (E) 21 Cb2 Channel 2 100 kW + (deg) 23 - 0.22 m F 2.7 kW OUT2 10 m F 19 FB2 150 W 22 VE2 16 -IN2 (C) VREF S
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MB3788 nnnn NOTES ON USE • Take account of common impedance when designing the earth line on a printed wiring board. • Take measures against static electricity. - For semiconductors, use antistatic or conductive containers. - When storing or carrying a printed circuit board after chip mounting, put it in a conductive bag or container. - The work table, tools and measuring instruments must be grounded. - The worker must put on a grounding device containing 250 kW to 1 MW resistors in series. • Do
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MB3788 nnnn PACKAGE DIMENSION Note 1) *1 : Resin protrusion. (Each side : +0.15 (.006) Max) . 24-pin plastic SSOP Note 2) *2 : These dimensions do not include resin protrusion. (FPT-24P-M03) Note 3) Pins width and pins thickness include plating thickness. Note 4) Pins width do not include tie bar cutting remainder. 1 0.17±0.03 * 7.75±0.10(.305±.004) (.007±.001) 24 13 2 * 5.60±0.10 7.60±0.20 (.220±.004) (.299±.008) INDEX Details of "A" part +0.20 1.25 –0.10 (Mounting height) +.008 .049 –.004 0.25
