Fujitsu MB3788 User Manual

FUJITSU SEMICONDUCTOR

DATA SHEET

ASSP

SWITCHING REGULATOR
CONTROLLER

MB3788

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 v o ltage po w er supply with a voltage
accuracy of ±1%. The maximum operating frequency is 1 MHz. It is designed f or a voltage-drop output s witching
regulator suitable for a logic power supply or speed control of a DC motor.
The MB3788 is compatible with all master ICs producing triangular wav es, sa w-tooth w a v es and sine wa v es with
an amplitude of 1.3 V to 1.9 V.
It can be used in high-performance portable equipment such as a video camcorder or notebook personal computer
(word processor).

DS04-27209-3E

FEATURES

■■■■

• Wide operating power supply voltage range: 3.6 V to 18 V

• Low power dissipation

• Operating: 1.9 mA (standard)
Standby: 10 µA Max

(Continued)

PACKAGE

■■■■

24-pin Plastic SSOP

(FPT-24P-M03)

This device contains circuitry to protect the inputs against damage due to high static voltages or electric fields. Howe ver, it is
advised that normal precautions be taken to avoid application of any v oltage higher than maxim um rated v oltages to this high
impedance circuit.

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 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

PIN ASSIGNMENT

■■■■

(TOP VIEW)

CC(out)

V

OUT1

VE1
Cb1
Ca1
FB1

-IN1(E)

+IN1(E)

-IN1(C)

-IN(PWN)
V

CC

VREF

1
2
3
4
5
6
7
8

9
10
11
12

CC

- 1.8 V

24
23
22
21
20
19
18
17
16
15
14
13

GND
OUT2
VE2
Cb2
Ca2
FB2

-IN2(E)
+IN2(E)

-IN2(C)
SCP
CTL2
CTL1

(FPT-24P-M03)

2

PIN DESCRIPTION

■■■■

Pin No. Pin name I/O Descriptions

MB3788

Channel 1

Channel 2

2OUT1O
3VE1I
4Ca1—
5Cb1—
6FB1O
7-IN1(E)I
8+IN1(E)

9-IN1(C)I
16 -IN2(C) I
17 +IN2(E) I
18 -IN2(E) I
19 FB2 O
20 Ca2 —
21 Cb2 —
22 VE2 I
23 OUT2 O

13 CTL1 I

Channel 1 push-pull type output
Channel 1 output current setting

Channel 1 output transistor OFF current setting: Output transistor OFF
The current is set by connecting a capacitor between pins Ca1 and Cb1.

Channel 1 error amplifier output
Channel 1 error amplifier inversion input
Channel 1 error amplifier non-inversion input
Channel 1 comparator inversion input
Channel 2 comparator inversion input
Channel 2 error amplifier non-inversion input
Channel 2 error amplifier inversion input
Channel 2 error amplifier output

Channel 2 output transistor OFF current setting: Output transistor OFF
The current is set by connecting a capacitor between pins Ca2 and Cb2.

Channel 2 output current setting
Channel 2 push-pull type output
Power and channel 1 control pin

H level: Power and channel 1 operating
L level: Standby

Control

circuit

Power

circuit

14 CTL2 I

15 SCP —

1

2

V

CC

10 -IN(PWM) I
11 VCC1 —
12 VREF O
24 GND —

Channel 2 control pin
When CTL1 pin = H level,
H level: Channel 2 operating
L level: Channel 2 OFF

Short-circuit protection circuit capacitor connection
Output circuit power pin

—

Master oscillating waveform input
Reference power and control circuit power
Reference voltage output
Ground

Note: The alphabetic characters in parenthesis above indicate the following input pins.

(C): Comparator
(E): Error amplifier

3

MB3788

BLOCK DIAGRAM

■■■■

+IN1 (E)

-IN1 (E)

-IN1 (C)

+IN2 (E)

-IN2 (E)

-IN2 (C)

FB1

FB2

8

7

6

9

17

18

19

16

0.6 V

0.6 V

Channel 1

Error amplifier 1

+

-

Comparator 1

+

-

Channel 2

Error amplifier 2

+

-

Comparator 2

+

-

PWM comparator 1

+

-

PWM comparator 2

+

-

OFF current setting

OFF current setting

1.5 V

1.5 V

20
21

23

22

4

5

1

2

3

Cb1

Ca1

CC(out)

V

OUT1

VE1

Ca2

Cb2

OUT2

VE2

Timer circuit

SCP

15

1 µA

SCP comparator

-

-

+

SR latch

circuit

VREF

2.1 V

Low input

voltage

protection

circuit

10 12

-IN(PWM)

Reference

voltage

power (2.5 V)

V

REF

1.9 V

1.3 V

Power/channel

ON/OFF

circuit

24

GND

CTL2

14

CC

V

11

CTL1

13

4

MB3788

FUNCTIONAL DESCRIPTION

■■■■

1. Major Functions

(1) Reference voltage power circuit

The reference voltage po wer supply produces a reference v oltage (≈ 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 V

(2) Error amplifier

The error amplifier detects the switching regulator output voltage and outputs a PWM control signal. It has a
wide in-phase input voltage range of -0.2 V to V

Connecting the output pin and inversion input pin of the error amplifier through a f eedback resistor and capacitor
allows setting of any loop gain to provide stable phase compensation.

(3) PWM comparator

The PWM comparator controls the output pulse ON time according to the input voltage.
The voltage input to the -IN pin (PWM) turns the output transistor on when it is lower than the output v oltage of

the error amplifier.

(4) Output circuit

CC

- 1.8 V to make setting from an external power supply easy.

REF

pin (pin12).

The output circuit is configured in a push-pull form and uses a PNP transistor drive system to drive a transistor
of up to 30 mA. (See

How to Set Output Current.

)

2. Channel Control Function

Channels can be set ON/OFF by combining the voltage levels at pin CTL1 (pin 13) and pin CTL2 (pin 14).

Channel ON/OFF Setting Conditions

Voltage level at CTL pin Channel ON/OFF status

CTL1 CTL2 Power circuit Channel 1 Channel

L × Stand by state*

HH

ON

LOFF

*: The power current in the standby state is 10 µA Max.

ON

5

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 cycle to 0 when the output
voltage lev el of the comparator does not return to the normal voltage lev el until the capacitor v oltage rises to the
base-emitter junction voltage VBE (≈ 0.65 V) of the transistor . (See

Type Short-Circuit Protection Circuit.

When the protection circuit operates, recycle the power to reset the circuit.

(2) Low input voltage malfunction fail-safe circ uit

A transient at power-on, or an instantaneous supply v oltage drop can cause a control IC malfunction, which may
damage the system. The low input voltage malfunction fail-safe circuit detects the internal reference voltage
level based on the supply voltage level, resets the latch circuit, turns off the output transistor, clears the duty
cycle to 0 and holds the SCP pin (pin 15) at Low level. All circuits are recovered when the supply voltage is
greater than the threshold voltage of the fail-safe circuit.

)

How to Set Time Constant for Timer & Latch-

6

■ ABSOLUTE MAXIMUM RATINGS

MB3788

A

= +25°C)

(T

Parameter Symbol Conditions

Unit

Min Max

Ratings

Supply voltage V
Control input voltage V
Allowable loss P
Operating ambient temperature T
Storage temperature T

CC

ICTL

D
OP
stg

——
——

Ta ≤ +25°C

—
—

—

-30 +85 °C

-55 +125 °C

20 V
20 V

500* mW

*: 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 absolute maximum ratings. Do not exceed these ratings.

■ RECOMMENDED OPERATING CONDITIONS

A

(T

= +25°C)

Parameter Symbol Conditions

Supply voltage V
Reference voltage output current I

CC

OR

Min Typ Max

—3.66.018V
—-1—0mA

Values

Unit

Error amplifier input voltage V
Error amplifier input voltage V
Control input voltage V
Output current I
Operating frequency f
Operating ambient temperature T

I
I

ICTL

O

osc

op

—-0.2—V
—-0.2—VCCV
—-0.2—18V
—3.0—30mA
— 100 300 1000 kHz
— -30 25 85 °C

CC

- 1.8 V

WARNING: The recommended operating conditions are required in order to ensure the normal operation of the

semiconductor device. All of the device’s electrical characteristics are warranted when the device is
operated within these ranges.

Always use semiconductor devices within their recommended operating condition ranges. Operation
outside these ranges may adversely affect reliability and could result in device failure.

No warranty is made with respect to uses, operating conditions, or combinations not represented on
the data sheet. Users considering application outside the listed conditions are advised to contact their
FUJITSU representatives beforehand.

7