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Circuit Descriptions, Abbreviation List, and IC Data Sheets

Block diagram

Figure 9-5 Block diagram DC/DC converter.

The advantage of using a "synchronous buck converter"
instead of a "classical buck converter" is a better efficiency
(about 90%). The difference between the two is that in a
synchronous buck converter the "low -side" diode is replaced
by a MOSFET (item 7U03). This, because the voltage drop
across the MOSFET is smaller than the forward voltage drop of
the diode. This second MOSFET conducts current during the
"off" times of the first MOSFET (at the input side). The upper
MOSFET conducts to transfers energy from the input to the
inductor and load, while the lower MOSFET conducts to
circulate the inductor current. The synchronous PWM (Pulse
Width Modulation) control block regulates the output voltage by
modulating the conduction intervals of the upper and lower
MOSFETs.

PWM Generator and Drivers

This circuit is build around transistors 7U01, 7U02, 7U08,
7U09, and the associated components. MOSFETs 7U04 and
7U03 are the switching transistors, they are conducting
alternatively.
•

Time sequence 1: 7U04 is conducting; energy is stored in
coil 5U01. The current is flowing from the +11V power
supply source.

•

Time sequence 2: 7U04 is blocked; energy is stored in coil
5U01.

•

Time sequence 3: 7U03 is conducting, and the current
circuit is now closed via 7U03, Coil 5U01, C2U08, and the
load. So the energy stored in the coil during time sequence
T1 is consumed during sequence T3. The signal on the
gate 7U03 is 180 degrees turned compared with the signal
on the gate 7U04.

The principle of operation of the PWM generators and drivers
is an astable circuit, which frequency is mainly given by the
values of capacitor 2U32 and resistor 3U03. Capacitors 2U05,
2U27, and 2U28 form a capacitive voltage divider to limit the
negative peak voltage on the base of transistor 7U09 at about
3 V. The same role have the capacitors 2U09 and 2U12 for the
transistor 7U08.
The resistor 3U14 and the capacitor 2U30 introduce a small
dead time between switching "off" of transistor 7U03 and
switching "on" of transistor 7U04 to prevent cross-conduction
through them.
The capacitors 2U07, 2U16, and 2U18 speed-up the switching
of the astable circuit. This circuit is blocked in "off" state (7U02
and 7U08 saturated and 7U01 and 7U09 blocked) when the
transistor 7U11 is conducting.
This happens when 5V has a low value (below 2 V), the
boost voltage drops below its minimum value, or the
3V3_FAULT signal is low (below 1 V) for more than 20 ...30 ms.

Voltage Booster

This circuit is build around capacitors 2U03, 2U04, 2U05,
resistor 3U51, and diodes 6U03 and 6U04.
It generates the boost voltage (approx. +20 V) for supplying the
"high-side" driver transistor for the power MOS-FET 7U04. The
voltage is generated only during normal operation of the
converter; therefore, any drop in its value means an internal
fault condition, which is sensed by the internal protection
circuit.
The A.C. component of the voltage on the source of transistor
7U04 is rectified by diodes 6U04 and 6U03 and added to the
input voltage, resulting into the boost voltage. The resistor
3U51 limits the peak current through the rectifier diodes to
about 300 mA.

Voltage Control Feedback

This circuit keeps the output voltage constant when the input
voltage and/or the output current vary. It is build around
transistors 7U06-A, 7U06-B, 7U05, diode D6U07, and
associated components.
A 6.9 V reference voltage is provided by zener diode D6U07
(via 7U15). Transistors 7U05 and 7U06A are comparators. E.g.
if the voltage on the base of 7U06-B increases, 7U06-B and
7U08 will conduct more, and via 7U08 the conducting time of
7U04 will be reduced, resulting in a decreasing output voltage.

Linefeed Forward

This circuit (resistor 3U46) improves the line regulation factor.

Stand-by Current Reduction

This circuit is build around transistors 7U14 and 7U15. It cuts
the quiescent current through the reference zener diode 6U07
in stand-by mode.

Adapter

This circuit is necessary for having proper start-up/shut-down
operation of the converter at various shapes of the +5V voltage.

Fault detection

In "on" mode, transistors 7U12, 7U10, 7U07, and 7U13 are
conducting and the transistor 7U11 is blocked.
In protection mode, transistors 7U11, 7U07, and 7U13 are
conducting (provided that the "+5V" voltage is higher than 4.5
V) and transistors 7U10 and 7U12 are blocked.
During start-up, the protections are bypassed for a short time
(tenths of ms) by capacitors 2U22 and 2U20.

+20V Detection

This circuit detects the boost voltage, and is build around
transistor 7U12, diode 6U10, and associated components. If
the boost voltage is lower than 15 V, transistors 7U12 and
7U10 are blocked, 7U11 is conducting, so via diodes 6U08 and
6U09 the "PWM and Driver" circuit is switched "off".
This is also the case when the input voltage "+11V" is less than
about 10 V (the under-voltage lock-out function).

+3V3 Detection

If the +3.3V and +5V are present, C2U26 is charged. Transistor
7U10 conducts, 7U11 is blocked, and the "PWM and Driver"
circuit is working.
If there is a voltage drop on the +3.3V, the voltage on C2U25
cannot be maintained, resulting that 7U10 will block and the
PWM and driver circuit will be switched "off".

+5V Detection

The same situations if the +5V drops: 7U07 and 7U13 will
block, the reference voltage on R3U41 will drop, and a switch
"off" of the PWM and Driver circuit is the result.
Diode 6U06 is foreseen to clamp the output voltage to 5 V in
case of fault conditions (like 7U04 short-circuited D-S). The
fuse 1U01 will open in such conditions.

2U08
470uF

47uH

5U01

CL 36532064_063.eps

211003

Voltage booster

Voltage control feedback

Standby current reduction

Line feed-forward

PWM generator

and drivers

Fault detection