Power Integrations TOP414G, TOP412G Datasheet

April 1999

®

TOP412/414

TOPSwitch

Family

Three-terminal DC to DC PWM Switch

®

Product Highlights

Low Cost Replacement for Discrete Switchers

• Up to 15 fewer components - cuts cost, increases reliability

• Allows for a smaller and lighter solution under 12 mm

height, all surface mount components

Over 80% Efficiency in Flyback Topology

• Built-in start-up and current limit reduce DC losses

• Low capacitance MOSFET cuts switching losses

• CMOS controller/gate driver consumes only 7 mW

• 70% maximum duty cycle minimizes conduction losses

Simplifies Design - Reduces Time to Market

• Integrated PWM Controller and high power MOSFET

• Only one external capacitor needed for compensation,

bypass and start-up/auto-restart functions

System Level Fault Protection Features

• Auto-restart and cycle by cycle current limiting functions
handle both primary and secondary faults

• On-chip latching thermal shutdown protects the entire
system against overload

Highly Versatile

• Implements Buck, Boost, Flyback or Forward topology

• Easily interfaces with both opto and primary feedback

• Supports continuous or discontinuous mode of operation

• Specified for operation down to 16 V DC input

Description

The TOPSwitch family implements, with only three terminals,
all functions necessary for a DC to DC, converter: high voltage
N-channel power MOSFET with controlled turn-on gate driver,
voltage mode PWM controller with integrated 120 kHz oscillator,
high voltage start-up bias circuit, bandgap derived reference,
bias shunt regulator/error amplifier for loop compensation and
fault protection circuitry. Compared to discrete MOSFET and
controller or self oscillating (RCC) switching converter solutions,
a TOPSwitch integrated circuit can reduce total cost, component
count, size, weight and at the same time increase efficiency and
system reliability. This device is well suited for Telecom,
Cablecom and other DC to DC converter applications up to

Figure 1. Typical Application.

MINIMUM

INPUT

VOLTAGE

ORDER PART NUMBER

Output Power Capability

1-4

18 VDC

3 W

9 W

TOP412G TOP414G

24 VDC

36 VDC

48 VDC
60 VDC

72 VDC

90 VDC

12 W

Table 1. TOP412/414 Output Power.

4 W

5 W 6 W
7 W 9 W

18 W

21 W

12 W 15 W
15 W 18 W

Notes: 1. Assumes maximum junction temperature of 100 °C

2. Assumes output of 5 V and KRP of 0.4 3. Soldered to 1 sq. inch

(645 mm2), 2 oz. copper clad (610 gm/mm2) 4. The continuous
power capability in a given application depends on thermal
environment, transformer design, efficiency required, input storage
capacity, etc.

PI-2371-120198

D

S

C

CONTROL

TOP414

V

IN

V

O

21W of output power. Internally, the lead frame of the
SMD-8 package uses six of its pins to transfer heat from the chip
directly to the board, eliminating the cost of a heat sink.

TOP412/414

A
4/99

2

PI-1746-011796

SHUTDOWN/

AUTO-RESTART

PWM

COMPARATOR

CLOCK

SAW

OSCILLATOR

CONTROLLED

TURN-ON

GATE

DRIVER

INTERNAL
SUPPLY

5.7 V

4.7 V

SOURCE

SRQ

Q

D

MAX

­+

CONTROL

­+

5.7 V

I

FB

R

E

Z

C

V

C

MINIMUM

ON-TIME

DELAY

+

-

V

I

LIMIT

LEADING

EDGE

BLANKING

POWER-UP

RESET

RSQ

Q

÷ 8

0

1

THERMAL

SHUTDOWN

EXTERNALLY

TRIGGERED
SHUTDOWN

SHUNT REGULATOR/

ERROR AMPLIFIER

+

-

DRAIN

Figure 2. Functional Block Diagram.

Pin Functional Description

DRAIN Pin:

Output MOSFET drain connection. Provides internal bias
current during start-up operation via an internal switched high­voltage current source. Internal current sense point.

CONTROL Pin:

Error amplifier and feedback current input pin for duty cycle
control. Internal shunt regulator connection to provide internal
bias current during normal operation. Trigger input for latching
shutdown. It is also used as the supply bypass and auto-restart/
compensation capacitor connection point.

SOURCE Pin:

Output MOSFET source connection. Primary-side circuit
common, power return, and reference point.

Figure 3. Pin Configuration.

PI-2208-120798

CONTROL

8

5

7

6

DRAIN

SOURCE (HV RTN)

SOURCE

SOURCE

1

4

2

3

SOURCE (HV RTN)

SOURCE (HV RTN)

SOURCE

G Package (SMD-8)

A

4/99

TOP412/414

3

TOPSwitch

Family Functional Description

TOPSwitch is a self biased and protected linear control current­to-duty cycle converter with an open drain output. High
efficiency is achieved through the use of CMOS and integration
of the maximum number of functions possible. CMOS
significantly reduces bias currents as compared to bipolar or
discrete solutions. Integration eliminates external power resistors
used for current sensing and/or supplying initial start-up bias
current.

During normal operation, the internal output MOSFET duty
cycle linearly decreases with increasing CONTROL pin current
as shown in Figure 4. To implement all the required control,
bias, and protection functions, the DRAIN and CONTROL pins
each perform several functions as described below. Refer to
Figure 2 for a block diagram and Figure 6 for timing and voltage
waveforms of the TOPSwitch integrated circuit.

Control Voltage Supply

CONTROL pin voltage V

C

is the supply or bias voltage for the
controller and driver circuitry. An external bypass capacitor
closely connected between the CONTROL and SOURCE pins
is required to supply the gate drive current. The total amount of
capacitance connected to this pin (CT) also sets the auto-restart
timing as well as control loop compensation. VC is regulated in
either of two modes of operation. Hysteretic regulation is used
for initial start-up and overload operation. Shunt regulation is
used to separate the duty cycle error signal from the control
circuit supply current. During start-up, VC current is supplied
from a high-voltage switched current source connected internally
between the DRAIN and CONTROL pins. The current source
provides sufficient current to supply the control circuitry as
well as charge the total external capacitance (CT).

PI-1691-112895

D

MAX

D

MIN

I

CD1

Duty Cycle (%)

IC (mA)

2.5 6.5 45

Slope = PWM Gain

-16%/mA

I

B

Auto-restart

Figure 4. Relationship of Duty Cycle to CONTROL Pin Current.

Figure 5. Start-up Waveforms for (a) Normal Operation and (b)

Auto-restart.

DRAIN

0

V

IN

V

C

0

4.7 V

5.7 V

8 Cycles

95%

5%

Off

Switching Switching

Off

I

C

Charging C

T

I

CD1

Discharging C

T

I

CD2

Discharging C

T

I

C

Charging C

T

Off

PI-1124A-060694

DRAIN

0

V

IN

V

C

0

4.7 V

5.7 V

Off

Switching

(b)

(a)

CT is the total external capacitance

connected to the CONTROL pin

TOP412/414

A
4/99

4

TOPSwitch

Family Functional Description (cont.)

The first time VC reaches the upper threshold, the high-voltage
current source is turned off and the PWM modulator and output
transistor are activated, as shown in Figure 5(a). During normal
operation (when the output voltage is regulated) feedback
control current supplies the V

C

supply current. The shunt
regulator keeps VC at typically 5.7 V by shunting CONTROL
pin feedback current exceeding the required DC supply current
through the PWM error signal sense resistor RE. The low
dynamic impedance of this pin (ZC) sets the gain of the error
amplifier when used in a primary feedback configuration. The
dynamic impedance of the CONTROL pin together with the
external resistance and capacitance determines the control loop
compensation of the power system.

If the CONTROL pins total external capacitance (CT) should
discharge to the lower threshold, the output MOSFET is turned
off and the control circuit is placed in a low-current standby
mode. The high-voltage current source is turned on and charges
the external capacitance again. Charging current is shown with
a negative polarity and discharging current is shown with a
positive polarity in Figure 6. The hysteretic auto-restart
comparator keeps VC within a window of typically 4.7 to 5.7 V
by turning the high-voltage current source on and off as shown
in Figure 5(b). The auto-restart circuit has a divide-by-8
counter which prevents the output MOSFET from turning on
again until eight discharge-charge cycles have elapsed. The
counter effectively limits TOPSwitch power dissipation by
reducing the auto-restart duty cycle to typically 5%. Auto­restart continues to cycle until output voltage regulation is again
achieved.

Bandgap Reference

All critical TOPSwitch internal voltages are derived from a
temperature-compensated bandgap reference. This reference
is also used to generate a temperature-compensated current
source which is trimmed to accurately set the oscillator frequency
and MOSFET gate drive current.

Oscillator

The internal oscillator linearly charges and discharges the
internal capacitance between two voltage levels to create a
sawtooth waveform for the pulse width modulator. The oscillator
sets the pulse width modulator/current limit latch at the beginning
of each cycle. The nominal frequency of 120 kHz was chosen
to minimize EMI and maximize efficiency in power supply
applications. Trimming of the current reference improves the
frequency accuracy.

Pulse Width Modulator

The pulse width modulator implements a voltage-mode control
loop by driving the output MOSFET with a duty cycle inversely
proportional to the current flowing into the CONTROL pin.
The error signal across RE is filtered by an RC network with a

typical corner frequency of 7 kHz to reduce the effect of
switching noise. The filtered error signal is compared with the
internal oscillator sawtooth waveform to generate the duty
cycle waveform. As the control current increases, the duty
cycle decreases. A clock signal from the oscillator sets a latch
which turns on the output MOSFET. The pulse width modulator
resets the latch, turning off the output MOSFET. The maximum
duty cycle is set by the symmetry of the internal oscillator. The
modulator has a minimum ON-time to keep the current
consumption of the TOPSwitch independent of the error signal.
Note that a minimum current must be driven into the CONTROL
pin before the duty cycle begins to change.

Gate Driver

The gate driver is designed to turn the output MOSFET on at a
controlled rate to minimize common-mode EMI. The gate
drive current is trimmed for improved accuracy.

Error Amplifier

The shunt regulator can also perform the function of an error
amplifier in primary feedback applications. The shunt regulator
voltage is accurately derived from the temperature compensated
bandgap reference. The gain of the error amplifier is set by the
CONTROL pin dynamic impedance. The CONTROL pin
clamps external circuit signals to the VC voltage level. The
CONTROL pin current in excess of the supply current is
separated by the shunt regulator and flows through RE as the
error signal.

Cycle-By-Cycle Current Limit

The cycle by cycle peak drain current limit circuit uses the
output MOSFET ON-resistance as a sense resistor. A current
limit comparator compares the output MOSFET ON-state
drain-source voltage, V

DS(ON),

with a threshold voltage. High

drain current causes V

DS(ON)

to exceed the threshold voltage and
turns the output MOSFET off until the start of the next clock
cycle. The current limit comparator threshold voltage is
temperature compensated to minimize variation of the effective
peak current limit due to temperature related changes in output
MOSFET R

DS(ON)

.

The leading edge blanking circuit inhibits the current limit
comparator for a short time after the output MOSFET is turned
on. The leading edge blanking time has been set so that current
spikes caused by primary-side capacitances and secondary-side
rectifier reverse recovery time will not cause premature
termination of the switching pulse.

Shutdown/Auto-restart

To minimize TOPSwitch power dissipation, the shutdown/
auto-restart circuit turns the power supply on and off at a duty
cycle of typically 5% if an out of regulation condition persists.
Loss of regulation interrupts the external current into the

A

4/99

TOP412/414

5

CONTROL pin. VC regulation changes from shunt mode to
the hysteretic auto-restart mode described above. When the
fault condition is removed, the power supply output becomes
regulated, VC regulation returns to shunt mode, and normal
operation of the power supply resumes.

Latching Shutdown

The output overvoltage protection latch is activated by a high­current pulse into the CONTROL pin. When set, the latch
turns off the TOPSwitch output. Activating the power-up
reset circuit by removing and restoring input power, or
momentarily pulling the CONTROL pin below the power-up
reset threshold resets the latch and allows TOPSwitch to
resume normal power supply operation. VC is regulated in
hysteretic mode when the power supply is latched off.

Over-Temperature Protection

Temperature protection is provided by a precision analog
circuit that turns the output MOSFET off when the junction

temperature exceeds the thermal shutdown temperature
(typically 145 °C). Activating the power-up reset circuit by
removing and restoring input power or momentarily pulling the
CONTROL pin below the power-up reset threshold resets the
latch and allows TOPSwitch to resume normal power supply
operation. VC is regulated in hysteretic mode when the power
supply is latched off.

High-voltage Bias Current Source

This current source biases TOPSwitch from the DRAIN pin and
charges the CONTROL pin external capacitance (CT) during
start-up or hysteretic operation. Hysteretic operation occurs
during auto-restart and latched shutdown. The current source
is switched on and off with an effective duty cycle of
approximately 35%. This duty cycle is determined by the ratio
of CONTROL pin charge (IC) and discharge currents (I

CD1

and

I

CD2

). This current source is turned off during normal operation

when the output MOSFET is switching.

PI-1119-110194

V

IN

V

OUT

0

I

OUT

0

1 2

143

DRAIN

0

V

IN

V

C

0

• • • • • •

12 12 81

0

I

C

• • • • • •

12

8

812 81

V

C(reset)

45 mA

Figure 6. Typical Waveforms for (1) Normal Operation, (2) Auto-restart, (3) Latching Shutdown, and (4) Power Down Reset.