OPERATING DESCRIPTION
GM3842A, GM3843A, GM3844A and GM3845A are high performance with fixed frequency, current mode
controllers. They are designed for off-ine and DC-to-DC converter applications offering great versatility with
minimal external components. A representative block diagram is shown on page 4.
Oscillator
The oscillator frequency is determined by the values of the timing components R
T
and C
T
. Capacitor C
T
is
charged from the 5.0 V reference through resistor R
T
to approximately 2.8 V and discharged to 1.2 V by an
internal current sink. During the of C
T
, the oscillator generates an internal blanking pulse that holds the center
input of the NOR gate high. This causes the output to be in a low state, thus producing a controlled amount of
Figure 2 show R
T
versus oscillator frequency and Figure 2, Output deadtime versus frequency, both for given
values of C
T
output deadtime.
Note that different values of R
T
and C
T
will give the same oscillator frequency, but only one combination will yield
a specific output deadtime at a given frequency. The oscillator thresholds are temperature compensated. These
interned circuit vefinements minimizes refinements of oscillator frequency and maximum output duty cycle. In
many noise sensitive applications, it may be desirable to frequency-lock the converter to an external system
clock. This can be accmplished by applying a clock signal to the circuit shown in Figure 9. For best locking
results, set the free-unning oscillator frequency to about 10% less than the clock frequency. A method for multi
unit synchronization is shown in Figure 10. You can get very accurate output duty cycle clamping by tweaking the
clock waveform.
Error Amplifier
GM384XA series has a fully compensated error amplifier with access to both the inverting input and output, and
providing DC voltage gain of 90 dB (typical). The noninverting input is internally biased at 2.5 V and is not pinned
out. The converter output voltage is typically divided down and monitored by the inverting input. The maximum
input bias current is -2.0 µA, which can cause an output voltage error that is equal to the product of the input bias
current and the equivalent input divider source resistance.
The Error Amplifier Output (Pin 1) allows external loop compensation. The output voltage is offset by the two
diode drops ( 1.4 V) and divided by three before it connects to the inverting input of the Current Sense
Comparator. This assures that no drive pulses appear at the Output (Pin 6) when Pin 1 is at its lowest state
(V
OL
). This happens when the power supply is operating and the load is removed, or at the beginning of a soft-
start interval (Figures 11, 12). The Error Amp minimum feedback resistance is limited by the amplifier's source
current (0.5 mA) and the required output voltage (V
OH
) to reach the comparator's 1.0 V clamp level:
R
f(min)
3.0(1.0V) + 1.4V
= 8800W
0.5mA
GM3842A, GM3843A, GM3844A, GM3845A
Current Sense Comparator and PWM Latch
GM384XA series operates as a current mode controller, whereby output switch conduction is initiated by the
oscillator and terminated when the peak inductor current reaches the threshold level established by the Error
Amplifier Output/Compensation (Pin1). The error signal controls the peak inductor current cycle-by-cycle basis.
The Current Sense Comparator PWM Latch configuration assures that only a single pulse appears at the Output
during any given oscillator cycle. The inductor current is converted to a voltage by inserting the ground-
referenced sense resistor R
S
in series with the source of output switch Q1. This voltage is monitored by the
Current Sense Input (Pin 3) and compared to a level derived from the Error Amp Output. The peak inductor
current under normal operating conditions is controlled by the voltage at pin 1 where:
I
pk
=
V
(PIN1)
- 1.4V
3 R
S
When the power supply output is overloaded or if output voltage sensing is lost, the chip operation is not normal.
In these situations, the Current Sense Comparator threshold will be internally clamped to 1.0 V and the
maximum peak switch current is:
1.0V
I
pk(mak)
= R
S
8
GAMMA [ GAMMA MICROELECTRONICS INC. ]
