CS5651
Operating Description: continued
outputs are disabled when the error amplifier output is at
its lowest state (V
OUT(LOW)
). This occurs when the power
supply is operating at light or no-load conditions, or at the
beginning of a soft-start interval.
The minimum allowable error amplifier feedback resis-
tance is limited by the amplifier’s source current capability
(0.5 mA) and the output voltage (V
OUT(High)
) required to
reach the current sense comparator 1.0V clamp level with
the error amplifier inverting input at ground. This condi-
tion happens during initial system start up or when the
sensed output is shorted:
R
F(min)
≈
(3 x 1.0V) + 1.4V = 8.8kΩ
0.5mA
comparator has built-in hysteresis to prevent erratic output
behavior as their respective thresholds are crossed. The
V
CC
comparator upper and lower thresholds are 14V and
10V for the CS5651. The V
REF
comparator disables the out-
puts until the internal circuitry is functional. This compara-
tor has upper and lower thresholds of 3.6V and 3.4V. The
guaranteed minimum operating voltage after turn-on is
11V for CS5651.
Outputs and Power Ground
Each channel contains a single totem-pole output stage
specifically designed for driving a power MOSFET. The
outputs have up to ±1.0A peak current capability and have
a typical rise and fall time of 28ns with a 1.0nF load.
Internal circuitry has been added to keep the outputs in
active pull-down mode whenever undervoltage lockout is
active. An external pull-down resistor is not needed.
Cross-conduction current in the totem-pole output stage
has been minimized for high speed operation. The average
added power due to cross-conduction with V
CC
= 15V is
only 60mW at 500kHz.
Although the outputs were optimized for MOSFET’s, they
can easily supply the negative base current required by
bipolar NPN transistors for enhanced turn-off. Because the
outputs do not contain internal current limiting circuitry,
an external series resistor may be required to prevent the
peak output current from exceeding the ±1.0A maximum
rating. The sink saturation voltage (V
OL
) is less than 0.4V at
100mA.
A separate Power Ground pin is provided and will signifi-
cantly reduce the level of switching transient noise
imposed on the control circuitry. This becomes particularly
important when the I
pk(max)
clamp level is reduced.
This input is used to switch V
OUT2
. V
OUT1
can be used to
control circuitry that runs continuously; e.g. volatile mem-
ENABLE
2
ory, the system clock, or a remote controlled receiver. The
V
OUT2
output can control the high power circuitry that can
be turned off when not needed.
Current Sense Comparator and PWM Latch
The CS5651 operates as a current mode controller. Output
switch conduction is initiated by the oscillator and termi-
nated when the peak inductor current reaches the thresh-
old level established by the error amplifier output. The
error signal controls the peak inductor current on a cycle-
by-cycle basis. The current sense comparator-PWM Latch
combination ensures that only a single pulse appears at the
output during any given oscillator cycle. The current is
converted to a voltage by connecting sense resistor R
Sense
in
series with the source of output switch Q1 and ground.
This voltage is monitored via the Sense
1,2
pins and com-
pared to a voltage derived from the error amp output. The
peak current under normal operating conditions is con-
trolled by the voltage at COMP where:
I
pk
=
V
COMP
– 1.4V
3R
Sense
Abnormal operating conditions occur when the power
supply output is overloaded or if output voltage is too
high. Under these conditions, the current sense comparator
threshold will be internally clamped to 1.0V. Therefore the
maximum peak switch current is:
I
pk(max)
=
1.0V
R
Sense
Voltage Reference
The 5.0V bandgap reference is trimmed to ±2.0% tolerance.
The reference has short circuit protection and is capable of
sourcing 30mA for powering any additional external cir-
cuitry.
Design Considerations
High frequency circuit layout techniques are imperative to
prevent pulse-width jitter. This is usually caused by exces-
sive noise pick-up imposed on the current sense or voltage
feed-back inputs. Noise immunity can be improved by
lowering circuit impedances at these points. The printed
circuit board layout should contain a ground plane with
low current signal and high current switch and output
grounds returning on separate paths back to the input fil-
6
Erratic operation due to noise pickup can result if there is
an excessive reduction of the I
pk(max)
clamp voltage.
A narrow spike on the leading edge of the current wave-
form can usually be observed and may cause the power
supply to exhibit an instability when the output is lightly
loaded. The addition of an RC filter on the current sense
input reduces this spike to an acceptable level.
Undervoltage Lockout
Two undervoltage lockout comparators have been incor-
porated to guarantee that the IC is fully functional before
the output stages are enabled. V
CC
and the reference out-
put V
REF
are monitored by separate comparators. Each
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
