LTC3721EGN-1#TRPBF Linear Technology, LTC3721EGN-1#TRPBF Datasheet - Page 11

• Manufacturer Part Number: LTC3721EGN-1#TRPBF
• Description: IC CTRLR PWM PUSH PULL HE 16SSOP
• Manufacturer: Linear Technology

Specifications of LTC3721EGN-1#TRPBF

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 50%
• Buck: No
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 16-SSOP
• Frequency-max: 1MHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -

OPERATIO
that the positive voltage square wave is produced while the
output inductor is freewheeling. An advantage of this
technique over the previous is that it does not require a
separate filter inductor and since the voltage is derived
from the well-regulated output voltage, it is also well
controlled. One disadvantage is that this winding will
require the same safety isolation that is required for the
main transformer. Another disadvantage is that a much
larger V
generate a voltage as the output is first starting up, or
during short-circuit conditions.
Programming the LTC3721-1 Oscillator
The high accuracy LTC3721-1 oscillator circuit provides
flexibility to program the switching frequency and slope
compensation required for current mode control. The
oscillator circuit produces a 2.35V peak-to-peak ampli-
tude ramp waveform on C
of 49% are possible. The oscillator is capable of operation
up to 1MHz by the following equation:
Note that this is the frequency seen on C
drivers switch at 1/2 of this frequency. Also note that
higher switching frequency and added driver dead-time
via DPRG will reduce the maximum duty cycle.
The LTC3721-1 derives a compensating slope current
from the oscillator ramp waveform and sources this
current out of CS. The desired level of slope compensation
is selected with an external resistor connected between CS
and the external current sense resistor, (Figure 5).
C
T
= 1/(14.8k • F
CC
Figure 5. Slope Compensation Circuitry
filter capacitor is needed, since it does not
C
T
U
LTC3721-1
OSC
33k
I =
)
V(C
33k
T
T
)
. Typical maximum duty cycles
CS
SLOPE
ADDED
CURRENT SENSE
WAVEFORM
R
SLOPE
37211 F05
R
SWITCH
CURRENT
CS
T
. The output
Current Sensing and Overcurrent Protection
Current sensing provides feedback for the current mode
control loop and protection from overload conditions. The
LTC3721-1 is compatible with either resistive sensing or
current transformer methods. Internally connected to CS
are two comparators that provide pulse-by-pulse and
overcurrent shutdown functions respectively, (Figure 6).
The pulse-by-pulse comparator has a 300mV nominal
threshold. If the 300mV threshold is exceeded, the PWM
cycle is terminated. The overcurrent comparator is set
approximately 2x higher than the pulse-by-pulse level. If
the current signal exceeds this level, the PWM cycle is
terminated, the soft-start capacitor is quickly discharged
and a soft-start cycle is initiated. If the overcurrent condi-
tion persists, the LTC3721-1 halts PWM operation and
waits for the soft-start capacitor to charge up to approxi-
mately 4V before a retry is allowed. The soft-start capaci-
tor is charged by an internal 13µA current source. If the
fault condition has not cleared when soft-start reaches 4V,
the soft-start pin is again discharged and a new cycle is
initiated. This is referred to as hiccup mode operation. In
normal operation and under most abnormal conditions,
the pulse-by-pulse comparator is fast enough to prevent
hiccup mode operation. In severe cases, however, with
high input voltage, very low R
shorted output, or with saturating magnetics, the
overcurrent comparator provides a means of protecting
the power converter.
R
CS
CS
300mV
650mV
Figure 6. Current Sense/Fault Circuitry Detail
PULSE BY PULSE
CURRENT LIMIT
CURRENT LIMIT
OVERLOAD
+
+
–
–
PWM
ENABLE
UVLO
LATCH
PWM
S
S Q
R
Q
PWM
LOGIC
DS(ON)
ENABLE
UVLO
LTC3721-1
MOSFETs and a
–
–
+
+
Q
S Q
R
0.4V
4.1V
sn37211 37211fs
H = SHUTDOWN
Q
11
OUTPUTS
13µA
37211 F06
SS
C
SS