LTC3731HGTR Linear Technology, LTC3731HGTR Datasheet - Page 12

• Manufacturer Part Number: LTC3731HGTR
• Manufacturer: Linear Technology

Specifications of LTC3731HGTR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIO S I FOR ATIO
OPERATIO
LTC3731H
Input Undervoltage Reset
The RUN/SS capacitor will be reset if the input voltage
(V
capacitor on the RUN/SS pin will be discharged until the
short-circuit arming latch is disarmed. The RUN/SS ca-
pacitor will attempt to cycle through a normal soft-start
The basic application circuit is shown in Figure 1 on the
first page of this data sheet. External component selection
is driven by the load requirement, and normally begins
with the selection of an inductance value based upon the
desired operating frequency, inductor current and output
voltage ripple requirements. Once the inductors and
operating frequency have been chosen, the current sens-
ing resistors can be calculated. Next, the power MOSFETs
and Schottky diodes are selected. Finally, C
are selected according to the voltage ripple requirements.
The circuit shown in Figure 1 can be configured for
operation up to a MOSFET supply voltage of 28V
(limited by the external MOSFETs and possibly the mini-
mum on-time).
Operating Frequency
The IC uses a constant frequency, phase-lockable archi-
tecture with the frequency determined by an internal
capacitor. This capacitor is charged by a fixed current plus
an additional current which is proportional to the voltage
applied to the PLLFLTR pin. Refer to the Phase-Locked
Loop and Frequency Synchronization section for addi-
tional information.
A graph for the voltage applied to the PLLFLTR pin versus
frequency is given in Figure 3. As the operating frequency
is increased the gate charge losses will be higher, reducing
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 680kHz.
Inductor Value Calculation and Output Ripple Current
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
12
CC
) is allowed to fall below approximately 4V. The
U
U
(Refer to Functional Diagram)
U
W
IN
U
and C
OUT
ramp up after the V
prevents power supply latchoff in the event of input power
switching break-before-make situations. The PGOOD pin
is held low during start-up until the RUN/SS capacitor
rises above the short-circuit latchoff arming threshold of
approximately 3.8V.
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge and transition losses. In addition to
this basic tradeoff, the effect of inductor value on ripple
current and low current operation must also be consid-
ered. The PolyPhase approach reduces both input and
output ripple currents while optimizing individual output
stages to run at a lower fundamental frequency, enhancing
efficiency.
The inductor value has a direct effect on ripple current. The
inductor ripple current ∆I
decreases with higher inductance or frequency and in-
creases with higher V
where f is the individual output stage operating frequency.
∆I
L
=
V
Figure 3. Operating Frequency vs V
700
600
500
400
300
200
OUT
fL
0
⎛
⎜
⎝
1
−
0.5
CC
PLLFLTR PIN VOLTAGE (V)
V
V
OUT
IN
IN
supply rises above 4V. This circuit
or V
1
⎞
⎟
⎠
L
OUT
per individual section, N,
1.5
:
2
3731H F03
PLLFLTR
2.5
3731hfa