LTC1773 LINER [Linear Technology], LTC1773 Datasheet - Page 7

• Manufacturer Part Number: LTC1773
• Description: Synchronous Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

OPERATIO
and forces the main switch to stay off for the same number
of cycles. Increasing the output load current slightly,
above the minimum required for discontinuous conduc-
tion mode, allows constant frequency PWM.
Frequency synchronization is inhibited when the feedback
voltage, V
clock from interfering with the frequency foldback for
short-circuit protection.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maxi-
mum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle until it reaches 100% duty cycle. The output voltage
will then be determined by the input voltage minus the IR
voltage drop across the external P-channel MOSFET,
sense resistor, and the inductor.
Undervoltage Lockout
A precision undervoltage lockout shuts down the LTC1773
when V
lithium-ion battery applications. In shutdown, the LTC1773
draws only several microamperes, which is low enough to
prevent deep discharge and possible damage to the lithium-
ion battery that’s nearing its end of charge. A 150mV
hysteresis ensures reliable operation with noisy supplies.
APPLICATIONS
The basic LTC1773 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement and begins with the selection of R
Once R
external power MOSFETs. Finally, C
lected
R
R
The LTC1773 current comparator has a maximum thresh-
old of 100mV/R
sets the peak of the inductor current, yielding a maximum
SENSE
SENSE
.
SENSE
IN
Selection for Output Current
is chosen based on the required output current.
FB
drops below 2.5V, making it ideal for single
, is below 0.6V. This prevents the external
is known, L can be chosen, followed by the
SENSE
U
U
(Refer to Functional Diagram)
. The current comparator threshold
INFORMATION
U
W
IN
and C
OUT
U
are se-
SENSE
.
Low Supply Operation
The LTC1773 is designed to operate down to a 2.65V
supply voltage. For proper operation at this low input
voltage, sub-logic level MOSFETs are required. When the
value of the output voltage is very close to the input
voltage, the converter is running at high duty cycles or in
dropout where the main switch is on continuously. See
Efficiency Considerations in the Applications Information
section.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability by preventing
subharmonic oscillations. It works by internally adding a
ramp to the inductor current signal at duty cycles in excess
of 30%. This causes the internal current comparator to trip
earlier. The I
conditions in which the duty cycle is below 30%. As a
result, the maximum inductor peak current is lower for
V
To compensate for this loss in maximum inductor peak
current during high duty cycles, the LTC1773 uses a
patent pending scheme that raises the I
(proportional to the amount of slope compensation) when
duty cycle is above 30%.
average output current I
half the peak-to-peak ripple current ∆I
Allowing a margin for variations in the LTC1773 and
external component values yields:
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC1773. The internal preset frequency is
550kHz, but can be externally synchronized up to 750kHz.
OUT
R
SENSE
/V
IN
> 0.3 than when V
= 70mV/I
TH
clamp level is also reached earlier than
MAX
MAX
OUT
equal to the peak value less
/V
IN
< 0.3.
L
.
LTC1773
TH
clamp level
1773fb
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