LT1777CS Linear Technology, LT1777CS Datasheet - Page 15

• Manufacturer Part Number: LT1777CS
• Description: IC SWREG STP-DWN LOWNOISE 16SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1777CS

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.24 ~ 30 V
• Current - Output: 700mA
• Frequency - Switching: 100kHz
• Voltage - Input: 12 ~ 48 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
However, remember that oscillator slowdown to achieve
short-circuit protection (discussed above) is dependent
on FB pin behavior, and this in turn, is sensitive to FB node
external impedance. The graph in Figure 7 shows the
typical relationship between FB pin voltage, driving im-
pedance and oscillator frequency. This shows that as
feedback network impedance increases beyond 10k, com-
plete oscillator slowdown is not achieved, and short-
circuit protection may be compromised. And as a practical
matter, the product of FB pin bias current and larger FB
network impedances will cause increasing output voltage
error. (Nominal cancellation for 10k of FB Thevenin im-
pedance is included internally.)
Thermal Considerations
Care should be taken to ensure that the worst-case input
voltage and load current conditions do not cause exces-
sive die temperatures. The SO16 package is rated at
50 C/W when the four corner package pins are connected
to a good ground plane. (These corner pins are internally
fused to the die paddle for improved thermal perfor-
mance.) Die junction temperature is then a function of
ambient temperature and internal dissipation as follows:
Total internally dissipated power is composed of three
parts, quiescent power, DC switch loss and AC switch
loss. The AC switch loss will often dominate the total
dissipation, and this is unfortunately difficult to estimate
accurately.
Two options are suggested to the potential user. The first
is to observe the graphical data presented in the Typical
Applications section. Internal LT1777 dissipation vs load
current is given for output voltages of 5V and 3.3V, with
input voltages of 12V, 24V and 36V, and with sense
inductors of 0 H, 1 H, and 2.2 H (Figures 9 and 11).
While it is true that the user’s ultimate circuit may use
somewhat different passive components than the ex-
amples given, it turns out that internal IC dissipation is not
very sensitive to these changes.
In cases where the user’s potential circuit differs signifi-
cantly from the examples given, an empirical method is
T
J
= T
A
+
JA
• P
U
INT
INFORMATION
U
W
U
suggested. Operate the proposed power supply over the
applicable input voltage and load current ranges. Measure
the input power and output power, and calculate the
difference as “lost power.” This measured lost power
minus estimated inductor and diode dissipation yields a
figure for internal LT1777 dissipation. Fortunately, as
LT1777 internal dissipation dominates total lost power,
inductor and diode power need not be estimated very
accurately. Inductor power may be estimated as I
I is the load current and R is the DC resistance of the
inductor. (Loss in the sense inductor is usually so small
that only the main inductor must be considered.) Diode
power may be estimated as 1/2 • V
diode forward voltage, I is the load current and DC is the
duty cycle percentage when the diode is conducting.
Frequency Compensation
Loop frequency compensation is performed by connect-
ing a capacitor, or in most cases a series R/C, from the
output of the error amplifier (V
loop compensation may be obtained by empirical meth-
ods as described in detail in Application Note AN19.
Briefly, this involves applying a load transient and observ-
ing the dynamic response over the expected range of V
and I
As a practical matter, a second small capacitor, directly
from the V
attenuate capacitive coupling from the V
typical value for this capacitor is 100pF. (See Switch Node
Considerations).
Switch Node Considerations
In spite of the fact that the LT1777 is a low noise converter,
it is still possible for the part to cause problems by
“coupling to itself.” Specifically, this can occur if the V
pin is allowed to capacitively couple in an uncontrolled
manner to the part’s high impedance nodes, i.e., SHDN,
SYNC, V
odd/even cycle behavior, pulse width “nervousness”, im-
proper output voltage and/or premature current limit
action.
LOAD
C
and FB. This can cause erratic operation such as
values.
C
pin to ground is generally recommended to
C
F
pin) to ground. Proper
• I • DC, where V
SW
and V
LT1777
2
D
R where
15
pins. A
F
is the
SW
IN