LTC1779 Datasheet, PDF (8/12 Pages) Linear Technology – 250mA Current Mode Step-Down DC/DC Converter in ThinSOT

LTC1779
APPLICATIO S I FOR ATIO
Under normal load conditions, the average current con-
ducted by the diode is:
I_D
=


V_IN − V_OUT
V_IN + V_D


I_OUT
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
V_F
≈
P_D
I_CL(MAX)
where P_D is the allowable power dissipation and will be
determined by efficiency and/or thermal requirements.
A fast switching diode must also be used to optimize
efficiency. Schottky diodes are a good choice for low
forward drop and fast switching times. Remember to keep
lead length short and observe proper grounding (see
Board Layout Checklist) to avoid ringing and increased
dissipation.
C_IN and C_OUT Selection
In continuous mode, the source current of the internal
P-channel MOSFET is a square wave of duty cycle
(V_OUT + V_D)/(V_IN + V_D). To prevent large voltage tran-
sients, a low ESR input capacitor sized for the maximum
RMS current must be used. The maximum RMS capaci-
tor current is given by:
[ ] ^{( )} _{CIN Required}
I_RMS
≈ I_MAX
V_OUT
V_IN − V_OUT
V_IN
1/ 2
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC1779, ceramic capacitors
can also be used for C_IN. Always consult the manufacturer
if there is any question.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple (∆V_OUT) is approximated by:
∆V_OUT
≈
I_RIPPLE^{}ESR
+
8
1
fC_OUT


where f is the operating frequency, C_OUT is the output
capacitance and I_RIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since ∆I_L increases with input voltage.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR (size)
product of any aluminum electrolytic at a somewhat
higher price. Once the ESR requirement for C_OUT has been
met, the RMS current rating generally far exceeds the
I_RIPPLE(P-P) requirement.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS, AVX TPSV and KEMET T510 series of surface mount
tantalum, available in case heights ranging from 2mm to
4mm. Other capacitor types include Sanyo OS-CON,
Nichicon PL series and Panasonic SP.
Low Supply Operation
Although the LTC1779 can function down to approxi-
mately 2.0V, the maximum allowable output current is
reduced when V_IN decreases below 3V. Figure 3 shows the
amount of change as the supply is reduced down to 2V.
Also shown in Figure 4 is the effect of V_IN on V_REF as V_IN
goes below 2.3V.
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