LTC1436ACGN-PLL View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1436ACGN-PLL
Linear Technology
'LTC1436ACGN-PLL' PDF : 28 Pages View PDF
LTC1436A
LTC1436A-PLL/LTC1437A
APPLICATIONS INFORMATION
The power dissipation on the topside MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in: RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input
voltage with T (estimated) = 50°C:
[ ] PMAIN
=
1.6V
22V
(3)2
1+
(0.005)(50°C
−
25°C)
(0.042Ω )
+ 2.5(22V)1.85(3A)(100pF)(250kHz) = 88mW
The most stringent requirement for the synchronous
N-channel MOSFET occurs when VOUT = 0 (i.e. short
circuit). In this case the worst-case dissipation rises to:
( ) ( ) ( ) PSYNC
=
ISC
AVG
2
1+
δ
RDS
ON
With the 0.033Ω sense resistor ISC(AVG) = 4A will result,
increasing the Si4412DY dissipation to 950mW at a die
temperature of 105°C.
CIN is chosen for an RMS current rating of at least 1.5A at
temperature. COUT is chosen with an ESR of 0.03Ω for low
output ripple. The output ripple in continuous mode will be
highest at the maximum input voltage. The output voltage
ripple due to ESR is approximately:
VORIPPLE = RESR (∆IL) = 0.03Ω(1.3 A) = 39mVP-P
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1436A/LTC1437A. These items are also illustrated
graphically in the layout diagram of Figure 15. Check the
following in your layout:
1. Are the signal and power grounds segregated? The
LTC1436A/LTC1437A signal ground pin must return to
the (–) plate of COUT. The power ground connects to the
source of the bottom N-channel MOSFET, anode of the
Schottky diode, and (–) plate of CIN, which should have
as short lead lengths as possible.
2. Does the LTC1436A/LTC1437A VOSENSE pin connect to
the (+) plate of COUT? In adjustable applications, the
resistive divider R1/R2 must be connected between the
(+) plate of COUT and signal ground. The 100pF capaci-
tor should be as close as possible to the LTC1436A/
LTC1437A.
3. Are the SENSE – and SENSE+ leads routed together with
minimum PC trace spacing? The filter capacitor be-
tween SENSE+ and SENSE– should be as close as
possible to the LTC1436A/LTC1437A.
4. Does the (+) plate of CIN connect to the drain of the
topside MOSFET(s) as closely as possible? This capaci-
tor provides the AC current to the MOSFET(s).
5. Is the INTVCC decoupling capacitor connected closely
between INTVCC and the power ground pin? This ca-
pacitor carries the MOSFET driver peak currents.
6. Keep the switching node SW away from sensitive small-
signal nodes. Ideally, the switch node should be placed
at the furthest point from the LTC1436A/LTC1437A.
7. Route the PLLIN line away from Boost and SW pins to
avoid unwanted pickup (Boost and SW pins have high
dV/dTs).
8. SGND should be used exclusively for grounding exter-
nal components on PLL LPF, COSC, ITH, LBI, SFB,
VOSENSE and AUXFB pins.
9. If operating close to the minimum on-time limit, is the
sense resistor oriented on the radial axis of the induc-
tor? See Figure 13.
23
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