LTC3802_15 Datasheet, PDF (26/28 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

LTC3802
APPLICATIO S I FOR ATIO
to ensure that inductance in the ground path doesn’t
cause a bigger spike than the transient signal being
measured. The typical probe tip ground clip is spaced just
right to span the leads of a typical output capacitor. In
general, it is best to take this measurement with the
20MHz bandwidth limit on the oscilloscope turned on to
limit high frequency noise. Note that microprocessor
manufacturers typically specify ripple ≤ 20MHz, as en-
ergy above 20MHz is generally radiated and not con-
ducted and will not affect the load even if it appears at the
output capacitor.
Now that we know how to measure the signal, we need to
have something to measure. The ideal situation is to use
the actual load for the test, switching it on and off while
watching the output. If this isn’t convenient, a current step
generator is needed. This generator needs to be able to
turn on and off in nanoseconds to simulate a typical
switching logic load, so stray inductance and long clip
leads between the LTC3802 and the transient generator
must be minimized.
Figure 9 shows an example of a simple transient genera-
tor. Be sure to use a noninductive resistor as the load
element—many power resistors use an inductive spiral
pattern and are not suitable for use here. A simple solution
is to take ten 1/4W film resistors and wire them in parallel
to get the desired value. This gives a noninductive resistive
load which can dissipate 2.5W continuously or 250W if
pulsed with a 1% duty cycle, enough for most LTC3802
circuits. Solder the MOSFET and the resistor(s) as close to
the output of the LTC3802 circuit as possible and set up
the signal generator to pulse at a 100Hz rate with a 1% duty
cycle. This pulses the LTC3802 with 100µs transients
10ms apart, adequate for viewing the entire transient
recovery time for both positive and negative transitions
while keeping the load resistor cool.
LTC3802
PULSE
GENERATOR
0V TO 10V
100Hz, 1%
DUTY CYCLE
50Ω
10k
V_OUT
R_LOAD
IRFZ44 OR
EQUIVALENT
LOCATE CLOSE TO THE OUTPUT
3802 F09
Figure 9. Transient Load Generator
PACKAGE DESCRIPTIO
GN Package
28-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.045 ±.005
.386 – .393*
(9.804 – 9.982)
28 27 26 25 24 23 22 21 20 19 18 17 1615
.033
(0.838)
REF
.254 MIN
.150 – .165
.229 – .244
(5.817 – 6.198)
.150 – .157**
(3.810 – 3.988)
26
.0165 ± .0015
RECOMMENDED SOLDER PAD LAYOUT
.0250 TYP
.0075 – .0098
(0.191 – 0.249)
.015 ± .004
(0.38 ± 0.10)
× 45°
0° – 8° TYP
1 2 3 4 5 6 7 8 9 10 11 12 13 14
.053 – .069
(1.351 – 1.748)
.004 – .009
(0.102 – 0.249)
.016 – .050
.008 – .012
.0250
(0.406 – 1.270)
(0.203 – 0.305)
(0.635)
BSC
NOTE:
1. CONTROLLING DIMENSION: INCHES *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
2.
DIMENSIONS
ARE
IN
INCHES
(MILLIMETERS)
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
3. DRAWING NOT TO SCALE
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
GN28 (SSOP) 0502
3802f