LTC1704 Linear Technology, LTC1704 Datasheet - Page 24

• Manufacturer Part Number: LTC1704
• Description: 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1704/LTC1704B
doesn’t cause a bigger spike than the transient signal
being measured. Conveniently, the typical probe tip ground
clip is spaced just right to span the leads of a typical output
capacitor. Make sure the bandwidth limit on the scope is
turned off, since a significant portion of the transient
energy occurs above the 20MHz cutoff.
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, and switch 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 LTC1704 and the transient generator
must be minimized.
Figure 12 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 50W if
pulsed with a 5% duty cycle, enough for most LTC1704
circuits. Solder the MOSFET and the resistor(s) as close to
the output of the LTC1704 circuit as possible and set up
the signal generator to pulse at a 100Hz rate with a 5% duty
cycle. This pulses the LTC1704 with 500 s transients
10ms apart, adequate for viewing the entire transient
recovery time for both positive and negative transitions
while keeping the load resistor cool.
24
Figure 12. Transient Load Generator
DUTY CYCLE
LOCATE CLOSE TO THE OUTPUT
GENERATOR
100Hz, 5%
0V TO 10V
LTC1704
PULSE
U
U
50
R
IRFZ44 OR
EQUIVALENT
LOAD
V
W
OUTSW
1704 F12
U
LINEAR REGULATOR SUPPLY
Linear Regulator Output Voltage
The linear regulator senses the output voltage at V
with an internal amplifier (see Figure 13). The amplifier
negative input is connected internally to an 800mV refer-
ence, while the positive input is connected to the REGFB
pin. The amplifier output drives a P-channel transistor
MREG, which is in turn connected to the external NPN pass
transistor. The linear regulator output voltage can be
obtained using the following equation:
Linear Regulator Supplies Requirement
The linear regulator operates with two supplies: V
LTC1704 and V
Both supplies must be higher than the minimum value
determined by the linear regulator output voltage, V
For a desired V
calculate the minimum required V
V
LTC1704
REF
Minimum V
V
OUTREG
+
–
–
+
REGFB
AMP
0 8
CC
INREG
.
Figure 13. Linear Regulator
OUTREG
= V
V
REG
ILM
1.9 A
2mA
for the external NPN transistor QEXT.
1
OUTREG
V
V
, use the following formula to
R
R
CC
CC
MREG
5
6
I
P
+ V
REGILM
REGDR
REGFB
MOFF
BE(QEXT)
CC
:
QEXT
R5
R6
C
OUTREG
+ V
R
C
DROPOUT
REGILM
DELAY
1704 F13
+
CC
OUTREG
OUTREG
for the
V
REGOFF
V
V
OUTREG
1704bfa
REGON
INREG
.