Datasheet

LT1074/LT1076
8
sn1074 1074fds
Figure 1. Input Capacitor Ripple Figure 2. Proper Ground Pin Connection
V
IN
PIN
The V
IN
pin is both the supply voltage for internal control
circuitry and one end of the high current switch. It is
important, especially at low input voltages , that this pin be
bypassed with a low ESR, and low inductance capacitor to
prevent transient steps or spikes from causing erratic
operation. At full switch current of 5A, the switching
transients at the regulator input can get very large as
shown in Figure 1. Place the input capacitor very close to
the regulator and connect it with wide traces to avoid extra
inductance. Use radial lead capacitors.
FEEDBACK PIN
The feedback pin is the inverting input of an error amplifier
which controls the regulator output by adjusting duty
cycle. The noninverting input is internally connected to a
trimmed 2.21V reference. Input bias current is typically
0.5µA when the error amplifier is balanced (I
OUT
= 0). The
error amplifier has asymmetrical G
M
for large input sig-
nals to reduce startup overshoot. This makes the amplifier
more sensitive to large ripple voltages at the feedback pin.
100mVp-p ripple at the feedback pin will create a 14mV
offset in the amplifier, equivalent to a 0.7% output voltage
shift. To avoid output errors, output ripple (P-P) should be
less than 4% of DC output voltage at the point where the
output divider is connected.
See the “Error Amplifier” section for more details.
Frequency Shifting at the Feedback Pin
The error amplifier feedback pin (FB) is used to downshift
the oscillator frequency when the regulator output voltage
is low. This is done to guarantee that output short-circuit
DESCRIPTIO S
U
PI
U
L
P
= Total inductance in input bypass connections
and capacitor.
“Spike” height (dI/dt • L
P
) is approximately 2V per
inch of lead length for LT1074 and 0.8V per inch for
LT1076.
“Step” for ESR = 0.05Ω and I
SW
= 5A is 0.25V.
“Ramp” for C = 200µF, T
ON
= 5µs, and I
SW
= 5A,
is 0.12V.
Input current on the V
IN
Pin in shutdown mode is the sum
of actual supply current (≈140µA, with a maximum of
300µA), and switch leakage current. Consult factory for
special testing if shutdown mode input current is critical.
GROUND PIN
It might seem unusual to describe a ground pin, but in the
case of regulators, the ground pin must be connected
properly to ensure good load regulation. The internal
reference voltage is referenced to the ground pin; so any
error in ground pin voltage will be multiplied at the output;
LT1074•PD01
()
I
SW
()
ESR
STEP =
()
I
SW
()
T
ON
RAMP =
()
()
L
P
dl
dt
C
FB
GND
LT1074
LT1074•PD02
R2
HIGH CURRENT
RETURN PATH
NEGATIVE OUTPUT NODE
WHERE LOAD REGULATION
WILL BE MEASURED
∆
∆
V
VV
OUT
GND OUT
=
()()
221.
To ensure good load regulation, the ground pin must be
connected directly to the proper output node, so that no
high currents flow in this path. The output divider resistor
should also be connected to this low current connection
line as shown in Figure 2.