MIC5156/5157/5158 Micrel, Inc.
MIC5156/5157/5158 10 August 2005
Current Limiting
Current sensing requires a low-value series resistance (R
s
)
between V
DD
and D (drain). Refer to the typical applications.
The internal current-limiting circuit limits the voltage drop
across the sense resistor to 35mV. Equation (2) provides the
sense resistor value required for a given maximum current.
2)
where:
R
S
= sense resistor value
I
LIM
= maximum output current
Most current-limited applications require low-value resistors.
See Application Hints 21 and 25 for construction hints.
Non-Current-Limited Applications
For circuits not requiring current limiting, do not use a sense
resistor between V
DD
and D (drain). See figure (3). The con-
troller will not limit current when it does not detect a 35mV
drop from V
DD
to D.
Figure 3. No Current Limit
3.3V Microprocessor Applications
For computer designs that use 3.3V microprocessors with 5V
logic, the FLAG output can be used to suppress the 5V sup-
ply until the 3.3V output is in regulation. Refer to the external
components shown with the MIC5156 Block Diagram.
SMPS Post Regulator Application
A Super LDO regulator can be used as a post regulator for a
switch-mode power supply. The Super LDO regulator can pro-
vide a significant reduction in peak-to-peak ripple voltage.
High-Current Switch Application
All versions of the MIC5156/7/8 may be used for current-lim-
ited, high-current, high-side switching with or without voltage
regulation. See figure (4a). Simply leave the “S” terminal
open. A 16V zener diode from the gate to the source of the
MOSFET protects the MOSFET from overdrive during fault
conditions.
G
MIC5156-x.x
D
V
DD
V
IN
V
G
Gate Supply
Enable
Shutdown
EN
GND
R
S
S
Figure 4a. High-Side Switch
If a MIC5157 or MIC5158 is used and is shutdown for a
given time, the charge pump reservoir V
CP
will bleed off. If
recharging the reservoir causes an unacceptable delay in the
load reaching its operating voltage, do not use the EN pin
for on/off control. Instead, use the MIC5158, hold EN high to
keep the charge pump in continuous operation, and switch
the MOSFET on or off by overriding the error amplifier input
as shown in figure (4b).
GND
S
EA
MIC5158
G
EN
Output Off
Output On
1N4148
V
IN
V
DD
Figure 4b. Fast High-Side Switch
Battery Charger Application
The MIC5158 may be used in constant-current applications
such as battery chargers. See figure (5). The regulator supplies
a constant-current (35mV ÷ R3) until the battery approaches
the float voltage:
V
FL
= 1 . 235 (1 + )
R1
R2
where:
V
FL
= float voltage
At float voltage, the MOSFET is shut off. A trickle charge is
supplied by R4.
