MIC5156 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – Super LDO™ Regulator Controller

MIC5156/5157/5158
V_IN
G
MIC5157 _S
or
MIC5158
EA
GND
Micrel
Adjustable Configurations
Micrel’s MIC5156 [adjustable] and MIC5158 require an exter-
nal resistive divider to set the output voltage from 1.235V to
36V. For best results, use a 10kΩ resistor for R2. See
equation (1) and figure (2).
1)
R₁
=
1× 10⁴


V_OUT
1. 235
− 1^{}
Figure 1b. Connections for Adjustable Output
G
MIC5156
EA
GND
V_IN
*
* Optional 16V zener diode
recommended in applications
where V_G is greater than 18V
Figure 1c. MIC5156 Connections for
Adjustable Output
MOSFET Gate-to-Source Protection
When using the adjustable version of the MIC5156, an
external 16V zener diode placed from gate-to-source is
recommended for MOSFET protection. All other versions of
the Super LDO regulator controller use the internal gate-to-
source clamp.
Output Voltage Configuration
Fixed Configurations
The MIC5156-3.3 and MIC5156-5.0 are preset for 3.3V and
5.0V respectively.
The MIC5157 operates at 3.3V when the 3.3V pin is con-
nected to the S (source) pin; 5.0V when the 5.0V pin is
connected to the S pin; or 12V if the 3.3V and 5.0V pins are
open.
The MIC5158 operates at a fixed 5V (without an external
resistive divider) if the 5V FB pin is connected to EA.
G
S
MIC5157/8
EA*
GND
V_OUT
R1
R2
10k
Figure 2. Typical Resistive Divider
Input Filter Capacitor
The Super LDO requires an input bypass capacitor for
accommodating wide changes in load current and for decou-
pling the error amplifier and charge pump. A medium to large
value low-ESR (equivalent series resistance) capacitor is
best, mounted close to the device.
Output Filter Capacitor
An output filter capacitor may be used to reduce ripple and
improve load regulation. Stable operation does not require a
large capacitor, but for transient load regulation the size of the
output capacitor may become a consideration. Common
aluminum electrolytic capacitors perform nicely; very low-
ESR capacitors are not necessary. Increased capacitance
(rather than reduced ESR) is preferred. The capacitor value
should be large enough to provide sufficient I = C × dV/dt
current consistent with the required transient load regulation
quality. For a given step increase in load current, the output
voltage will drop by about dV = I × dt/C, where I represents the
increase in load current over time t. This relationship as-
sumes that all output current was being supplied via the
MOSFET pass device prior to the load increase. Small
(0.01µF to 10µF) film capacitors parallel to the load will further
improve response to transient loads.
Some linear regulators specify a minimum required output
filter capacitance because the capacitor determines the
dominant pole of the system, and thereby stabilizes the
system. This is not the situation for the MIC5156/7/8; its
dominant pole is determined within its error amplifier.
3-120
March 1999