MIC5156 Micrel Semiconductor, MIC5156 Datasheet - Page 8

• Manufacturer Part Number: MIC5156
• Description: Super LDO Regulator Controller
• Manufacturer: Micrel Semiconductor

Charge Pump (MIC5157/5158 only)
The charge pump tripler creates a dc voltage across reservoir
capacitor C3. External capacitors C1 and C2 provide the
necessary storage for the stages of the charge pump tripler.
The tripler’s approximate dc output voltage is:
where:
The V
above V
as required. The charge pump oscillator operates at 160kHz.
The error amplifier uses the charge pump voltage to drive the
gate of the external MOSFET. It provides a constant load of
about 1mA to the charge pump. The error amplifier output can
swing to within 1V of V
Although the MIC5157/8 is designed to provide gate drive
using its internal charge pump, an external gate drive supply
voltage can be applied to V
drive supply, V
than V
When constant loads are driven, the ON/OFF switching of the
charge pump may be evident on the output waveform. This is
caused by the charge pump switching ON and rapidly in-
creasing the supply voltage to the error amplifier. The period
of this small charge pump excitation is determined by a
number of factors: the input voltage, the 1mA op-amp load,
any dc leakage associated with the MOSFET gate circuit, the
size of the charge pump capacitors, the size of the charge
pump reservoir capacitor, and the characteristics of the input
voltage and load. The period is lengthened by increasing the
charge pump reservoir capacitor (C3). The amplitude is
reduced by weakening the charge pump—this is accom-
plished by reducing the size of the pump capacitors (C1 and
C2). If this small burst is a problem in the application, use a
10 F reservoir capacitor at C3 and 0.01 F pump capacitors
Application Information
MOSFET Selection
Standard N-channel enhancement-mode MOSFETs are ac-
ceptable for most Super LDO regulator applications.
Logic-level N-channel enhancement-mode MOSFETs may
be necessary if the external gate drive voltage is too low
(MIC5156), or the input voltage is too low, to provide ad-
equate charge pump voltage (MIC5157/8) to enhance a
standard MOSFET.
Circuit Layout
For the best voltage regulation, place the source, ground, and
error amplifier connections as close as possible to the load.
See figures (1a) and (1b).
March 1999
MIC5156/5157/5158
V
V
V
CP
DD
CP
CP
DD
DD
clamp circuit limits the charge pump voltage to 16V
.
= charge pump output voltage
= supply voltage
by gating the charge pump oscillator ON or OFF
3 (V
CP
DD
must not be forced more than 14V higher
– 1)
CP
.
CP
. When using an external gate
3-119
at C1 and C2. Note that the recovery time to repetitive load
transients may be affected with small pump capacitors.
Gate-to-Source Clamp
A gate-to-source protective voltage clamp of 16.6V protects
the MOSFET in the event that the output voltage is suddenly
forced to zero volts. This prevents damage to the external
MOSFET during shorted load conditions. Refer to “Charge
Pump” for normal clamp circuit operation.
The source connection required by the gate-to-source clamp
is not available on the adjustable version of the MIC5156.
Output Regulation
At start-up, the error amplifier feedback voltage (EA), or
internal feedback on fixed versions, is below nominal when
compared to the internal 1.235V bandgap reference. This
forces the error amplifier output high which turns on external
MOSFET Q1. Once the output reaches regulation, the con-
troller maintains constant output voltage under changing
input and load conditions by adjusting the error amplifier
output voltage (gate enhancement voltage) according to the
feedback voltage.
Out-of-Regulation Detection
When the output voltage is 8% or more below nominal, the
open-collector FLAG output (normally high) is forced low to
signal a fault condition. The FLAG output can be used to
signal or control external circuitry. The FLAG output can also
be used to shut down the regulator using the EN control.
Current Limiting
Super LDO Regulators perform constant-current limiting (not
foldback). To implement current limiting, a sense resistor
(R
(drain).
If the voltage drop across the sense resistor reaches 35mV,
the current limit comparator reduces the error amplifier out-
put. The error amplifier output is decreased only enough to
reduce the output current, keeping the voltage across the
sense resistor from exceeding 35mV.
S
) must be placed in the “power” path between V
Figure 1a. Connections for Fixed Output
MIC515x
GND
G
S
V
IN
DD
and D
Micrel
3