MIC5156 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – Super LDO™ Regulator Controller

MIC5156/5157/5158
Micrel
Charge Pump (MIC5157/5158 only)
at C1 and C2. Note that the recovery time to repetitive load
The charge pump tripler creates a dc voltage across reservoir ^{transients may be affected with small pump capacitors.}
capacitor C3. External capacitors C1 and C2 provide the Gate-to-Source Clamp
necessary storage for the stages of the charge pump tripler. _{A gate-to-source protective voltage clamp of 16.6V protects}
The tripler’s approximate dc output voltage is:
the MOSFET in the event that the output voltage is suddenly
V_CP ≈ 3 (V_DD – 1)
where:
V_CP = charge pump output voltage
V_DD = supply voltage
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.
The V_CP clamp circuit limits the charge pump voltage to 16V
above V_DD by gating the charge pump oscillator ON or OFF
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_CP.
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
Although the MIC5157/8 is designed to provide gate drive input and load conditions by adjusting the error amplifier
using its internal charge pump, an external gate drive supply output voltage (gate enhancement voltage) according to the
voltage can be applied to V_CP . When using an external gate feedback voltage.
drive supply, V_CP must not be forced more than 14V higher
than V_DD.
Out-of-Regulation Detection
When the output voltage is 8% or more below nominal, the
3
When constant loads are driven, the ON/OFF switching of the _{open-collector FLAG output (normally high) is forced low to}
charge pump may be evident on the output waveform. This is _{signal a fault condition. The FLAG output can be used to}
caused by the charge pump switching ON and rapidly in- _{signal or control external circuitry. The FLAG output can also}
creasing the supply voltage to the error amplifier. The period _{be used to shut down the regulator using the EN control.}
of this small charge pump excitation is determined by a
number of factors: the input voltage, the 1mA op-amp load,
Current Limiting
any dc leakage associated with the MOSFET gate circuit, the ^{Super LDO Regulators perform constant-current limiting (not}
size of the charge pump capacitors, the size of the charge ^{foldback). To implement current limiting, a sense resistor}
pump reservoir capacitor, and the characteristics of the input ^(RS^{) must be placed in the “power” path between V}DD ^{and D}
voltage and load. The period is lengthened by increasing the ^(drain).
charge pump reservoir capacitor (C3). The amplitude is If the voltage drop across the sense resistor reaches 35mV,
reduced by weakening the charge pump—this is accom- the current limit comparator reduces the error amplifier out-
plished by reducing the size of the pump capacitors (C1 and put. The error amplifier output is decreased only enough to
C2). If this small burst is a problem in the application, use a reduce the output current, keeping the voltage across the
10µF reservoir capacitor at C3 and 0.01µF pump capacitors sense resistor from exceeding 35mV.
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).
V_IN
G
S
MIC515x
GND
Figure 1a. Connections for Fixed Output
March 1999
3-119