lm2770sdx-12157 National Semiconductor Corporation, lm2770sdx-12157 Datasheet - Page 10

• Manufacturer Part Number: lm2770sdx-12157
• Description: High Efficiency Switched Capacitor Step-down Dc/dc Regulator With Sleep Mode
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation Description
OVERVIEW
The LM2770 is a switched capacitor converter that produces
a regulated low voltage output. The core of the part is a
highly efficient charge pump that utilizes multiple fractional
gains and pulse-frequency-modulated (PFM) switching to
minimize power losses over wide input voltage and output
current ranges. A description of the principal operational
characteristics of the LM2770 is broken up into the following
sections: PFM Regulation, Fractional Multi-Gain Charge
Pump, and Multi-Gain Efficiency Performance. Each of
these sections refers to the Block Diagram.
PFM REGULATION
The LM2770 achieves tightly regulated output voltages with
pulse-frequency-modulated (PFM) regulation. PFM simply
means the part only pumps when charge needs to be deliv-
ered to the output in order to keep the output voltage in
regulation. When the output voltage is above the target
regulation voltage, the part idles and consumes minimal
supply-current. In this state, the load current is supplied
solely by the charge stored on the output capacitor. As this
capacitor discharges and the output voltage falls below the
target regulation voltage, the charge pump activates, and
charge is delivered to the output. This charge supplies the
load current and boosts the voltage on the output capacitor.
The primary benefit of PFM regulation is when output cur-
rents are light and the part is predominantly in the low-
supply-current idle state. Net supply current is minimal be-
cause the part only occasionally needs to recharge the
output capacitor by activating the charge pump. With PFM
regulation, input and output ripple frequencies vary signifi-
cantly, and are dependent on output current, input voltage,
and, to a lesser degree, other factors such as temperature
and internal switch characteristics.
FRACTIONAL MULTI-GAIN CHARGE PUMP
The core of the LM2770 is a two-phase charge pump con-
trolled by an internally generated non-overlapping clock. The
charge pump operates by using the external flying capaci-
tors, C1 and C2, to transfer charge from the input to the
output.
The two phases of the switching cycle will be referred to as
the "charge phase" and the "hold/rest phase". During the
charge phase, the flying capacitors are charged by the input
supply. After charging the flying capacitors for half of a
switching cycle [ t = 1/(2xF
hold/rest phase. In this configuration, the charge that was
stored on the flying capacitors in the charge phase is trans-
ferred to the output. If the voltage on the output is below the
target regulation voltage at completion of the switching
cycle, the charge pump will switch back to the charge phase.
But if the output voltage is above the target regulation volt-
age at the end of the switching cycle, the charge pump will
remain in the hold/rest state. It will idle in this mode until the
output voltage drops below the target regulation voltage.
When this finally occurs, the LM2770 will switch back to the
charge phase.
Input, output, and intermediary connections of the flying
capacitors are made with internal MOS switches. The
LM2770 utilizes two flying capacitors and a versatile switch
network to achieve three distinct fractional voltage gains:
1
LM2770 is three-charge-pumps-in-one. The "active" charge
⁄
2
, and
2
⁄
3
. With this gain-switching ability, it is as if the
SW
) ], the LM2770 switches to the
1
⁄
3
,
10
pump at any given time is the one that yields the highest
efficiency based on the input and output conditions present.
MULTI-GAIN EFFICIENCY PERFORMANCE
The ability to switch gains based on input and output condi-
tions results in optimal efficiency throughout the operating
ranges of the LM2770. Charge-pump efficiency is derived in
the following two ideal equations (supply current and other
losses are neglected for simplicity):
In the equations, G represents the charge pump gain. Effi-
ciency is at its highest as GxV
the efficiency graphs in the Typical Perfromance Charac-
teristics section for detailed efficiency data. The gain re-
gions are clearly distinguished by the sharp discontinuities in
the efficiency curves and are identified at the bottom of each
graph (G =
DYNAMIC OUTPUT VOLTAGE SELECTION
The output voltage of the LM2770 can be dynamically ad-
justed for the purpose of improving system efficiency. Each
LM2770 version contains two built-in output voltage options:
a high level and a low level (1.5V and 1.2V, for example).
With the simple V
be switched between these two voltages.
Dynamic voltage selection can be used to improve overall
system efficiency. When comparing system efficiency be-
tween two different output voltages, evaluating power con-
sumption often lends more insight than actually comparing
converter efficiencies. An application powered with a Li-Ion
battery is a good example to illustrate this. Referring to the
LM2770 efficiency curves (see Typical Performance Char-
actersitics), all LM2770 output voltage options operate with
G =
3.9V). Thus, the LM2770 circuit will draw an input current
that is approximately half the output current in the core Li-Ion
voltage range, regardless of the output voltage (I
I
While varying the LM2770 output voltage does not directly
improve system efficiency, it can have a secondary effect.
Different output voltages often will result in different LM2770
load currents. This is where system efficiency can benefit
from dynamic output voltage selection: the LM2770 load
circuit can run at lower currents. This reduces LM2770 input
current and improves overall system efficiency.
SLEEP MODE BYPASS LDO
The LM2770 offers a bypass low-dropout linear regulator
(LDO) for low-noise performance under light loads. Capable
of delivering up to 20mA of output current, this LDO has low
ground pin current and is ideal for stand-by operation. The
LDO is activated with the SLEEP logic input pin. When
SLEEP is active, the charge pump is disabled and the LDO
supplies all load current.
SHUTDOWN
The LM2770 is in shutdown mode when the voltage on the
enable pin (EN) is logic-low. In shutdown, the LM2770 draws
virtually no supply current. When in shutdown, the output of
the LM2770 is completely disconnected from the input. The
internal feedback resistors will pull the output voltage down
to 0V (unless the output is driven by an outside source).
In some applications, it may be desired to disable the
LM2770 and drive the output pin with another voltage
OUT
) .
E = (V
1
⁄
2
over the core Li-Ion battery voltage range (3.5V -
OUT
2
⁄
3
, G =
x I
SEL
OUT
1
⁄
2
) ÷ (V
, and G =
logic input pin, the output voltage can
I
IN
= G x I
IN
x I
IN
IN
1
approaches V
OUT
⁄
3
) = V
).
OUT
÷ (G X V
OUT
. Refer to
IN
IN
= G x
)