U5 Charge-Pump Regulator Overview

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U5
Here’s a detailed overview of U5 in your design:
Component: U5 – LM27762DSSR charge-pump switching regulator
1. Purpose & Core Function
  • Generates both a regulated positive rail (OUT⁺) and negative rail (OUT⁻) from a single input (VIN).
  • Ideal when you need small bipolar supplies (e.g. ±2.5 V, ±5 V) but don’t want an inductor-based solution.
2. Key Features
  • Dual charge-pump architecture with internal 2 MHz oscillator and MOSFET switch array.
  • Adjustable outputs via external resistor dividers on FB⁺ and FB⁻ pins.
  • Built-in LDO regulators for low-noise, tightly regulated rails.
  • Power-Good (PGOOD) flag (open-drain) to indicate when both outputs are in regulation.
  • Shutdown pins (EN⁺, EN⁻) enable individual disabling of each pump, reducing quiescent current.
  • Capable of up to 250 mA per output.
3. How It Operates in the Circuit
  1. Oscillator & Pumping Stage
    • A 2 MHz clock drives internal switches that shuttle charge into flying capacitors (C1+, C1–).
  2. Switch-to-Output Stage
    • Pumped voltage is steered through an internal LDO stage to produce regulated OUT⁺ and OUT⁻.
  3. Feedback & Regulation
    • External resistor dividers on FB⁺/FB⁻ set the exact output voltages.
    • The chip adjusts switch duty-cycle and LDO gate drive to maintain regulation.
  4. Monitoring & Control
    • PGOOD asserts low when both rails are within regulation bands.
    • EN pins allow power-down or sequencing.
4. Common Applications
  • Hi-Fi audio headphone amplifiers (bias rails)
  • Op-amp power biasing in precision analog circuits
  • Powering data converters (ADC/DAC supply rails)
  • Interface supplies for mixed-signal or wireless modules
  • Hand-held instrumentation where ± rails are needed without large inductors
5. Notable Characteristics
  • High switching frequency (2 MHz) allows use of small SMD capacitors (0603/0805).
  • Low-ESR pump caps yield better efficiency and lower output impedance.
  • Exposed pad WSON package for superior thermal performance.
  • Wide temperature range (–40 °C to +85 °C).
6. Series & Family Context
The LM27762 belongs to TI’s family of high-frequency, dual-output charge-pump regulators.
  • Fixed-output variants (e.g. LM27762-5.0) provide factory-set ±5 V with no external divider.
  • Adjustable versions (LM27762DSSR) let you pick any ± output within the device’s range (typically ±1.5 V to ±6 V).
  • Key differences across the family:
    • Output voltage options (fixed vs. adjustable)
    • Maximum output current (200–300 mA)
    • Package styles (WSON vs. QFN)
  • Use-case trade-offs:
    • Choose fixed versions for simpler BOM when only one bipolar voltage is needed.
    • Choose the adjustable LM27762DSSR for flexibility in mixed-supply or multi-rail systems.
By using U5, your signal generator gains compact, inductor-less ± rails that are essential for driving the op-amps (e.g., U1–U3) and delivering clean, stable waveforms.
Specify output voltages
Add pump caps
Set shutdown behavior
Enable PGOOD pin
Thermal considerations
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how does the feedback loop work on U1?
nico

nico

@copilot how does the feedback loop work on U1?
2y

copilot

The feedback on the Operational Amplifier (Op-Amp) U1 is implemented through a resistor R1 and a resistor R3.
In this case, U1 is configured in a non-inverting amplifier configuration. The input signal is applied to the non-inverting input (U1:+), while the output (U1:OUTPUT) is fed back to the inverting input (U1:-) via resistors R1 and R3, forming a voltage divider.
The feedback helps to stabilize the amplifier’s gain and improve its performance. It works as follows:
  • The Op-Amp produces an output signal in response to the difference between its input signals.
  • A portion of the output signal is fed back to the inverting input via resistors R3 and R1 that act as voltage dividers, hence the name 'feedback'.
  • Since the feedback signal is in phase with the input, it counteracts any increase or decrease in the input signal to maintain a stable output.
This feedback configuration gives us a voltage gain of:
Gain=1+R3R1 Gain = 1 + \frac{R3} {R1}
The exact values of the resistors (R3 and R1) in the divider help define the amplifier's closed-loop gain. Thus, by adjusting R1 or R3, you can control the gain of the op-amp circuit.
Also, in this case, R1 is connected to the ground, forming negative feedback, which is essential for amplifier stability, accuracy, and predictability. Negative feedback tends to decrease the output (due to the inverse phase) and is used in most amplifiers to improve bandwidth and distortion while stabilizing the gain.
2y
Someone is typing...
what is c2 accomplishing in the feedback loop?
nico

nico

@copilot what is c2 accomplishing in the feedback loop?
2y

copilot

The capacitor C2, according to the netlist, is directly connected between the output (OUTPUT pin) and the inverting input (- pin) of the operational amplifier U2. This setup constitutes a feedback loop.
C2 effectively operates as an integral component of a high-pass filter which is part of the configuration of the op-amp U2. In the feedback loop of an amplifier, having a capacitor allows the circuit to preferentially amplify higher frequencies more than lower frequencies, creating a 'high-pass' effect.
The exact cut-off frequency of the high-pass filter is determined by the values of the capacitor C2 and the resistor R5, in line with the formula:
fc=12πR5C2 f_{c} = \frac{1} {2 \pi R5 C2}
Where:
  • $f_{c} $ is the cut-off frequency,
  • $R5$ is the resistance connected to the capacitor in feedback loop, and
  • $C2$ is the capacitance value of the capacitor in the feedback loop.
In the Fourier domain, the phase of the output signal is shifted by 90 degrees by the capacitor which can be useful for specific applications where a phase shift or waveform shaping is required. However, more information about the whole circuit and its intended application is needed to say more about the exact influence and role of C2 in this circuit.
2y
Someone is typing...
3
-2.7V
+2.7V
+5V
VBUS
+5V
+5V
R9
Resistance
51.1k Ω
C20
Capacitance
10u F
C6
Capacitance
10u F
R10
Resistance
41.2k Ω
C21
Capacitance
4.7u F
R14
Resistance
Resistance
Resistance
5.1k Ω
C24
Capacitance
4.7u F
C4
Capacitance
4.7u F
C5
Capacitance
1u F
R12
Resistance
49.9k Ω
R11
Resistance
41.2k Ω
C19
Capacitance
0.1u F
R13
Resistance
Resistance
Resistance
5.1k Ω
U5
Manufacturer Part Number
LM27762DSSR
CC2
CC1
D1
CC2
RV1
Resistance
100k Ω
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